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 Timezone: CET
Creation date: 20200121
Creation time: 073744
 Number of references
133
article
Pogorzalek_Nat.Comm.2019
Secure quantum remote state preparation of squeezed microwave states
NatureCommunications
2019
6
13
10
2604
Quantum communication protocols based on nonclassical correlations can be more efficient than known classical methods and offer intrinsic security over direct state transfer. In particular, remote state preparation aims at the creation of a desired and known quantum state at a remote location using classical communication and quantum entanglement. We present an experimental realization of deterministic continuousvariable remote state preparation in the microwave regime over a distance of 35 cm. By employing propagating twomode squeezed microwave states and feed forward, we achieve the remote preparation of squeezed states with up to 1.6 dB of squeezing below the vacuum level. Finally, security of remote state preparation is investigated by using the concept of the onetime pad and measuring the von Neumann entropies. We find nearly identical values for the entropy of the remotely prepared state and the respective conditional entropy given the classically communicated information and, thus, demonstrate closetoperfect security.
https://www.nature.com/articles/s41467019107277
10.1063/1.5052414
article
Verresen_Nat.Phy.2019
Avoided quasiparticle decay from strong quantum interactions
Nature Physics
2019
5
27
Quantum states of matter—such as solids, magnets and topological phases—typically exhibit collective excitations (for example, phonons, magnons and anyons). These involve the motion of many particles in the system, yet, remarkably, act like a single emergent entity—a quasiparticle. Known to be long lived at the lowest energies, quasiparticles are expected to become unstable when encountering the inevitable continuum of manyparticle excited states at high energies, where decay is kinematically allowed. Although this is correct for weak interactions, we show that strong interactions generically stabilize quasiparticles by pushing them out of the continuum. This general mechanism is straightforwardly illustrated in an exactly solvable model. Using stateoftheart numerics, we find it at work in the spin1/2 triangularlattice Heisenberg antiferromagnet (TLHAF). This is surprising given the expectation of magnon decay in this paradigmatic frustrated magnet. Turning to existing experimental data, we identify the detailed phenomenology of avoided decay in the TLHAF material Ba3CoSb2O9, and even in liquid helium, one of the earliest instances of quasiparticle decay. Our work unifies various phenomena above the universal lowenergy regime in a comprehensive description. This broadens our window of understanding of manybody excitations, and provides a new perspective for controlling and stabilizing quantum matter in the strongly interacting regime.
https://www.nature.com/articles/s4156701905353
10.1038/s4156701905353
article
Bohrdt_Nat.Phy.2019
Classifying snapshots of the doped Hubbard model with machine learning
Nature Physics
2019
1
7
Quantum gas microscopes for ultracold atoms can provide highresolution realspace snapshots of complex manybody systems. We implement machine learning to analyse and classify such snapshots of ultracold atoms. Specifically, we compare the data from an experimental realization of the twodimensional Fermi–Hubbard model to two theoretical approaches: a doped quantum spin liquid state of resonating valence bond type, and the geometric string theory, describing a state with hidden spin order. This technique considers all available information without a potential bias towards one particular theory by the choice of an observable and can therefore select the theory that is more predictive in general. Up to intermediate doping values, our algorithm tends to classify experimental snapshots as geometricstringlike, as compared to the doped spin liquid. Our results demonstrate the potential for machine learning in processing the wealth of data obtained through quantum gas microscopy for new physical insights.
https://www.nature.com/articles/s415670190565x
10.1038/s415670190565x
article
Zukovic_Phys.Rev.Lett2018
Dynamical Quantum Phase Transitions in Spin Chains with LongRange Interactions: Merging Different Concepts of Nonequilibrium Criticality
Physical Review Letters
2018
3
27
120
130601
We theoretically study the dynamics of a transversefield Ising chain with powerlaw decaying interactions characterized by an exponent α, which can be experimentally realized in ion traps. We focus on two classes of emergent dynamical critical phenomena following a quantum quench from a ferromagnetic initial state: The first one manifests in the timeaveraged order parameter, which vanishes at a critical transverse field. We argue that such a transition occurs only for longrange interactions α≤2. The second class corresponds to the emergence of timeperiodic singularities in the return probability to the groundstate manifold which is obtained for all values of α and agrees with the order parameter transition for α≤2. We characterize how the two classes of nonequilibrium criticality correspond to each other and give a physical interpretation based on the symmetry of the timeevolved quantum states.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.130601
10.1103/PhysRevLett.120.130601
BojanZunkovic
MarkusHeyl
MichaelKnap
AlessandroSilva
article
Pancotti_Phys.Rev.B2018
Almost conserved operators in nearly manybody localized systems
Physical Review B
2018
3
23
97
094206
We construct almost conserved local operators, that possess a minimal commutator with the Hamiltonian of the system, near the manybody localization transition of a onedimensional disordered spin chain. We collect statistics of these slow operators for different support sizes and disorder strengths, both using exact diagonalization and tensor networks. Our results show that the scaling of the average of the smallest commutators with the support size is sensitive to Griffiths effects in the thermal phase and the onset of manybody localization. Furthermore, we demonstrate that the probability distributions of the commutators can be analyzed using extreme value theory and that their tails reveal the difference between diffusive and subdiffusive dynamics in the thermal phase.
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.97.094206
10.1103/PhysRevB.97.094206
NicolaPancotti
MichaelKnap
David A.Huse
J. IgnacioCirac
MariCarmenBañuls
article
Bohrdt_Phys.Rev.B2018
Angleresolved photoemission spectroscopy with quantum gas microscopes
Physical Review B
2018
3
13
97
12
125117
Quantum gas microscopes are a promising tool to study interacting quantum manybody systems and bridge the gap between theoretical models and real materials. So far, they were limited to measurements of instantaneous correlation functions of the form ⟨ˆO(t)⟩, even though extensions to frequencyresolved response functions ⟨ˆO(t)ˆO(0)⟩ would provide important information about the elementary excitations in a manybody system. For example, singleparticle spectral functions, which are usually measured using photoemission experiments in electron systems, contain direct information about fractionalization and the quasiparticle excitation spectrum. Here, we propose a measurement scheme to experimentally access the momentum and energyresolved spectral function in a quantum gas microscope with currently available techniques. As an example for possible applications, we numerically calculate the spectrum of a single hole excitation in onedimensional t−J models with isotropic and anisotropic antiferromagnetic couplings. A sharp asymmetry in the distribution of spectral weight appears when a hole is created in an isotropic Heisenberg spin chain. This effect slowly vanishes for anisotropic spin interactions and disappears completely in the case of pure Ising interactions. The asymmetry strongly depends on the total magnetization of the spin chain, which can be tuned in experiments with quantum gas microscopes. An intuitive picture for the observed behavior is provided by a slavefermion meanfield theory. The key properties of the spectra are visible at currently accessible temperatures.
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.97.125117
10.1103/PhysRevB.97.125117
A.Bohrdt
D.Greif
EugeneDemler
MichaelKnap
F.Grusdt
article
Hütten_Nat.Comm.2018
Ultrafast quantum control of ionization dynamics in krypton
Nature Communications
2018
2
19
9
719
Ultrafast spectroscopy with attosecond resolution has enabled the real time observation of ultrafast electron dynamics in atoms, molecules and solids. These experiments employ attosecond pulses or pulse trains and explore dynamical processes in a pump–probe scheme that is selectively sensitive to electronic state of matter via photoelectron or XUV absorption spectroscopy or that includes changes of the ionic state detected via photoion mass spectrometry. Here, we demonstrate how the implementation of combined photoion and absorption spectroscopy with attosecond resolution enables tracking the complex multidimensional excitation and decay cascade of an Auger autoionization process of a few femtoseconds in highly excited krypton. In tandem with theory, our study reveals the role of intermediate electronic states in the formation of multiply charged ions. Amplitude tuning of a dressing laser field addresses different groups of decay channels and allows exerting temporal and quantitative control over the ionization dynamics in rare gas atoms.
https://www.nature.com/articles/s41467018031221
10.1038/s41467018031221
KonradHütten
MichaelMittermair
Sebastian O.Stock
RandolfBeerwerth
VaheShirvanyan
JohannRiemensberger
AndreasDuensing
RupertHeider
Martin S.Wagner
AlexanderGuggenmos
StephanFritzsche
Nikolay M.Kabachnik
ReinhardKienberger
BrigittaBernhardt
article
Welte_Phys.Rev.X2018
PhotonMediated Quantum Gate between Two Neutral Atoms in an Optical Cavity
Physics Review X
2018
2
6
8
011018
Quantum logic gates are fundamental building blocks of quantum computers. Their integration into quantum networks requires strong qubit coupling to network channels, as can be realized with neutral atoms and optical photons in cavity quantum electrodynamics. Here we demonstrate that the longrange interaction mediated by a flying photon performs a gate between two stationary atoms inside an optical cavity from which the photon is reflected. This single step executes the gate in 2 μs. We show an entangling operation between the two atoms by generating a Bell state with 76(2)% fidelity. The gate also operates as a cnot. We demonstrate 74.1(1.6)% overlap between the observed and the ideal gate output, limited by the state preparation fidelity of 80.2(0.8)%. As the atoms are efficiently connected to a photonic channel, our gate paves the way towards quantum networking with multiqubit nodes and the distribution of entanglement in repeaterbased longdistance quantum networks.
https://journals.aps.org/prx/abstract/10.1103/PhysRevX.8.011018
10.1103/PhysRevX.8.011018
StephanWelte
BastianHacker
SeverinDaiss
StephanRitter
GerhardRempe
article
Seifert_NatComm2018
Spin Hall photoconductance in a threedimensional topological insulator at room temperature
Nature Communications
2018
1
23
9
331
Threedimensional topological insulators are a class of Dirac materials, wherein strong spinorbit coupling leads to twodimensional surface states. The latter feature spinmomentum locking, i.e., each momentum vector is associated with a spin locked perpendicularly to it in the surface plane. While the principal spin generation capability of topological insulators is well established, comparatively little is known about the interaction of the spins with external stimuli like polarized light. We observe a helical, biasdependent photoconductance at the lateral edges of topological Bi2Te2Se platelets for perpendicular incidence of light. The same edges exhibit also a finite biasdependent Kerr angle, indicative of spin accumulation induced by a transversal spin Hall effect in the bulk states of the Bi2Te2Se platelets. A symmetry analysis shows that the helical photoconductance is distinct to common longitudinal photoconductance and photocurrent phenomena, but consistent with optically injected spins being transported in the side facets of the platelets.
https://www.nature.com/articles/s41467017026711
10.1038/s41467017026711
PaulSeifert
KristinaVaklinova
SergeyGanichev
KlausKern
MarkoBurghard
Alexander W.Holleitner
report
Schmidt_Reports_on_Progress2018
Universal manybody response of heavy impurities coupled to a Fermi sea: a review of recent progress
Reports on Progress in Physics
2018
1
5
81
2
38
In this report we discuss the dynamical response of heavy quantum impurities immersed in a Fermi gas at zero and at finite temperature. Studying both the frequency and the time domain allows one to identify interaction regimes that are characterized by distinct manybody dynamics. From this theoretical study a picture emerges in which impurity dynamics is universal on essentially all time scales, and where the highfrequency fewbody response is related to the longtime dynamics of the Anderson orthogonality catastrophe by Tan relations. Our theoretical description relies on different and complementary approaches: functional determinants give an exact numerical solution for time and frequencyresolved responses, bosonization provides accurate analytical expressions at low temperatures, and the theory of Toeplitz determinants allows one to analytically predict response up to high temperatures. Using these approaches we predict the thermal decoherence rate of the fermionic system and prove that within the considered model the fastest rate of longtime decoherence is given by γ=πkB T∕4. We show that Feshbach resonances in cold atomic systems give access to new interaction regimes where quantum effects can prevail even in the thermal regime of manybody dynamics. The key signature of this phenomenon is a crossover between different exponential decay rates of the realtime Ramsey signal. It is shown that the physics of the orthogonality catastrophe is experimentally observable up to temperatures T∕TF≤ 0.2 where it leaves its fingerprint in a powerlaw temperature dependence of thermal spectral weight and we review how this phenomenon is related to the physics of heavy ions in liquid 3 He and the formation of Fermi polarons. The presented results are in excellent agreement with recent experiments on LiK mixtures, and we predict several new phenomena that can be tested using currently available experimental technology.
http://iopscience.iop.org/article/10.1088/13616633/aa9593/meta
10.1088/13616633/aa9593
RichardSchmidt
MichaelKnap
Dmitri AIvanov
JhihShihYou
MarkoCetina
EugeneDemler
article
Lohse_Nature2018
Exploring 4D quantum Hall physics with a 2D topological charge pump
Nature
2018
1
4
553
5558
The discovery of topological states of matter has greatly improved our understanding of phase transitions in physical systems. Instead of being described by local order parameters, topological phases are described by global topological invariants and are therefore robust against perturbations. A prominent example is the twodimensional (2D) integer quantum Hall effect1: it is characterized by the first Chern number, which manifests in the quantized Hall response that is induced by an external electric field2. Generalizing the quantum Hall effect to fourdimensional (4D) systems leads to the appearance of an additional quantized Hall response, but one that is nonlinear and described by a 4D topological invariant—the second Chern number3,4. Here we report the observation of a bulk response with intrinsic 4D topology and demonstrate its quantization by measuring the associated second Chern number. By implementing a 2D topological charge pump using ultracold bosonic atoms in an angled optical superlattice, we realize a dynamical version of the 4D integer quantum Hall effect5,6. Using a small cloud of atoms as a local probe, we fully characterize the nonlinear response of the system via in situ imaging and siteresolved band mapping. Our findings pave the way to experimentally probing higherdimensional quantum Hall systems, in which additional strongly correlated topological phases, exotic collective excitations and boundary phenomena such as isolated Weyl fermions are predicted4.
https://www.nature.com/articles/nature25000
10.1038/nature25000
MichaelLohse
ChristianSchweizer
Hannah M.Price
OdedZilberberg
ImmanuelBloch
article
Bausch_PNAS2018
Sizedriven quantum phase transitions
Proceedings of the National Academy of Sciences
2018
1
115
1
1923
Can the properties of the thermodynamic limit of a manybody quantum system be extrapolated by analyzing a sequence of finitesize cases? We present models for which such an approach gives completely misleading results: translationally invariant, local Hamiltonians on a square lattice with open boundary conditions and constant spectral gap, which have a classical product ground state for all system sizes smaller than a particular threshold size, but a ground state with topological degeneracy for all system sizes larger than this threshold. Starting from a minimal case with spins of dimension 6 and threshold lattice size 15×15, we show that the latter grows faster than any computable function with increasing local spin dimension. The resulting effect may be viewed as a unique type of quantum phase transition that is driven by the size of the system rather than by an external field or coupling strength. We prove that the construction is thermally robust, showing that these effects are in principle accessible to experimental observation.
http://www.pnas.org/content/115/1/19
10.1073/pnas.1705042114
JohannesBausch
TobyCubitt
AngeloLucia
DavidPerezGarcia
MichaelWolf
article
Beaud_J.Math.Phys2018
Bounds on the entanglement entropy of droplet states in the XXZ spin chain
Journal of Mathematical Physics
2018
1
59
012109
We consider a class of onedimensional quantum spin systems on the finite lattice Λ⊂ℤ, related to the XXZ spin chain in its Ising phase. It includes in particular the socalled droplet Hamiltonian. The entanglement entropy of energetically lowlying states over a bipartition Λ = B ∪ Bc is investigated and proven to satisfy a logarithmic bound in terms of min{n, B, Bc}, where n denotes the maximal number of down spins in the considered state. Upon addition of any (positive) random potential, the bound becomes uniformly constant on average, thereby establishing an area law. The proof is based on spectral methods: a deterministic bound on the local (manybody integrated) density of states is derived from an energetically motivated Combes–Thomas estimate.
https://aip.scitation.org/doi/abs/10.1063/1.5007035
10.1063/1.5007035
V.Beaud
SimoneWarzel
article
Körber_Nat.Photonics2018
Decoherenceprotected memory for a singlephoton qubit
Nature Photonics
2017
12
11
12
1821
Distributed quantum computation in a quantum network is based on the idea that qubits can be preserved and efficiently exchanged between longlived, stationary network nodes via photonic links4. Although long qubit lifetimes have been observed and nonqubit excitations have been memorized the longlived storage and efficient retrieval of a photonic qubit by means of a light–matter interface remains an outstanding challenge. Here, we report on a qubit memory based on a single atom coupled to a highfinesse optical resonator. By mapping the qubit between an interface basis with strong light–matter coupling and a memory basis with low decoherence, we achieve a coherence time exceeding 100 ms with a timeindependent storageandretrieval efficiency of 22%. The former constitutes an improvement by two orders of magnitude and thus implements an efficient photonic qubit memory with a coherence time that exceeds the lower bound needed for direct qubit teleportation in a global quantum internet.
https://www.nature.com/articles/s415660170050y
doi:10.1038/s415660170050y
MatthiasKörber
O.Morin
S.Langenfeld
A.Neuzner
StephanRitter
GerhardRempe
article
Aizenman_ActaPhysicaPolonicaA2017
Edge Switching Transformations of Quantum Graphs
Acta Physica Polonica A
2017
12
132
6
16991703
Discussed here are the effects of basics graph transformations on the spectra of associated quantum graphs. In particular it is shown that under an edge switch the spectrum of the transformed Schrödinger operator is interlaced with that of the original one. By implication, under edge swap the spectra before and after the transformation, denoted by {Eₙ}^{∞}ₙ₌₁ and {Ẽₙ}^{∞}ₙ₌₁ correspondingly, are level2 interlaced, so that Eₙ₂ ≤ Ẽₙ ≤ Eₙ₊₂. The proofs are guided by considerations of the quantum graphs' discrete analogs.
http://psjd.icm.edu.pl/psjd/element/bwmeta1.element.bwnjournalarticleappv132n6p09kz?q=bwmeta1.element.bwnjournalnumberappola20171326;8&qt=CHILDRENSTATELESS
10.12693/APhysPolA.132.1699
MichaelAizenman
H.Schanz
U.Smilansky
SimoneWarzel
article
Bordia_Phys.Rev.X.2017
Probing Slow Relaxation and ManyBody Localization in TwoDimensional Quasiperiodic Systems
Physical Review X
2017
11
28
7
041047
In a manybody localized (MBL) quantum system, the ergodic hypothesis breaks down, giving rise to a fundamentally new manybody phase. Whether and under which conditions MBL can occur in higher dimensions remains an outstanding challenge both for experiments and theory. Here, we experimentally explore the relaxation dynamics of an interacting gas of fermionic potassium atoms loaded in a twodimensional optical lattice with different quasiperiodic potentials along the two directions. We observe a dramatic slowing down of the relaxation for intermediate disorder strengths. Furthermore, beyond a critical disorder strength, we see negligible relaxation on experimentally accessible time scales, indicating a possible transition into a twodimensional MBL phase. Our experiments reveal a distinct interplay of interactions, disorder, and dimensionality and provide insights into regimes where controlled theoretical approaches are scarce.
https://journals.aps.org/prx/abstract/10.1103/PhysRevX.7.041047
10.1103/PhysRevX.7.041047
PranjalBordia
HenrikLüschen
SebastianScherg
SarangGopalakrishnan
MichaelKnap
UlrichSchneider
ImmanuelBloch
article
Weidinger_Phys.Rev.B.2017
Dynamical quantum phase transitions in systems with continuous symmetry breaking
Physics Review B
2017
10
30
96
134313
Interacting manybody systems that are driven far away from equilibrium can exhibit phase transitions between dynamically emerging quantum phases, which manifest as singularities in the Loschmidt echo. Whether and under which conditions such dynamical transitions occur in higherdimensional systems with spontaneously broken continuous symmetries is largely elusive thus far. Here, we study the dynamics of the Loschmidt echo in the threedimensional O(N) model following a quantum quench from a symmetrybreaking initial state. The O(N) model exhibits a dynamical transition in the asymptotic steady state, separating two phases with a finite and vanishing order parameter, that is associated with the broken symmetry. We analytically calculate the rate function of the Loschmidt echo and find that it exhibits periodic kink singularities when this dynamical steadystate transition is crossed. The singularities arise exactly at the zero crossings of the oscillating order parameter. As a consequence, the appearance of the kink singularities in the transient dynamics is directly linked to a dynamical transition in the order parameter. Furthermore, we argue, that our results for dynamical quantum phase transitions in the O(N) model are general and apply to generic systems with continuous symmetry breaking.
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.134313
10.1103/PhysRevB.96.134313
Simon A.Weidinger
MarkusHeyl
AlessandroSilva
MichaelKnap
article
Joas_NatComm2017
Quantum sensing of weak radiofrequency signals by pulsed Mollow absorption spectroscopy
Nat. Commun.
2017
10
17
8
964
Quantum sensors—qubits sensitive to external fields—have become powerful detectors for various small acoustic and electromagnetic fields. A major key to their success have been dynamical decoupling protocols which enhance sensitivity to weak oscillating (AC) signals. Currently, those methods are limited to signal frequencies below a few MHz. Here we harness a quantumoptical effect, the Mollow triplet splitting of a strongly driven twolevel system, to overcome this limitation. We microscopically understand this effect as a pulsed dynamical decoupling protocol and find that it enables sensitive detection of fields close to the driven transition. Employing a nitrogenvacancy center, we detect GHz microwave fields with a signal strength (Rabi frequency) below the current detection limit, which is set by the center’s spectral linewidth 1∕T2*. Pushing detection sensitivity to the much lower 1/T2 limit, this scheme could enable various applications, most prominently coherent coupling to single phonons and microwave photons.
10.1038/s41467017011583
T.Joas
A. M.Waeber
G.Braunbeck
FriedemannReinhard
article
Bravyi_arXiv2017
Correcting coherent errors with surface codes
2017
10
6
We study how well topological quantum codes can tolerate coherent noise caused by systematic unitary errors such as unwanted Zrotations. Our main result is an efficient algorithm for simulating quantum error correction protocols based on the 2D surface code in the presence of coherent errors. The algorithm has runtime O(n2), where n is the number of physical qubits. It allows us to simulate systems with more than one thousand qubits and obtain the first error threshold estimates for several toy models of coherent noise. Numerical results are reported for storage of logical states subject to Zrotation errors and for logical state preparation with general SU(2) errors. We observe that for large code distances the effective logicallevel noise is wellapproximated by random Pauli errors even though the physicallevel noise is coherent. Our algorithm works by mapping the surface code to a system of Majorana fermions.
https://arxiv.org/abs/1710.02270
submitted
SergeyBravyi
MatthiasEnglbrecht
RobertKönig
NolanPeard
article
Gross_Science2017
Quantum simulations with ultracold atoms in optical lattices
Science
2017
9
8
357
6355
9951001
Abstract
Quantum simulation, a subdiscipline of quantum computation, can provide valuable insight into difficult quantum problems in physics or chemistry. Ultracold atoms in optical lattices represent an ideal platform for simulations of quantum manybody problems. Within this setting, quantum gas microscopes enable single atom observation and manipulation in large samples. Ultracold atom–based quantum simulators have already been used to probe quantum magnetism, to realize and detect topological quantum matter, and to study quantum systems with controlled longrange interactions. Experiments on manybody systems out of equilibrium have also provided results in regimes unavailable to the most advanced supercomputers. We review recent experimental progress in this field and comment on future directions.
http://science.sciencemag.org/content/357/6355/995.full
10.1126/science.aal3837
ChristianGross
ImmanuelBloch
article
vonSoosten_mathph2017
NonErgodic Delocalization in the RosenzweigPorter Model
Mathematical Physics
2017
9
We consider the RosenzweigPorter model H=V+T−−√Φ, where V is a N×N diagonal matrix, Φ is drawn from the N×N Gaussian Orthogonal Ensemble, and N−1≪T≪1. We prove that the eigenfunctions of H are typically supported in a set of approximately NT sites, thereby confirming the existence of a previously conjectured nonergodic delocalized phase. Our proof is based on martingale estimates along the characteristic curves of the stochastic advection equation satisfied by the local resolvent of the Brownian motion representation of H.
https://arxiv.org/abs/1709.10313
submitted
P.von Soosten
SimoneWarzel
article
Knap_PRL2017
Noiseinduced subdiffusion in strongly localized quantum systems
Phys. Rev. Lett.
2017
7
26
119
046601
We consider the dynamics of strongly localized systems subject to dephasing noise with arbitrary correlation time. Although noise inevitably induces delocalization, transport in the noiseinduced delocalized phase is subdiffusive in a parametrically large intermediatetime window. We argue for this intermediatetime subdiffusive regime both analytically and using numerical simulations on singleparticle localized systems. Furthermore, we show that normal diffusion is restored in the longtime limit, through processes analogous to variablerange hopping. With numerical simulations based on Lanczos exact diagonalization, we demonstrate that our qualitative conclusions are also valid for interacting systems in the manybody localized phase.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.046601
10.1103/PhysRevLett.119.046601
SarangGopalakrishnan
K. RanjibulIslam
MichaelKnap
article
quantumsensing2017
Quantum sensing
Rev. Mod. Phys.
2017
7
25
89
3
“Quantum sensing” describes the use of a quantum system, quantum properties, or quantum phenomena to perform a measurement of a physical quantity. Historical examples of quantum sensors include magnetometers based on superconducting quantum interference devices and atomic vapors or atomic clocks. More recently, quantum sensing has become a distinct and rapidly growing branch of research within the area of quantum science and technology, with the most common platforms being spin qubits, trapped ions, and flux qubits. The field is expected to provide new opportunities—especially with regard to high sensitivity and precision—in applied physics and other areas of science. This review provides an introduction to the basic principles, methods, and concepts of quantum sensing from the viewpoint of the interested experimentalist.
10.1103/RevModPhys.89.035002
C. L.Degen
FriedemannReinhard
PCappellaro
article
Knap_PRB_2017
Theory of parametrically amplified electronphonon superconductivity
Phys. Rev. B
2017
7
19
96
014512
Ultrafast optical manipulation of ordered phases in strongly correlated materials is a topic of significant theoretical, experimental, and technological interest. Inspired by a recent experiment on lightinduced superconductivity in fullerenes [M. Mitrano et al., Nature (London) 530, 461 (2016)], we develop a comprehensive theory of lightinduced superconductivity in driven electronphonon systems with lattice nonlinearities. In analogy with the operation of parametric amplifiers, we show how the interplay between the external drive and lattice nonlinearities lead to significantly enhanced effective electronphonon couplings. We provide a detailed and unbiased study of the nonequilibrium dynamics of the driven system using the realtime Green's function technique. To this end, we develop a Floquet generalization of the MigdalEliashberg theory and derive a numerically tractable set of quantum FloquetBoltzmann kinetic equations for the coupled electronphonon system. We study the role of parametric phonon generation and electronic heating in destroying the transient superconducting state. Finally, we predict the transient formation of electronic Floquet bands in time and angleresolved photoemission spectroscopy experiments as a consequence of the proposed mechanism.
Editors' Suggestion
http://arxiv.org/abs/1702.02531
doi.org/10.1103/PhysRevB.96.014512
MehrtashBabadi
MichaelKnap
IvarMartin
GilRefael
EugeneDemler
article
Beaud_Ann.HenriPolonica2017
LowEnergy FockSpace Localization for Attractive HardCore Particles in Disorder
Annales Henri Poincaré
2017
6
20
18
10
3143–3166
We study a onedimensional quantum system with an arbitrary number of hardcore particles on the lattice, which are subject to a deterministic attractive interaction as well as a random potential. Our choice of interaction is suggested by the spectral analysis of the XXZ quantum spin chain. The main result concerns a version of highdisorder Fockspace localization expressed here in the configuration space of hardcore particles. The proof relies on an energetically motivated Combes–Thomas estimate and an effective oneparticle analysis. As an application, we show the exponential decay of the twopoint function in the infinite system uniformly in the particle number.
https://link.springer.com/article/10.1007/s0002301705910
10.1007/s0002301705910
V.Beaud
SimoneWarzel
article
brandtdonor2017
MultipleQuantum Transitions and ChargeInduced Decoherence of Donor Nuclear Spins in Silicon
PRL
2017
6
15
118
We study single and multiquantum transitions of the nuclear spins of an ensemble of ionized arsenic donors in silicon and find quadrupolar effects on the coherence times, which we link to fluctuating electrical field gradients present after the application of light and bias voltage pulses. To determine the coherence times of superpositions of all orders in the 4dimensional Hilbert space, we use a phasecycling technique and find that, when electrical effects were allowed to decay, these times scale as expected for a fieldlike decoherence mechanism such as the interaction with surrounding 29Si nuclear spins.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.246401
10.1103/PhysRevLett.118.246401
David P.Franke
Moritz P. D.Pflüger
Kohei M.Itoh
Martin S.Brandt
Bohrst_NJP_2017
Scrambling and thermalization in a diffusive quantum manybody system
New J. Phys.
2017
6
2
19
063001
Outoftime ordered (OTO) correlation functions describe scrambling of information in correlated quantum matter. They are of particular interest in incoherent quantum systems lacking well defined quasiparticles. Thus far, it is largely elusive how OTO correlators spread in incoherent systems with diffusive transport governed by a few globally conserved quantities. Here, we study the dynamical response of such a system using highperformance matrixproductoperator techniques. Specifically, we consider the nonintegrable, onedimensional Bose–Hubbard model in the incoherent hightemperature regime. Our system exhibits diffusive dynamics in timeordered correlators of globally conserved quantities, whereas OTO correlators display a ballistic, lightcone spreading of quantum information. The slowest process in the global thermalization of the system is thus diffusive, yet information spreading is not inhibited by such slow dynamics. We furthermore develop an experimentally feasible protocol to overcome some challenges faced by existing proposals and to probe timeordered and OTO correlation functions. Our study opens new avenues for both the theoretical and experimental exploration of thermalization and information scrambling dynamics.
http://arxiv.org/abs/1612.02434
dx.doi.org/10.1088/13672630/aa719b
A.Bohrdt
C. B.Mendl
ManuelEndres
MichaelKnap
article
Meinert_Science_2017
Bloch oscillations in the absence of a lattice.
Science
2017
6
2
356
945
The interplay of strong quantum correlations and farfromequilibrium conditions can give rise to striking dynamical phenomena. We experimentally investigated the quantum motion of an impurity atom immersed in a strongly interacting onedimensional Bose liquid and subject to an external force. We found that the momentum distribution of the impurity exhibits characteristic Bragg reflections at the edge of an emergent Brillouin zone. Although Bragg reflections are typically associated with lattice structures, in our strongly correlated quantum liquid they result from the interplay of shortrange crystalline order and kinematic constraints on the manybody scattering processes in the onedimensional system. As a consequence, the impurity exhibits periodic dynamics, reminiscent of Bloch oscillations, although the quantum liquid is translationally invariant. Our observations are supported by largescale numerical simulations.
10.1126/science.aah6616
FlorianMeinert
MichaelKnap
EmilKirilov
KatharinaJagLauber
Mikhail B.Zvonarev
EugeneDemler
HannsChristophNägerl
article
vonSoosten_mathph20170
The Localization Transition in the Ultrametric Ensemble
Mathematical Physics
2017
5
11
We study the hierarchical analogue of powerlaw random band matrices, a symmetric ensemble of random matrices with independent entries whose variances decay exponentially in the metric induced by the tree topology on N. We map out the entirety of the localization regime by proving the localization of eigenfunctions and Poisson statistics of the suitably scaled eigenvalues. Our results complement existing works on complete delocalization and random matrix universality, thereby proving the existence of a phase transition in this model.
https://arxiv.org/abs/1705.04165
submitted
P.von Soosten
SimoneWarzel
article
knapfloquet20170
Floquet prethermalization and regimes of heating in a periodically driven, interacting quantum system.
Sci. Rep.
2017
4
3
7
45382
We study the regimes of heating in the periodically driven O(N)model, which represents a generic model for interacting quantum manybody systems. By computing the absorbed energy with a nonequilibrium Keldysh Green's function approach, we establish three dynamical regimes: at short times a singleparticle dominated regime, at intermediate times a stable Floquet prethermal regime in which the system ceases to absorb, and at parametrically late times a thermalizing regime. Our simulations suggest that in the thermalizing regime the absorbed energy grows algebraically in time with an the exponent that approaches the universal value of 1/2, and is thus significantly slower than linear Joule heating. Our results demonstrate the parametric stability of prethermal states in a generic manybody system driven at frequencies that are comparable to its microscopic scales. This paves the way for realizing exotic quantum phases, such as time crystals or interacting topological phases, in the prethermal regime of interacting Floquet systems.
10.1038/srep45382
Simon A.Weidinger
MichaelKnap
article
Bravyi_arXiv20170
Quantum advantage with shallow circuits
2017
4
3
We prove that constantdepth quantum circuits are more powerful than their classical counterparts. To this end we introduce a nonoracular version of the BernsteinVazirani problem which we call the 2D Hidden Linear Function problem. An instance of the problem is specified by a quadratic form q that maps nbit strings to integers modulo four. The goal is to identify a linear boolean function which describes the action of q on a certain subset of nbit strings. We prove that any classical probabilistic circuit composed of bounded fanin gates that solves the 2D Hidden Linear Function problem with high probability must have depth logarithmic in n. In contrast, we show that this problem can be solved with certainty by a constantdepth quantum circuit composed of one and twoqubit gates acting locally on a twodimensional grid.
Quantum advantage with shallow circuits
submitted
SergeyBravyi
DavidGosset
RobertKönig
article
Knapmbl2017
Rare region effects and dynamics near the manybody localization transition.
Annalen der Physik, Special issue on ManyBody Localization
2017
1
12
The lowfrequency response of systems near the manybody localization phase transition, on either side of the transition, is dominated by contributions from rare regions that are locally “in the other phase”, i.e., rare localized regions in a system that is typically thermal, or rare thermal regions in a system that is typically localized. Rare localized regions affect the properties of the thermal phase, especially in one dimension, by acting as bottlenecks for transport and the growth of entanglement, whereas rare thermal regions in the localized phase act as local “baths” and dominate the lowfrequency response of the MBL phase. We review recent progress in understanding these rareregion effects, and discuss some of the open questions associated with them: in particular, whether and in what circumstances a single rare thermal region can destabilize the manybody localized phase.
10.1002/andp.201600326
KartiekAgarwal
EhudAltman
EugeneDemler
SarangGopalakrishnan
David A.Huse
MichaelKnap
article
demler2016
Dynamical Cooper pairing in nonequilibrium electronphonon systems.
Phys. Rev. B
2016
12
2
94
We analyze Cooper pairing instabilities in strongly driven electronphonon systems. The lightinduced nonequilibrium state of phonons results in a simultaneous increase of the superconducting coupling constant and the electron scattering. We demonstrate that the competition between these effects leads to an enhanced superconducting transition temperature in a broad range of parameters. Our results may explain the observed transient enhancement of superconductivity in several classes of materials upon irradiation with high intensity pulses of terahertz light, and may pave new ways for engineering hightemperature lightinduced superconducting states.
10.1103/PhysRevB.94.214504
MichaelKnap
MehrtashBabadi
GilRefael
IvarMartin
EugeneDemler
article
Punk2016
Finitetemperature scaling close to Isingnematic quantum critical points in twodimensional metals
Phys. Rev. B
2016
11
7
94
195113
We study finitetemperature properties of metals close to an Isingnematic quantum critical point in two spatial dimensions. In particular we show that at any finite temperature there is a regime where order parameter fluctuations are characterized by a dynamical critical exponent z=2, in contrast to z=3 found at zero temperature. Our results are based on a simple Eliashbergtype approach, which gives rise to a boson selfenergy proportional to Ω/γ(T) at small momenta, where γ(T) is the temperature dependent fermion scattering rate. These findings might shed some light on recent Monte Carlo simulations at finite temperature, where results consistent with z=2 were found.
10.1103/PhysRevB.94.195113
MatthiasPunk
article
CetinaJLFWGLPSKD2016
Ultrafast manybody interferometry of impurities coupled to a Fermi sea
Science
2016
10
7
354
6308
9699
The fastest possible collective response of a quantum manybody system is related to its excitations at the highest possible energy. In condensed matter systems, the time scale for such “ultrafast” processes is typically set by the Fermi energy. Taking advantage of fast and precise control of interactions between ultracold atoms, we observed nonequilibrium dynamics of impurities coupled to an atomic Fermi sea. Our interferometric measurements track the nonperturbative quantum evolution of a fermionic manybody system, revealing in real time the formation dynamics of quasiparticles and the quantum interference between attractive and repulsive states throughout the full depth of the Fermi sea. Ultrafast timedomain methods applied to strongly interacting quantum gases enable the study of the dynamics of quantum matter under extreme nonequilibrium conditions.
10.1126/science.aaf5134
MarkoCetina
MichaelJag
Rianne S.Lous
IsabellaFritsche
Jook T. M.Walraven
RudolfGrimm
JesperLevinsen
Meera M.Parish
RichardSchmidt
MichaelKnap
EugeneDemler
article
WildGKYL2016
Adiabatic Quantum Search in Open Systems
Phys. Rev. Lett.
2016
10
6
117
150501
Adiabatic quantum algorithms represent a promising approach to universal quantum computation. In isolated systems, a key limitation to such algorithms is the presence of avoided level crossings, where gaps become extremely small. In open quantum systems, the fundamental robustness of adiabatic algorithms remains unresolved. Here, we study the dynamics near an avoided level crossing associated with the adiabatic quantum search algorithm, when the system is coupled to a generic environment. At zero temperature, we find that the algorithm remains scalable provided the noise spectral density of the environment decays sufficiently fast at low frequencies. By contrast, higher order scattering processes render the algorithm inefficient at any finite temperature regardless of the spectral density, implying that no quantum speedup can be achieved. Extensions and implications for other adiabatic quantum algorithms will be discussed.
10.1103/PhysRevLett.117.150501
Dominik S.Wild
SarangGopalakrishnan
MichaelKnap
Norman Y.Yao
Mikhail D.Lukin
article
König_Phys.Rev.Lett2016
Matrix Product Approximations to Multipoint Functions in TwoDimensional Conformal Field Theory
Physical Review Letters
2016
9
14
117
121601
Matrix product states (MPSs) illustrate the suitability of tensor networks for the description of interacting manybody systems: ground states of gapped 1D systems are approximable by MPSs, as shown by Hastings [M. B. Hastings, J. Stat. Mech. (2007) P08024]. By contrast, whether MPSs and more general tensor networks can accurately reproduce correlations in critical quantum systems or quantum field theories has not been established rigorously. Ample evidence exists: entropic considerations provide restrictions on the form of suitable ansatz states, and numerical studies show that certain tensor networks can indeed approximate the associated correlation functions. Here, we provide a complete positive answer to this question in the case of MPSs and 2D conformal field theory: we give quantitative estimates for the approximation error when approximating correlation functions by MPSs. Our work is constructive and yields an explicit MPS, thus providing both suitable initial values and a rigorous justification of variational methods.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.117.121601
10.1103/PhysRevLett.117.121601
RobertKönig
Volkher B.Scholz
article
GopalakrishnanKD2016
Regimes of heating and dynamical response in driven manybody localized systems
Phys. Rev. B
2016
9
14
94
094201
We explore the response of manybody localized (MBL) systems to periodic driving of arbitrary amplitude, focusing on the rate at which they exchange energy with the drive. To this end, we introduce an infinitetemperature generalization of the effective “heating rate” in terms of the spread of a random walk in energy space. We compute this heating rate numerically and estimate it analytically in various regimes. When the drive amplitude is much smaller than the frequency, this effective heating rate is given by linear response theory with a coefficient that is proportional to the optical conductivity; in the opposite limit, the response is nonlinear and the heating rate is a nontrivial power law of time. We discuss the mechanisms underlying this crossover in the MBL phase. We comment on implications for the subdiffusive thermal phase near the MBL transition, and for response in imperfectly isolated MBL systems.
10.1103/PhysRevB.94.094201
SarangGopalakrishnan
MichaelKnap
EugeneDemler
article
HalimehP2016
Spin structure factors of chiral quantum spin liquids on the kagome lattice
Phys. Rev. B
2016
9
13
94
104413
We calculate dynamical spin structure factors for gapped chiral spin liquid states in the spin1/2 Heisenberg antiferromagnet on the kagome lattice using Schwingerboson meanfield theory. In contrast to static (equaltime) structure factors, the dynamical structure factor shows clear signatures of timereversal symmetry breaking for chiral spin liquid states. In particular, momentum inversion k→−k symmetry as well as the sixfold rotation symmetry around the Γ point are lost. We highlight other interesting features, such as a relatively flat onset of the twospinon continuum for the cuboc1 state. Our work is based on the projective symmetry group classification of timereversal symmetry breaking Schwingerboson meanfield states by Messio, Lhuillier, and Misguich.
10.1103/PhysRevB.94.104413
Jad C.Halimeh
MatthiasPunk
article
FournaisLLO2016
Coulomb potentials and Taylor expansions in timedependent densityfunctional theory
Phys. Rev. A
2016
6
23
92
062510
We investigate when Taylor expansions can be used to prove the RungeGross theorem, which is at the foundation of timedependent densityfunctional theory (TDDFT). We start with a general analysis of the conditions for the RungeGross argument, especially the time differentiability of the density. The latter should be questioned in the presence of singular (e.g., Coulomb) potentials. Then we show that a singular potential in a onebody operator considerably decreases the class of timedependent external potentials to which the original argument can be applied. A twobody singularity has an even stronger impact and an external potential is essentially incompatible with it. For the Coulomb interaction and all reasonable initial manybody states, the Taylor expansion only exists to a finite order, except for constant external potentials. Therefore, highorder Taylor expansions are not the right tool to study atoms and molecules in TDDFT.
10.1103/PhysRevA.93.062510
SørenFournais
JonasLampert
MathieuLewin
ThomasØstergaard Sørensen
article
HofmannNKCH2016
Ubiquity of Exciton Localization in Cryogenic Carbon Nanotubes
Nano Lett.
2016
4
22
16
5
2958–2962
We present photoluminescence studies of individual semiconducting singlewall carbon nanotubes at room and cryogenic temperatures. From the analysis of spatial and spectral features of nanotube photoluminescence, we identify characteristic signatures of unintentional exciton localization. Moreover, we quantify the energy scale of exciton localization potentials as ranging from a few to a few tens of millielectronvolts and stemming from both environmental disorder and shallow covalent sidewall defects. Our results establish disorderinduced crossover from the diffusive to the localized regime of nanotube excitons at cryogenic temperatures as a ubiquitous phenomenon in micelleencapsulated and asgrown carbon nanotubes.
10.1021/acs.nanolett.5b04901
MatthiasHofmann
JonathanNoe
AlexanderKneer
JaredCrochet
AlexanderHögele
article
GopalakrishnanADHK2016
Griffiths effects and slow dynamics in nearly manybody localized systems
Phys. Rev. B
2016
4
11
93
134206
The lowfrequency response of systems near a manybody localization transition can be dominated by rare regions that are locally critical or “in the other phase.” It is known that in one dimension, these rare regions can cause the dc conductivity and diffusion constant to vanish even inside the delocalized thermal phase. Here, we present a general analysis of such Griffiths effects in the thermal phase near the manybody localization transition: we consider both onedimensional and higherdimensional systems, subject to quenched randomness, and discuss both linear response (including the frequency and wavevectordependent conductivity) and more general dynamics. In all the regimes we consider, we identify observables that are dominated by rareregion effects. In some cases (onedimensional systems and Floquet systems with no extensive conserved quantities), essentially all longtime local observables are dominated by rareregion effects; in others, generic observables are instead dominated by hydrodynamic longtime tails throughout the thermal phase, and one must look at specific probes, such as spin echo, to see Griffiths behavior.
SarangGopalakrishnan
KartiekAgarwal
Eugene A.Demler
David A.Huse
MichaelKnap
Beverland_J.Maths.Phys._2016
Protected gates for topological quantum field theories
Journal of Mathematical Physics
2016
1
13
57
022201
We study restrictions on localitypreserving unitary logical gates for topological quantum codes in two spatial dimensions. A localitypreserving operation is one which maps local operators to local operators — for example, a constantdepth quantum circuit of geometrically local gates, or evolution for a constant time governed by a geometrically local boundedstrength Hamiltonian. Localitypreserving logical gates of topological codes are intrinsically fault tolerant because spatially localized errors remain localized, and hence sufficiently dilute errors remain correctable. By invoking general properties of twodimensional topological field theories, we find that the localitypreserving logical gates are severely limited for codes which admit nonabelian anyons, in particular, there are no localitypreserving logical gates on the torus or the sphere with M punctures if the braiding of anyons is computationally universal. Furthermore, for Ising anyons on the Mpunctured sphere, localitypreserving gates must be elements of the logical Pauli group. We derive these results by relating logical gates of a topological code to automorphisms of the Verlinde algebra of the corresponding anyon model, and by requiring the logical gates to be compatible with basis changes in the logical Hilbert space arising from local Fmoves and the mapping class group.
https://aip.scitation.org/doi/10.1063/1.4939783
10.1063/1.4939783
Michael E.Beverland
OliverBuerschaper
RobertKönig
article
TsvelikY2015
Quantum Phase Transition and Protected Ideal Transport in a Kondo Chain
PRL
2015
11
20
115
216402
We study the low energy physics of a Kondo chain where electrons from a onedimensional band interact
with magnetic moments via an anisotropic exchange interaction. It is demonstrated that the anisotropy
gives rise to two different phases which are separated by a quantum phase transition. In the phase with easy
plane anisotropy, Z2 symmetry between sectors with different helicity of the electrons is broken. As a
result, localization effects are suppressed and the dc transport acquires (partial) symmetry protection.
This effect is similar to the protection of the edge transport in timereversal invariant topological insulators.
The phase with easy axis anisotropy corresponds to the TomonagaLuttinger liquid with a pronounced
spincharge separation. The slow charge density wave modes have no protection against localization.
10.1103/PhysRevLett.115.216402
A. M.Tsvelik
O. M.Yevtushenko
article
LiuCDEPP2015
Massive Goldstone (Higgs) mode in twodimensional ultracold atomic lattice systems
Phys. Rev. B
2015
11
15
92
174521
We discuss how to reveal the massive Goldstone mode, often referred to as the Higgs amplitude mode, near the superfluidtoinsulator quantum critical point (QCP) in a system of twodimensional ultracold bosonic atoms in optical lattices. The spectral function of the amplitude response is obtained by analytic continuation of the kinetic energy correlation function calculated by Monte Carlo methods. Our results enable a direct comparison with the recent experiment [M. Endres, T. Fukuhara, D. Pekker, M. Cheneau, P. Schauß, C. Gross, E. Demler, S. Kuhr, and I. Bloch, Nature (London) 487, 454 (2012)] and demonstrate a good agreement for temperature shifts induced by lattice modulation. Based on our numerical analysis, we formulate the necessary conditions in terms of homogeneity, detuning from the QCP and temperature in order to reveal the massive Goldstone resonance peak in spectral functions experimentally. We also propose to apply a local modulation at the trap center to overcome the inhomogeneous broadening caused by the parabolic trap confinement.
10.1103/PhysRevB.92.174521
LongxiangLiu
KunChen
YoujinDeng
ManuelEndress
LodePollet
NikolayProkof'ev
article
BukovGKD2015
Prethermal Floquet Steady States and Instabilities in the Periodically Driven, Weakly Interacting BoseHubbard Model
Phys. Rev. Lett.
2015
11
11
115
205301
We explore prethermal Floquet steady states and instabilities of the weakly interacting twodimensional BoseHubbard model subject to periodic driving. We develop a description of the nonequilibrium dynamics, at arbitrary drive strength and frequency, using a weakcoupling conserving approximation. We establish the regimes in which conventional (zeromomentum) and unconventional [(π,π)momentum] condensates are stable on intermediate time scales. We find that condensate stability is enhanced by increasing the drive strength, because this decreases the bandwidth of quasiparticle excitations and thus impedes resonant absorption and heating. Our results are directly relevant to a number of current experiments with ultracold bosons.
10.1103/PhysRevLett.115.205301
MarinBukov
SarangGopalakrishnan
MichaelKnap
EugeneDemler
article
BabadiDK2015
FarfromEquilibrium Field Theory of ManyBody Quantum Spin Systems: Prethermalization and Relaxation of Spin Spiral States in Three Dimensions
Phys. Rev. X
2015
10
12
5
041005
We study theoretically the farfromequilibrium relaxation dynamics of spin spiral states in the threedimensional isotropic Heisenberg model. The investigated problem serves as an archetype for understanding quantum dynamics of isolated manybody systems in the vicinity of a spontaneously broken continuous symmetry. We present a fieldtheoretical formalism that systematically improves on the mean field for describing the realtime quantum dynamics of generic spin1/2 systems. This is achieved by mapping spins to Majorana fermions followed by a 1/N expansion of the resulting twoparticleirreducible effective action. Our analysis reveals rich fluctuationinduced relaxation dynamics in the unitary evolution of spin spiral states. In particular, we find the sudden appearance of longlived prethermalized plateaus with diverging lifetimes as the spiral winding is tuned toward the thermodynamically stable ferro or antiferromagnetic phases. The emerging prethermalized states are characterized by different bosonic modes being thermally populated at different effective temperatures and by a hierarchical relaxation process reminiscent of glassy systems. Spinspin correlators found by solving the nonequilibrium BetheSalpeter equation provide further insight into the dynamic formation of correlations, the fate of unstable collective modes, and the emergence of fluctuationdissipation relations. Our predictions can be verified experimentally using recent realizations of spin spiral states with ultracold atoms in a quantum gas microscope [S. Hild et al., Phys. Rev. Lett. 113, 147205 (2014)].
10.1103/PhysRevX.5.041005
MehrtashBabadi
EugeneDemler
MichaelKnap
article
SchaussZFHCMPBG2015
Microscopic Characterization of Scalable Coherent Rydberg Superatoms
Physical Review X
2015
8
12
5
031015
Strong interactions can amplify quantum effects such that they become important on macroscopic scales. Controlling these coherently on a singleparticle level is essential for the tailored preparation of strongly correlated quantum systems and opens up new prospects for quantum technologies. Rydberg atoms offer such strong interactions, which lead to extreme nonlinearities in lasercoupled atomic ensembles. As a result, multiple excitation of a micrometersized cloud can be blocked while the lightmatter coupling becomes collectively enhanced. The resulting twolevel system, often called a “superatom,” is a valuable resource for quantum information, providing a collective qubit. Here, we report on the preparation of 2 orders of magnitude scalable superatoms utilizing the large interaction strength provided by Rydberg atoms combined with precise control of an ensemble of ultracold atoms in an optical lattice. The latter is achieved with subshotnoise precision by local manipulation of a twodimensional Mott insulator. We microscopically confirm the superatom picture by in situ detection of the Rydberg excitations and observe the characteristic squareroot scaling of the optical coupling with the number of atoms. Enabled by the full control over the atomic sample, including the motional degrees of freedom, we infer the overlap of the produced manybody state with a W state from the observed Rabi oscillations and deduce the presence of entanglement. Finally, we investigate the breakdown of the superatom picture when two Rydberg excitations are present in the system, which leads to dephasing and a loss of coherence.
10.1103/PhysRevX.5.031015
J.Zeiher
P.Schauss
S.Hild
T.Macri
I.Bloch
C.Gross
article
FukuharaHZSBEG2015_2
Spatially Resolved Detection of a SpinEntanglement Wave in a BoseHubbard Chain
Physical Review Letters
2015
7
13
115
035302
Entanglement is an essential property of quantum manybody systems. However, its local detection is challenging and was so far limited to spin degrees of freedom in ion chains. Here we measure entanglement between the spins of atoms located on two lattice sites in a onedimensional BoseHubbard chain which features both local spin and particlenumber ﬂuctuations. Starting with an initially localized spin impurity, we observe an outwards propagating entanglement wave and show quantitatively how entanglement in the spin sector rapidly decreases with increasing particlenumber ﬂuctuations in the chain.
10.1103/PhysRevLett.115.035302
T.Fukuhara
S.Hild
J.Zeiher
P.Schauss
I.Bloch
M.Endres
C.Gross
article
BruognolovW2015
Symmetric minimally entangled typical thermal states
Phys. Rev. B
2015
6
12
92
115105
We extend White's minimally entangled typically thermal states approach (METTS) to allow Abelian and nonAblian symmetries to be exploited when computing finitetemperature response functions in onedimensional (1D) quantum systems. Our approach, called SYMETTS, starts from a METTS sample of states that are not symmetry eigenstates, and generates from each a symmetry eigenstate. These symmetry states are then used to calculate dynamic response functions. SYMETTS is ideally suited to determine the lowtemperature spectra of 1D quantum systems with high resolution. We employ this method to study a generalized diamond chain model for the natural mineral azurite Cu3(CO3)2(OH)2, which features a plateau at 13 in the magnetization curve at low temperatures. Our calculations provide new insight into the effects of temperature on magnetization and excitation spectra in the plateau phase, which can be fully understood in terms of the microscopic model.
10.1103/PhysRevB.92.115105
BenediktBruognolo
Janvon Delft
AndreasWeichselbaum
article
FukuharaHZSBEG2015
Crystallization in Ising quantum magnets
Science
2015
3
27
347
6229
14551458
Dominating finiterange interactions in manybody systems can lead to intriguing selfordered phases of matter. For quantum magnets, Ising models with powerlaw interactions are among the most elementary systems that support such phases. These models can be implemented by laser coupling ensembles of ultracold atoms to Rydberg states. Here, we report on the experimental preparation of crystalline ground states of such spin systems. We observe a magnetization staircase as a function of the system size and show directly the emergence of crystalline states with vanishing susceptibility. Our results demonstrate the precise control of Rydberg manybody systems and may enable future studies of phase transitions and quantum correlations in interacting quantum magnets.
10.1126/science.1258351
P.Schauss
J.Zeiher
T.Fukuhara
S.Hild
M.Cheneau
T.Macri
T.Pohl
I.Bloch
C.Gross
article
Abdi2015
Quantum State Engineering with Circuit Electromechanical ThreeBody Interactions
Phys. Rev. Lett.
2015
114
173602
We propose a hybrid system with quantum mechanical threebody interactions between photons, phonons, and qubit excitations. These interactions take place in a circuit quantum electrodynamical architecture with a superconducting microwave resonator coupled to a transmon qubit whose shunt capacitance is free to mechanically oscillate. We show that this system design features a threemode polaritonmechanical mode and a nonlinear transmonmechanical mode interaction in the strong coupling regime. Together with the strong resonatortransmon interaction, these properties provide intriguing opportunities for manipulations of this hybrid quantum system. We show, in particular, the feasibility of cooling the mechanical motion down to its ground state and preparing various nonclassical states including mechanical Fock and cat states and hybrid tripartite entangled states.
10.1103/PhysRevLett.114.173602
MehdiAbdi
MatthiasPernpeintner
RudolfGross
HansHuebl
MichaelJ. Hartmann
article
Caneva2015
Quantum dynamics of propagating photons with strong interactions: a generalized inputoutput formalism
2015
There has been rapid development of systems that yield strong interactions between freely propagating photons in one dimension via controlled coupling to quantum emitters. This raises interesting possibilities such as quantum information processing with photons or quantum manybody states of light, but treating such systems generally remains a difficult task theoretically. Here, we describe a novel technique in which the dynamics and correlations of a few photons can be exactly calculated, based upon knowledge of the initial photonic state and the solution of the reduced effective dynamics of the quantum emitters alone. We show that this generalized "inputoutput" formalism allows for a straightforward numerical implementation regardless of system details, such as emitter positions, external driving, and level structure. As a specific example, we apply our technique to show how atomic systems with infiniterange interactions and under conditions of electromagnetically induced transparency enable the selective transmission of correlated multiphoton states.
http://arxiv.org/abs/1501.04427
TommasoCaneva
MarcoT. Manzoni
TaoShi
JamesS. Douglas
J.Ignacio Cirac
DarrickE. Chang
article
Cubitt2015
Undecidability of the Spectral Gap (short version)
2015
The spectral gap  the difference in energy between the ground state and the first excited state  is one of the most important properties of a quantum manybody system. Quantum phase transitions occur when the spectral gap vanishes and the system becomes critical. Much of physics is concerned with understanding the phase diagrams of quantum systems, and some of the most challenging and longstanding open problems in theoretical physics concern the spectral gap, such as the Haldane conjecture that the Heisenberg chain is gapped for integer spin, proving existence of a gapped topological spin liquid phase, or the YangMills gap conjecture (one of the Millennium Prize problems). These problems are all particular cases of the general spectral gap problem: Given a quantum manybody Hamiltonian, is the system it describes gapped or gapless? Here we show that this problem is undecidable, in the same sense as the Halting Problem was proven to be undecidable by Turing. A consequence of this is that the spectral gap of certain quantum manybody Hamiltonians is not determined by the axioms of mathematics, much as Goedels incompleteness theorem implies that certain theorems are mathematically unprovable. We extend these results to prove undecidability of other low temperature properties, such as correlation functions. The proof hinges on simple quantum manybody models that exhibit highly unusual physics in the thermodynamic limit.
TobyCubitt
DavidPerezGarcia
MichaelM. Wolf
article
Cui2015
Variational matrix product operators for the steady state of dissipative quantum systems
2015
We present a new variational method, based on the matrix product operator (MPO) ansatz, for finding the steady state of dissipative quantum chains governed by master equations of the Lindblad form. Instead of requiring an accurate representation of the system evolution until the stationary state is attained, the algorithm directly targets the final state, thus allowing for a faster convergence when the steady state is a MPO with small bond dimension. Our numerical simulations for several dissipative spin models over a wide range of parameters illustrate the performance of the method and show that indeed the stationary state is often well described by a MPO of very moderate dimensions.
JianCui
J.Ignacio Cirac
MariCarmen Ba
article
Erhard2015a
Optical control of internal electric fields in bandgap graded InGaN nanowires
Nano Lett.
2015
15
1
332–338
InGaN nanowires are suitable building blocks for many future optoelectronic devices. We show that a linear grading of the indium content along the nanowire axis from GaN to InN introduces an internal electric field evoking a photocurrent. Consistent with quantitative band structure simulations we observe a sign change in the measured photocurrent as a function of photon flux. This negative differential photocurrent opens the path to a new type of nanowirebased photodetector. We demonstrate that the photocurrent response of the nanowires is as fast as 1.5 ps.
http://pubs.acs.org/doi/abs/10.1021/nl503616w
N.Erhard
A.T. M. Golam Sarwar
F.Yang
D.W. McComb
R.C. Myers
A.W. Holleitner
article
Erhard2015
Ultrafast photocurrents and THz generation in single InAsnanowires
2015
To clarify the ultrafast temporal interplay of the different photocurrent mechanisms occurring in single InAsnanowirebased circuits, an onchip photocurrent pumpprobe spectroscopy based on coplanar striplines was utilized. The data are interpreted in terms of a photothermoelectric current and the transport of photogenerated holes to the electrodes as the dominating ultrafast photocurrent contributions. Moreover, it is shown that THz radiation is generated in the optically excited InAsnanowires, which is interpreted in terms of a dominating photoDember effect. The results are relevant for nanowirebased optoelectronic and photovoltaic applications as well as for the design of nanowirebased THz sources.
http://arxiv.org/abs/1502.03782
NadineErhard
PaulSeifert
LeonhardPrechtel
SimonHertenberger
HelmutKarl
GerhardAbstreiter
GregorKoblmüller
AlexanderW. Holleitner
article
Flassig2015
Towards onchip generation, routing and detection of nonclassical light
Proc. SPIE 9373
2015
We fabricate an integrated photonic circuit with emitter, waveguide and detector on one chip, based on a hybrid superconductorsemiconductor system. We detect photoluminescence from selfassembled InGaAs quantum dots onchip using NbN superconducting nanowire single photon detectors. Using the fast temporal response of these detectors we perform timeresolved studies of nonresonantly excited quantum dots. By introducing a temporal ?ltering to the signal, we are able to resonantly excite the quantum dot and detect its resonance uorescence onchip with the integrated superconducting single photon detector.
http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=2190947
FabianFlassig
MichaelKaniber
GReithmaier
KaiM
AlexanderAndrejew
RudolfGross
JelenaVu
JonathanFinley
article
Haegeman2015
Quantum GrossPitaevskii Equation
2015
We introduce a noncommutative generalization of the GrossPitaevskii equation for onedimensional quantum field theories. This generalization is obtained by applying the DiracFrenkel timedependent variational principle to the variational manifold of continuous matrix product states. This allows for a full quantum description of the many body system including entanglement and correlations and thus extends significantly beyond the usual meanfield description of the GrossPitaevskii equation, which is known to fail for onedimensional systems.
http://arxiv.org/abs/1501.06575
JuthoHaegeman
DamianDraxler
VidStojevic
J.Ignacio Cirac
TobiasJ. Osborne
FrankVerstraete
article
Huegel2015
Thermodynamics of the BoseHubbard model in a Bogoliubov+U theory
2015
We derive the Bogoliubov+U formalism to study the thermodynamical properties of the BoseHubbard model. The framework can be viewed as the zerofrequency limit of bosonic dynamical meanfield theory (BDMFT), but equally well as an extension of the meanfield decoupling approximation in which pair creation and annihilation of depleted particles is taken into account. The selfenergy on the impurity site is treated variationally, minimizing the grand potential. The theory containing just 3 parameters that are determined selfconsistently reproduces the T=0 phase diagrams of the 3d and 2d BoseHubbard model with an accuracy of 1 % or better. The superfluid to normal transition at finite temperature is also reproduced well and only slightly less accurately than in BDMFT.
http://arxiv.org/abs/1501.07849
DarioH
LodePollet
article
Kastl2015
Ultrafast helicity control of surface currents in topological insulators with nearunity fidelity
Nature Communications
2015
6
6617
In recent years, a class of solid state materials, called threedimensional topological insulators, has emerged. In the bulk, a topological insulator behaves like an ordinary insulator with a band gap. At the surface, conducting gapless states exist showing remarkable properties such as helical Dirac dispersion and suppression of backscattering of spinpolarized charge carriers. The characterization and control of the surface states via transport experiments is often hindered by residual bulk contributions yet at cryogenic temperatures. Here, we show that surface currents in Bi2Se3 can be controlled by circularly polarized light on a picosecond time scale with a fidelity near unity even at room temperature. We reveal the temporal separation of such ultrafast helicitydependent surface currents from photoinduced thermoelectric and drift currents in the bulk. Our results uncover the functionality of ultrafast optoelectronic devices based on surface currents in topological insulators.
10.1038/ncomms7617
ChristophKastl
ChristophKarnetzky
HelmutKarl
AlexanderW. Holleitner
article
MuellerHermes2015
Positivity of linear maps under tensor powers
2015
We investigate linear maps between matrix algebras that remain positive under tensor powers, i.e., under tensoring with n copies of themselves. Completely positive and completely copositive maps are trivial examples of this kind. We show that for every n∈\mathbb<prt>N</prt> there exist nontrivial maps with this property and that for twodimensional Hilbert spaces there is no nontrivial map for which this holds for all n. For higher dimensions we reduce the existence question of such nontrivial "tensorstable positive maps" to a oneparameter family of maps and show that an affirmative answer would imply the existence of NPPT bound entanglement. As an application we show that any tensorstable positive map that is not completely positive yields an upper bound on the quantum channel capacity, which for the transposition map gives the wellknown cbnorm bound. We furthermore show that the latter is an upper bound even for the LOCCassisted quantum capacity, and that moreover it is a strong converse rate for this task.
http://arxiv.org/abs/1502.05630
AlexanderM
DavidReeb
MichaelM. Wolf
article
Schraml2015
Linear and Nonlinear Response of Lithographically Defined Plasmonic Nanoantennas
Proceedings of SPIE 9371, 93711D (2015)
2015
We present numerical studies, nanofabrication and optical characterization of bowtie nanoantennas demonstrating their superior performance with respect to the electric field enhancement as compared to other Au nanoparticle shapes. For optimized parameters, we found mean intensity enhancement factors >2300x in the feedgap of the antenna, decreasing to 1300x when introducing a 5nm titanium adhesion layer. Using electron beam lithography we fabricated gold bowties on various substrates with feedgaps and tip radii as small as 10nm. In polarization resolved measurement we experimentally observed a blue shift of the surface plasmon resonance from 1.72eV to 1.35eV combined with a strong modification of the electric field enhancement in the feedgap. Under excitation with a 100fs pulsed laser source, we observed nonlinear light emission arising from twophoton photoluminescence and second harmonic generation from the gold. The bowtie nanoantenna shows a high potential for outstanding conversion efficiencies and the enhancement of other optical effects which could be exploited in future nanophotonic devices.
10.1117/12.2079104
K.Schraml
M.Kaniber
J.Bartl
G.Glashagen
A.Regler
T.Campbell
J.J. Finley
article
Schreiber2015
Observation of manybody localization of interacting fermions in a quasirandom optical lattice
2015
We experimentally observe manybody localization of interacting fermions in a onedimensional quasirandom optical lattice. We identify the manybody localization transition through the relaxation dynamics of an initiallyprepared charge density wave. For sufficiently weak disorder the time evolution appears ergodic and thermalizing, erasing all remnants of the initial order. In contrast, above a critical disorder strength a significant portion of the initial ordering persists, thereby serving as an effective order parameter for localization. The stationary density wave order and the critical disorder value show a distinctive dependence on the interaction strength, in agreement with numerical simulations. We connect this dependence to the ubiquitous logarithmic growth of entanglement entropy characterizing the generic manybody localized phase.
http://arxiv.org/abs/1501.05661
MichaelSchreiber
SeanS. Hodgman
PranjalBordia
HenrikP. L
MarkH. Fischer
RonenVosk
EhudAltman
UlrichSchneider
ImmanuelBloch
article
deVegaB2015
Thermofieldbased chain mapping approach for open quantum systems
2015
We consider a thermofield approach to analyze the evolution of an open quantum system coupled to an environment at finite temperature. In this approach, the finite temperature environment is exactly mapped onto two virtual environments at zero temperature. These two environments are then unitarily transformed into two different chains of oscillators, leading to a one dimensional structure that can be numerically studied using tensor network techniques.
http://arxiv.org/abs/1504.07228
Inesde Vega
MariCarmenBañuls
article
piazza2013a
Umklapp Superradiance from a Collisionless Quantum Degenerate Fermi Gas
Phys. Rev. Lett.
2014
4
8
112,
143003
The quantum dynamics of the electromagnetic light mode of an optical cavity filled with a coherently driven Fermi gas of ultracold atoms strongly depends on geometry of the Fermi surface. Superradiant light generation and selforganization of the atoms can be achieved at low pumping threshold due to resonant atomphoton Umklapp processes, where the fermions are scattered from one side of the Fermi surface to the other by exchanging photon momenta. The cavity spectrum exhibits sidebands, that, despite strong atomlight coupling and cavity decay, retain narrow linewidth, due to absorptionless transparency windows outside the atomic particlehole continuum and the suppression of inhomogeneous broadening and thermal fluctuations in the collisionless Fermi gas.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.143003
10.1103/PhysRevLett.112.143003
FrancescoPiazza
PhilippStrack
article
kuhlen2013
Unambiguous determination of spin dephasing times in ZnO
Phys. Status Solidi B
2014
4
3
251,
9
1861
Timeresolved magnetooptics is a wellestablished optical pump probe technique to generate and to probe spin coherence in semiconductors. By this method, spin dephasing times T_2^* can easily be determined if their values are comparable to the available pumpprobedelays. If T_2^* exceeds the laser repetition time, however, resonant spin amplification (RSA) can equally be used to extract T_2^*. We demonstrate that in ZnO these techniques have several tripping hazards resulting in deceptive values for T_2^* and show how to avoid them. We show that the temperature dependence of the amplitude ratio of two separate spin species can easily be misinterpreted as a strongly temperature dependent T_2^* of a single spin ensemble, while the two spin species have T_2^* values which are nearly independent of temperature. Additionally, consecutive pump pulses can significantly diminish the spin polarization, which remains from previous pump pulses. While this barely affects T_2^* values extracted from delay line scans, it results in seemingly shorter T_2^* values in RSA.
http://onlinelibrary.wiley.com/doi/10.1002/pssb.201350201/abstract;jsessionid=0E7964788D941B5E670C8F6C297B537B.f01t01
10.1002/pssb.201350201
SebastianKuhlen
RalphLedesch
Robinde Winter
MatthiasAlthammer
SebastianT. B. Goennenwein
MatthiasOpel
RudolfGross
ThomasA. Wassner
MartinS. Brandt
BerndBeschoten
article
aizenman2014
Resonances and Partial Delocalization on the Complete Graph
2014
Random operators may acquire extended states formed from a multitude of mutually resonating local quasimodes. This mechanics is explored here in the context of the random Schrödinger operator on the complete graph. The operators exhibits local quasi modes mixed through a single channel. While most of its spectrum consists of localized eigenfunctions, under appropriate conditions it includes also bands of states which are delocalized in the \ell^1though not in \ell^2sense, where the eigenvalues have the statistics of \v<prt>S</prt>eba spectra. The analysis proceeds through some general observations on the scaling limits of random functions in the HerglotzPick class. The results are in agreement with a heuristic condition for the emergence of resonant delocalization, which is stated in terms of the tunneling amplitude among quasimodes.
http://arxiv.org/abs/1405.3951
MichaelAizenman
MiraShamis
SimoneWarzel
article
buesser2014
Decoherence of an entangled state of a stronglycorrelated double quantum dot structure through tunneling processes
2014
We consider two quantum dots described by the Andersonimpurity model with one electron per dot. The goal of our work is to study the decay of a maximally entangled state between the two electrons localized in the dots. We prepare the system in a perfect singlet and then tunnelcouple one of the dots to leads, which induces the nonequilibrium dynamics. We identify two cases: if the leads are subject to a sufficiently large voltage and thus a finite current, then direct tunneling processes cause decoherence and the entanglement as well as spin correlations decay exponentially fast. At zero voltage or small voltages and beyond the mixedvalence regime, virtual tunneling processes dominate and lead to a slower loss of coherence. We analyze this problem by studying the realtime dynamics of the spin correlations and the concurrence using two techniques, namely the timedependent density matrix renormalization group method and a masterequation method. The results from these two approaches are in excellent agreement in the directtunneling regime for the case in which the dot is weakly tunnelcoupled to the leads. We present a quantitative analysis of the decay rates of the spin correlations and the concurrence as a function of tunneling rate, interaction strength, and voltage.
http://arxiv.org/abs/1406.4093
C.A. Büsser
I.de Vega
F.HeidrichMeisner
article
bukov2014
Meanfield phase diagram of the BoseFermi Hubbard model
Phys. Rev. B
2014
89
094502
We analyze the groundstate properties of mixtures consisting of scalar bosons and spin12 fermions using a meanfield treatment of the local bosonfermion interaction on a simple cubic lattice. In the deep superfluid limit of the boson sector and the BCS regime of the fermion sector, we derive BCStype equations to determine the phase diagram of the system. We find a competition between a charge density wave and a superconducting phase. In the opposite limit, we study the Mottinsulatortosuperfluid transition of the boson sector in the presence of a staggered densityinduced alternating potential provided by the fermions, and determine the meanfield transition line. In the twosuperfluids phase of the mixture, we restrict to nearestneighborinduced interactions between the fermions and consider the extended Hubbard model. We perform a meanfield analysis of the critical temperature for the formation of bosonassisted s, extended s−, d, and pwave pairs at fermionic halffilling. We compare our results with a recent dynamical meanfield study [P. Anders et al., Phys. Rev. Lett. 109, 206401 (2012)].
http://link.aps.org/doi/10.1103/PhysRevB.89.094502
American Physical Society
10.1103/PhysRevB.89.094502
MarinBukov
LodePollet
article
kun2014
Universal Conductivity in a TwoDimensional SuperfluidtoInsulator Quantum Critical System
Phys. Rev. Lett.
2014
112
030402
We compute the universal conductivity of the (2+1)dimensional XY universality class, which is realized for a superfluidtoMott insulator quantum phase transition at constant density. Based on largescale Monte Carlo simulations of the classical (2+1)dimensional Jcurrent model and the twodimensional BoseHubbard model, we can precisely determine the conductivity on the quantum critical plateau, σ(∞)=0.359(4)σQ with σQ the conductivity quantum. The universal conductivity curve is the standard example with the lowest number of components where the bottomsup AdS/CFT correspondence from string theory can be tested and made to use [R. C. Myers, S. Sachdev, and A. Singh, Phys. Rev. D 83, 066017 (2011)]. For the first time, the shape of the σ(iωn)−σ(∞) function in the Matsubara representation is accurate enough for a conclusive comparison and establishes the particlelike nature of charge transport. We find that the holographic gaugegravity duality theory for transport properties can be made compatible with the data if temperature of the horizon of the black brane is different from the temperature of the conformal field theory. The requirements for measuring the universal conductivity in a cold gas experiment are also determined by our calculation.
http://link.aps.org/doi/10.1103/PhysRevLett.112.030402
American Physical Society
10.1103/PhysRevLett.112.030402
KunChen
LongxiangLiu
YoujinDeng
LodePollet
NikolayProkof'ev
article
cuenin2014
Dipoles in Graphene Have Infinitely Many Bound States
2014
We show that in graphene charge distributions with nonvanishing dipole moment have infinitely many bound states. The corresponding eigenvalues accumulate at the edges of the gap faster than any power.
http://arxiv.org/abs/1403.7160
JeanClaudeCuenin
HeinzSiedentop
article
duca2014
An AharonovBohm interferometer for determining Bloch band topology
2014
The geometric structure of an energy band in a solid is fundamental for a wide range of manybody phenomena in condensed matter and is uniquely characterized by the distribution of Berry curvature over the Brillouin zone. In analogy to an AharonovBohm interferometer that measures the magnetic flux penetrating a given area in real space, we realize an atomic interferometer to measure Berry flux in momentum space. We demonstrate the interferometer for a graphenetype hexagonal lattice, where it has allowed us to directly detect the singular π Berry flux localized at each Dirac point. We show that the interferometer enables one to determine the distribution of Berry curvature with high momentum resolution. Our work forms the basis for a general framework to fully characterize topological band structures and can also facilitate holonomic quantum computing through controlled exploitation of the geometry of Hilbert space.
http://arxiv.org/abs/1407.5635
LuciaDuca
TracyLi
MartinReitter
ImmanuelBloch
MonikaSchleierSmith
UlrichSchneider
article
Dumas2014
Polynomial cubic differentials and convex polygons in the projective plane
2014
We construct and study a natural homeomorphism between the moduli space of polynomial cubic differentials of degree d on the complex plane and the space of projective equivalence classes of oriented convex polygons with d+3 vertices. This map arises from the construction of a complete hyperbolic affine sphere with prescribed Pick differential, and can be seen as an analogue of the LabourieLoftin parameterization of convex RP^2 structures on a compact surface by the bundle of holomorphic cubic differentials over Teichmuller space.
arxiv.org/abs/1407.8149
DavidDumas
MichaelWolf
article
fauser2014
Multiparticle localization for disordered systems on continuous space via the fractional moment method
2014
We investigate spectral and dynamical localization of a quantum system of n particles on \mathbb<prt>R</prt>^d which are subject to a random potential and interact through a pair potential which may have infinite range. We establish two conditions which ensure spectral and dynamical localization near the bottom of the spectrum of the n particle system: i)localization is established in the regime of weak interactions supposing oneparticle localization, and ii)localization is also established under a Lifshitztail type condition on the sparsity of the spectrum. In case of polynomially decaying interactions, we provide an upper bound on the number of particles up to which these conditions apply.
http://arxiv.org/abs/1402.5832
MichaelFauser
SimoneWarzel
article
fukuda2014
Quantum channels with polytopic images and image additivity
2014
We study quantum channels with respect to their image, i.e., the image of the set of density operators under the action of the channel. We first characterize the set of quantum channels having polytopic images and show that additivity of the minimal output entropy can be violated in this class. We then provide a complete characterization of quantum channels T that are universally image additive in the sense that for any quantum channel S, the image of T \otimes S is the convex hull of the tensor product of the images of T and S. These channels turn out to form a strict subset of entanglement breaking channels with polytopic images and a strict superset of classicalquantum channels.
http://arxiv.org/abs/1408.2340
MotohisaFukuda
IonNechita
MichaelM. Wolf
article
gebert2014
The exponent in the orthogonality catastrophe for Fermi gases
2014
We quantify the asymptotic vanishing of the groundstate overlap of two noninteracting Fermi gases in ddimensional Euclidean space in the thermodynamic limit. Given two oneparticle Schrödinger operators in finitevolume which differ by a compactly supported bounded potential, we prove a powerlaw upper bound on the groundstate overlap of the corresponding noninteracting Nparticle systems. We interpret the decay exponent γ in terms of scattering theory and find γ = π<sup>2</sup>{\lVert\arcsin<prt>\lvert T_E/2\rvert}\rVert</prt>_{\mathrm<prt>HS}</prt>^2, where T_E is the transition matrix at the Fermi energy E. This exponent reduces to the one predicted by Anderson [Phys. Rev. 164, 352359 (1967)] for the exact asymptotics in the special case of a pointlike perturbation. We therefore expect the upper bound to coincide with the exact asymptotics of the overlap.
http://arxiv.org/abs/1407.2512
MartinGebert
HeinrichKüttler
PeterMüller
PeterOtte
article
germinet2014
Ergodicity and dynamical localization for DeloneAnderson operators
2014
We study the ergodic properties of DeloneAnderson operators, using the framework of randomly coloured Delone sets and Delone dynamical systems. In particular, we show the existence of the integrated density of states and, under some assumptions on the geometric complexity of the underlying Delone sets, we obtain information on the almostsure spectrum of the family of random operators. We then exploit these results to study the Lifshitztail behaviour of the integrated density of states of a DeloneAnderson operator at the bottom of the spectrum. This is used as an input for the multi scale analysis to prove dynamical localization. We also estimate the size of the spectral region where dynamical localization occurs.
http://arxiv.org/abs/1405.4233
FrancoisGerminet
PeterMüller
ConstanzaRojasMolina
article
glasser2014
Construction of spin models displaying quantum criticality from quantum field theory
Nuclear Physics B
2014
886,
6374
We provide a method for constructing finite temperature states of onedimensional spin chains displaying quantum criticality. These models are constructed using correlators of products of quantum fields and have an analytical purification. Their properties can be investigated by MonteCarlo simulations, which enable us to study the lowtemperature phase diagram and to show that it displays a region of quantum criticality. The mixed states obtained are shown to be close to the thermal state of a simple nearest neighbour Hamiltonian.
http://arxiv.org/abs/1405.4135
10.1016/j.nuclphysb.2014.06.016
IvanGlasser
J.Ignacio Cirac
GermanSierra
AnneE. B. Nielsen
article
goulko2014
The effect of spinorbit interactions on the 0.7anomaly in quantum point contacts
2014
We study how the conductance of a quantum point contact is affected by spinorbit interactions, for systems at zero temperature both with and without electronelectron interactions. In the presence of spinorbit coupling, tuning the strength and direction of an external magnetic field can change the dispersion relation and hence the local density of states in the point contact region. This modifies the effect of electronelectron interactions, implying striking changes in the shape of the 0.7anomaly and introducing additional distinctive features in the first conductance step.
http://arxiv.org/abs/1408.0746
OlgaGoulko
FlorianBauer
JanHeyder
Janvon Delft
article
hanl2014
Equilibrium Fermiliquid coefficients for the fully screened Nchannel Kondo model
Phys. Rev. B
2014
89,
195131
We analytically and numerically compute three equilibrium Fermiliquid coefficients of the fully screened Nchannel Kondo model, namely c_B, c_T and c_\varepsilon, characterizing the magnetic field and temperature dependence of the resisitivity, and the curvature of the equilibrium Kondo resonance, respectively. We present a compact, unified derivation of the Ndependence of these coefficients, combining elements from various previous treatments of this model. We numerically compute these coefficients using the numerical renormalization group, with nonAbelian symmetries implemented explicitly, finding agreement with Fermiliquid predictions on the order of 5% or better.
http://arxiv.org/abs/1403.0497
10.1103/PhysRevB.89.195131
MarkusHanl
AndreasWeichselbaum
Janvon Delft
MikhailKiselev
article
moritz2014
Low dimensionality of the surface conductivity of diamond
Phys. Rev. B
2014
89
115426
Undoped diamond, a remarkable bulk electrical insulator, exhibits a high surface conductivity in air when the surface is hydrogen terminated. Although theoretical models have claimed that a twodimensional hole gas is established as a result of surface energyband bending, no definitive experimental demonstration has been reported so far. Here, we prove the twodimensional character of the surface conductivity by lowtemperature characterization of diamond inplane gated fieldeffect transistors that enable the lateral confinement of the transistor's drainsource channel to nanometer dimensions. In these devices, we observe Coulomb blockade effects of multiple quantum islands varying in size with the gate voltage. The charging energy and thus the size of these zerodimensional islands exhibit a gatevoltage dependence which is the direct result of the twodimensional character of the conductive channel formed at hydrogenterminated diamond surfaces.
http://link.aps.org/doi/10.1103/PhysRevB.89.115426
American Physical Society
10.1103/PhysRevB.89.115426
Moritz V.Hauf
PatrickSimon
MaxSeifert
Alexander W.Holleitner
MartinStutzmann
Jose A.Garrido
article
hild2014
Farfromequilibrium spin transport in Heisenberg quantum magnets
2014
We study experimentally the farfromequilibrium dynamics in ferromagnetic Heisenberg quantum magnets realized with ultracold atoms in an optical lattice. After controlled imprinting of a spin spiral pattern with adjustable wave vector, we measure the decay of the initial spin correlations through singlesite resolved detection. On the experimentally accessible timescale of several exchange times we find a profound dependence of the decay rate on the wave vector. In onedimensional systems we observe diffusionlike spin transport with a dimensionless diffusion coefficient of 0.22(1). We show how this behavior emerges from the microscopic properties of the closed quantum system. In contrast to the onedimensional case, our transport measurements for twodimensional Heisenberg systems indicate anomalous superdiffusion.
http://arxiv.org/abs/1407.6934
SebastianHild
TakeshiFukuhara
PeterSchauss
JohannesZeiher
MichaelKnap
EugeneDemler
ImmanuelBloch
ChristianGross
article
hochstaettler2014
Semicircle law for a matrix ensemble with dependent entries
2014
We study ensembles of random symmetric matrices whose entries exhibit certain correlations. Examples are distributions of CurieWeisstype. We provide a criterion on the correlations ensuring the validity of Wigner's semicircle law for the eigenvalue distribution measure. In case of CurieWeiss distributions this criterion applies above the critical temperature (i.e. β<1). We also investigate the largest eigenvalue of certain ensembles of CurieWeiss type and find a transition in its behavior at the critical temperature.
http://arxiv.org/abs/1401.6636
WinfriedHochstättler
WernerKirsch
SimoneWarzel
article
hocke2014
Determination of effective mechanical properties of a doublelayer beam by means of a nanoelectromechanical transducer
2014
We investigate the mechanical properties of a doublyclamped, doublelayer nanobeam embedded into an electromechanical system. The nanobeam consists of a highly prestressed silicon nitride and a superconducting niobium layer. By measuring the mechanical displacement spectral density both in the linear and the nonlinear Duffing regime, we determine the prestress and the effective Young's modulus of the nanobeam. An analytical doublelayer model quantitatively corroborates the measured values. This suggests that this model can be used to design mechanical multilayer systems for electro and optomechanical devices, including materials controllable by external parameters such as piezoelectric, magnetrostrictive, or in more general multiferroic materials.
http://arxiv.org/abs/1407.6867
FredrikHocke
MatthiasPernpeintner
XiaoqingZhou
AlbertSchliesser
TobiasJ. Kippenberg
HansHuebl
RudolfGross
article
idel2014
Sinkhorn normal form for unitary matrices
2014
Sinkhorn proved that every entrywise positive matrix can be made doubly stochastic by multiplying with two diagonal matrices. In this note we prove a recently conjectured analogue for unitary matrices: every unitary can be decomposed into two diagonal unitaries and one whose row and column sums are equal to one. The proof is nonconstructive and based on a reformulation in terms of symplectic topology. As a corollary, we obtain a decomposition of unitary matrices into an interlaced product of unitary diagonal matrices and discrete Fourier transformations. This provides a new decomposition of linear optics arrays into phase shifters and canonical multiports described by Fourier transformations.
http://arxiv.org/abs/1408.5728
MartinIdel
MichaelM. Wolf
article
Keller2014
An invitation to trees of finite cone type: random and deterministic operators
2014
Trees of finite cone type have appeared in various contexts. In particular, they come up as simplified models of regular tessellations of the hyperbolic plane. The spectral theory of the associated Laplacians can thus be seen as induced by geometry. Here we give an introduction focusing on background and then turn to recent results for (random) perturbations of trees of finite cone type and their spectral theory.
http://arxiv.org/abs/1403.4426
MatthiasKeller
DanielLenz
SimoneWarzel
article
keyl2014
Controlling several atoms in a cavity
New Journal of Physics
2014
16
6
065010
We treat control of several twolevel atoms interacting with one mode of the electromagnetic field in a cavity. This provides a useful model to study pertinent aspects of quantum control in infinite dimensions via the emergence of infinitedimensional system algebras. Hence we address problems arising with infinitedimensional Lie algebras and those of unbounded operators. For the models considered, these problems can be solved by splitting the set of control Hamiltonians into two subsets: the first obeys an Abelian symmetry and can be treated in terms of infinitedimensional Lie algebras and strongly closed subgroups of the unitary group of the system Hilbert space. The second breaks this symmetry, and its discussion introduces new arguments. Yet, full controllability can be achieved in a strong sense: e.g., in a time dependent Jaynes Cummings model we show that, by tuning coupling constants appropriately, every unitary of the coupled system (atoms and cavity) can be approximated with arbitrarily small error.
http://stacks.iop.org/13672630/16/i=6/a=065010
MichaelKeyl
RobertZeier
ThomasSchulte Herbrüggen
article
klein2014
Acconductivity and electromagnetic energy absorption for the Anderson model in linear response theory
2014
We continue our study of the acconductivity in linear response theory for the Anderson model using the conductivity measure. We establish further properties of the conductivity measure, including nontriviality at nonzero temperature, the high temperature limit, and asymptotics with respect to the disorder. We also calculate the electromagnetic energy absorption in linear response theory in terms of the conductivity measure.
http://arxiv.org/abs/1403.0286
AbelKlein
PeterMüller
article
kumar2014
Fewcycle, Broadband, Midinfrared Optical Parametric Oscillator Pumped by a 20fs Ti:sapphire Laser
2014
We report a fewcycle, broadband, singlyresonant optical parametric oscillator (OPO) for the midinfrared based on MgOdoped periodicallypoled LiNbO3 (MgO:PPLN), synchronously pumped by a 20fs Ti:sapphire laser. By using crystal interaction lengths as short as 250 um, and careful dispersion management of input pump pulses and the OPO resonator, neartransformlimited, fewcycle idler pulses tunable across the midinfrared have been generated, with as few as 3.7 optical cycles at 2682 nm. The OPO can be continuously tuned over 21793732 nm by cavity delay tuning, providing up to 33 mW of output power at 3723 nm. The idler spectra exhibit stable broadband profiles with bandwidths spaning over 422 nm (FWHM) recorded at 3732 nm. We investigate the effect of crystal length on spectral bandwidth and pulse duration at a fixed wavelength, confirming neartransformlimited idler pulses for all grating interaction lengths. By locking the repetition frequency of the pump laser to a radiofrequency reference, and without active stabilization of the OPO cavity length, an idler power stability better than 1.6% rms over >2.75 hours is obtained when operating at maximum output power, in excellent spatial beam quality with TEM00 mode profile.
http://arxiv.org/abs/1404.1893
SuddapalliChaitanya Kumar
AdolfoEstebanMartin
TakuroIdeguchi
MingYan
SimonHolzner
TheodorW. Haensch
NathaliePicque
MajidEbrahimZadeh
article
liu2014
Simplex valencebond crystal in the spin1 kagome Heisenberg antiferromagnet
2014
We investigate the ground state properties of a spin1 kagome antiferromagnetic Heisenberg (KAH) model using tensornetwork methods. We find a trimerized ground state, with energy per site e_0\simeq1.409 obtained by accurate calculations directly in the thermodynamic limit. The symmetry between left and right triangles is spontaneously broken, with a relative energy difference of δ ≈ 20%. The spinspin, dimerdimer, and chiral correlation functions are found to decay exponentially with a rather short correlation length, showing that the ground state is gapped. Based on this unambiguous numerical evidence, we identify the ground state of the spin1 KAH model to be a simplex valencebond crystal (SVBC). Besides the KAH model, we also discuss the spin1 bilinearbiquadratic Heisenberg model on a kagome lattice, and determine its ground state phase diagram. In particular, we find a quantum phase transition between the SVBC and ferroquadrupolar nematic states.
http://arxiv.org/abs/1406.5905
TaoLiu
WeiLi
AndreasWeichselbaum
Janvon Delft
GangSu
article
lubasch2014
Algorithms for finite Projected Entangled Pair States
Phys. Rev. B
2014
90,
064425
Projected Entangled Pair States (PEPS) are a promising ansatz for the study of strongly correlated quantum manybody systems in two dimensions. But due to their high computational cost, developing and improving PEPS algorithms is necessary to make the ansatz widely usable in practice. Here we analyze several algorithmic aspects of the method. On the one hand, we quantify the connection between the correlation length of the PEPS and the accuracy of its approximate contraction, and discuss how purifications can be used in the latter. On the other, we present algorithmic improvements for the update of the tensor that introduce drastic gains in the numerical conditioning and the efficiency of the algorithms. Finally, the stateoftheart general PEPS code is benchmarked with the Heisenberg and quantum Ising models on lattices of up to 21 × 21 sites.
http://arxiv.org/abs/1405.3259
10.1103/PhysRevB.90.064425
MichaelLubasch
J.Ignacio Cirac
MariCarmenBañuls
article
:/content/aip/journal/apl/105/8/10.1063/1.4894239
A carrier relaxation bottleneck probed in single InGaAs quantum dots using integrated superconducting single photon detectors
Applied Physics Letters
2014
105
8

Using integrated superconducting single photon detectors, we probe ultraslow exciton capture and relaxation dynamics in single selfassembled InGaAs quantum dots embedded in a GaAs ridge waveguide. Timeresolved luminescence measurements performed with on and offchip detection reveal a continuous decrease in the carrier relaxation time from 1.22 ± 0.07 ns to 0.10 ± 0.07 ns upon increasing the number of nonresonantly injected carriers. By comparing offchip timeresolved spectroscopy with spectrally integrated onchip measurements, we identify the observed dynamics in the rise time (τr ) as arising from a relaxation bottleneck at low excitation levels. From the comparison with the temporal dynamics of the single exciton transition with the onchip emission signal, we conclude that the relaxation bottleneck is circumvented by the presence of charge carriers occupying states in the bulk material and the twodimensional wetting layer continuum. A characteristic τr ∝ P −2∕3 power law dependence is observed suggesting Augertype scattering between carriers trapped in the quantum dot and the twodimensional wetting layer continuum which circumvents the phonon relaxation bottleneck.
http://scitation.aip.org/content/aip/journal/apl/105/8/10.1063/1.4894239
http://dx.doi.org/10.1063/1.4894239
G.Reithmaier
F.Flassig
P.Hasch
S.Lichtmannecker
K.Müller
J.Vučković
R.Gross
M.Kaniber
J. J.Finley
article
PSSR:PSSR201308305
Polarization dependent, surface plasmon induced photoconductance in gold nanorod arrays
physica status solidi (RRL) – Rapid Research Letters
2014
8
3
264268
We report on the photoconductance in twodimensional arrays of gold nanorods. The arrays are formed by a combination of droplet deposition and stamping methods. We find that the plasmon induced photoconductance is sensitive to the linear polarization of the exciting photons consistent with the excitation of the longitudinal surface plasmon resonance of the nanorods.
surface plasmons, optical sensors, nanorods, photoconductance, gold
http://dx.doi.org/10.1002/pssr.201308305
WILEYVCH Verlag
18626270
10.1002/pssr.201308305
S.Diefenbach
N.Erhard
J.Schopka
A.Martin
C.Karnetzky
D.Iacopino
A. W.Holleitner
article
PhysRevApplied.2.024002
Optical Thermometry of an Electron Reservoir Coupled to a Single Quantum Dot in the Millikelvin Range
Phys. Rev. Applied
2014
2
024002
We show how resonant laser spectroscopy of the trion optical transitions in a selfassembled quantum dot can be used to determine the temperature of a nearby electron reservoir. At finite magnetic field, the spinstate occupation of the Zeemansplit quantumdot electron ground states is governed by thermalization with the electron reservoir via cotunneling. With resonant spectroscopy of the corresponding excited trion states, we map out the spin occupation as a function of magnetic field to establish optical thermometry for the electron reservoir. We demonstrate the implementation of the technique in the subkelvin temperature range where it is most sensitive and where the electron temperature is not necessarily given by the cryostat base temperature.
http://arxiv.org/abs/1405.2261
http://link.aps.org/doi/10.1103/PhysRevApplied.2.024002
American Physical Society
10.1103/PhysRevApplied.2.024002
F.Seilmeier
M.Hauck
E.Schubert
G. J.Schinner
S. E.Beavan
A.Högele
article
seilmeier2014
SubKelvin optical thermometry of an electron reservoir coupled to a selfassembled InGaAs quantum dot
Phys. Rev. Applied
2014
2,
024002
We show how resonant laser spectroscopy of the trion optical transitions in a selfassembled quantum dot can be used to determine the temperature of a nearby electron reservoir. At finite magnetic field the spinstate occupation of the Zeemansplit quantum dot electron ground states is governed by thermalization with the electron reservoir via cotunneling. With resonant spectroscopy of the corresponding excited trion states we map out the spin occupation as a function of magnetic field to establish optical thermometry for the electron reservoir. We demonstrate the implementation of the technique in the subKelvin temperature range where it is most sensitive, and where the electron temperature is not necessarily given by the cryostat base temperature.
F.Seilmeier
M.Hauck
E.Schubert
G.J. Schinner
S.E. Beavan
A.H
article
PhysRevB.90.235306
Locating environmental charge impurities with confluent laser spectroscopy of multiple quantum dots
Phys. Rev. B
2014
90
235306
http://link.aps.org/doi/10.1103/PhysRevB.90.235306
American Physical Society
10.1103/PhysRevB.90.235306
M.Hauck
F.Seilmeier
S. E.Beavan
A.Badolato
P. M.Petroff
A.Högele
article
Adame2014
Exponential vanishing of the groundstate gap of the QREM via adiabatic quantum computing
2014
In this note we compile and slightly generalise ideas of Farhi, Goldstone, Gosset, Gutmann, Nagaj and Shor by discussing a lower bound on the run time of their quantum adiabatic search algorithm and its use for an upper bound on the energy gap above the groundstate of the generators of this algorithm. We illustrate these ideas by applying them to the quantum random energy model (QREM). Our main result is a simple proof of the conjectured exponential vanishing of the energy gap of the QREM.
JuanAdame
SimoneWarzel
article
Bruognolo2014
Twobath spinboson model: Phase diagram and critical properties
Phys. Rev. B
2014
90,
245130
The spinboson model, describing a twolevel system coupled to a bath of harmonic oscillators, is a generic model for quantum dissipation, with manifold applications. It has also been studied as a simple example for an impurity quantum phase transition. Here we present a detailed study of a U(1)symmetric twobath spinboson model, where two different components of an SU(2) spin 1/2 are coupled to separate dissipative baths. Nontrivial physics arises from the competition of the two dissipation channels, resulting in a variety of phases and quantum phase transitions. We employ a combination of analytical and numerical techniques to determine the properties of both the stable phases and the quantum critical points. In particular, we find a critical intermediatecoupling phase which is bounded by a continuous quantum phase transition which violates the quantumtoclassical correspondence.
BenediktBruognolo
AndreasWeichselbaum
ChengGuo
Janvon Delft
ImkeSchneider
MatthiasVojta
article
Cai2014
Identifying a bathinduced Bose liquid in interacting spinboson models
Phys. Rev. Lett.
2014
113,
260403
We study the ground state phase diagram of a onedimensional hardcore bosonic model with nearestneighbor interactions (XXZ model) where every site is coupled Ohmically to an independent but identical reservoir, hereby generalizing spinboson models to interacting spinboson systems. We show that a bathinduced Bose metal phase can occur in the ground state phase diagram away from half filling. This phase is compressible, gapless, and conducting but not superfluid. At haffilling, only a Luttinger liquid and a charge density wave are found. The phase transition between them is of KosterlitzThouless type where the Luttinger parameter takes a nonuniversal value.The applied quantum Monte Carlo method can be used for all open bosonic and unfrustrated spin systems, regardless of their dimension, filling factor and spectrum of the dissipation as long as the quantum system couples to the bath via the density operators.
ZiCai
UlrichSchollwöck
LodePollet
article
Ehberger2014
Highly coherent electron beam from a lasertriggered tungsten needle tip
2014
We report on a quantitative measurement of the spatial coherence of electrons emitted from a sharp metal needle tip. We investigate the coherence in photoemission using nearultraviolet laser triggering with a photon energy of 3.1 eV and compare it to DCfield emission. A carbonnanotube is brought in close proximity to the emitter tip to act as an electrostatic biprism. From the resulting electron matter wave interference fringes we deduce an upper limit of the effective source radius both in lasertriggered and DCfield emission mode, which quantifies the spatial coherence of the emitted electron beam. We obtain (0.80\pm 0.05)\,nm in lasertriggered and (0.55\pm 0.02)\,nm in DCfield emission mode, revealing that the outstanding coherence properties of electron beams from needle tip field emitters are largely maintained in laserinduced emission. In addition, the relative coherence width of 0.36 of the photoemitted electron beam is the largest observed so far. The preservation of electronic coherence during emission as well as ramifications for timeresolved electron imaging techniques are discussed.
DominikEhberger
JakobHammer
MaxEisele
MichaelKr
JonathanNoe
AlexanderH
PeterHommelhoff
article
Glaetzle2014
Frustrated Quantum Magnetism with LaserDressed Rydberg Atoms
Phys. Rev. Lett.
2014
114
173002
We show how a broad class of lattice spin1/2 models with angular and distancedependent couplings can be realized with cold alkali atoms stored in optical or magnetic trap arrays. The effective spin1/2 is represented by a pair of atomic ground states, and spinspin interactions are obtained by admixing van der Waals interactions between finestructure split Rydberg states with laser light. The strengths of the diagonal spin interactions as well as the "flipflop", and "flipflip" and "flopflop" interactions can be tuned by exploiting quantum interference, thus realizing different spin symmetries. The resulting energy scales of interactions compare well with typical temperatures and decoherence timescales, making the exploration of exotic forms of quantum magnetism, including emergent gauge theories and compass models, accessible within stateoftheart experiments.
10.1103/PhysRevLett.114.173002
AlexanderW. Glaetzle
MarcelloDalmonte
RejishNath
ChristianGross
ImmanuelBloch
PeterZoller
article
Gloeckner2014
Rotational state detection of electrically trapped polyatomic molecules
2014
Detecting the internal state of polar molecules is a substantial challenge when standard techniques such as resonanceenhanced multi photon ionization (REMPI) or laserinduced fluorescense (LIF) do not work. As this is the case for most polyatomic molecule species, we here investigate an alternative based on state selective removal of molecules from an electrically trapped ensemble. Specifically, we deplete molecules by driving rotational and/or vibrational transitions to untrapped states. Fully resolving the rotational state with this method can be a considerable challenge as the frequency differences between various transitions is easily substantially less than the Stark broadening in an electric trap. However, making use of a unique trap design providing homogeneous fields in a large fraction of the trap volume, we successfully discriminate all rotational quantum numbers, including the rotational Msubstate.
RosaGl
AlexanderPrehn
GerhardRempe
MartinZeppenfeld
article
Handrek2014
The Ground State Energy of Heavy Atoms: the Leading Correction
2014
For heavy atoms (large atomic number Z) described by nopair operators in the Furry picture we find the ground state's leading energy correction. We compare the result with (semi)empirical values and Schwinger's prediction showing more than qualitative agreement.
MichaelHandrek
HeinzSiedentop
article
Knips2014
Multipartite entanglement dectection with minimal effort
2014
Certifying entanglement in a multipartite state is a demanding task. A state of N qubits is parametrized by 4^N1 real numbers, so, at first glimpse, one may expect that the measurement complexity of generic entanglement detection is also exponential with N. However, here we show how to design indicators for genuine multipartite quantum entanglement which require only two correlation measurements for prominent quantum states. We introduce a constructive method to derive such criteria and apply them in experiments for fourqubit GreenbergerHorneZeilinger states, cluster states and Dicke states.
LukasKnips
ChristianSchwemmer
NicoKlein
MarcinWie?niak
HaraldWeinfurter
article
Kroiss2014
Diagrammatic Monte Carlo study of massimbalanced Fermipolaron system
2014
We apply the diagrammatic Monte Carlo approach to threedimensional Fermipolaron systems with massimbalance, where an impurity interacts resonantly with a noninteracting Fermi sea whose atoms have a different mass. This method allows to go beyond frequently used variational techniques by stochastically summing all relevant impurity Feynman diagrams up to a maximum expansion order limited by the sign problem. Polaron energy and quasiparticle residue can be accurately determined over a broad range of impurity masses. Furthermore, the spectral function of an imbalanced polaron demonstrates the stability of the quasiparticle and allows to locate in addition also the repulsive polaron as an excited state. The quantitative exactness of twoparticlehole wavefunctions is investigated, resulting in a relative lowering of polaronic energies in the massimbalance phase diagram. Tan's contact coefficient for the massbalanced polaron system is found in good agreement with variational methods. Massimbalanced systems can be studied experimentally by ultracold atom mixtures like ^6Li<sup>40</sup>K.
PeterKroiss
LodePollet
article
Raczkowski2014
Spin and charge dynamics of a quasionedimensional antiferromagnetic metal
Phys. Rev. B
2014
91,
045137
We use quantum Monte Carlo simulations to study a finitetemperature dimensionalcrossoverdriven evolution of spin and charge dynamics in weakly coupled Hubbard chains with a halffilled band. The lowtemperature behavior of the charge gap indicates a crossover between two distinct energy scales: a highenergy onedimensional (1D) Mott gap due to the umklapp process and a lowenergy gap which stems from longrange antiferromagnetic (AF) fluctuations. Away from the 1D regime and at temperature scales above the charge gap, the emergence of a zerofrequency Drudelike feature in the interchain optical conductivity \sigma<sub>\perp</sub>(ω) implies the onset of a higherdimensional metal. In this metallic phase, enhanced quasiparticle scattering off finiterange AF fluctuations results in incoherent singleparticle dynamics. The coupling between spin and charge fluctuations is also seen in the spin dynamical structure factor S(<prt>\pmb q</prt>,ω) displaying damped spin excitations (paramagnons) close to the AF wavevector <prt>\pmb q</prt>=(π,π) and particlehole continua near 1D momentum transfers spanning quasiparticles at the Fermi surface. We relate our results to the charge deconfinement in quasi1D organic BechgaardFabre salts.
MarcinRaczkowski
FakherF. Assaad
LodePollet
article
Reiserer2014a
A Quantum Gate between a Flying Optical Photon and a Single Trapped Atom
Nature
2014
508,
237240
The steady increase in control over individual quantum systems has backed the dream of a quantum technology that provides functionalities beyond any classical device. Two particularly promising applications have been explored during the past decade: First, photonbased quantum communication, which guarantees unbreakable encryption but still has to be scaled to high rates over large distances. Second, quantum computation, which will fundamentally enhance computability if it can be scaled to a large number of quantum bits. It was realized early on that a hybrid system of light and matter qubits could solve the scalability problem of both fields  that of communication via quantum repeaters, that of computation via an optical interconnect between smaller quantum processors. To this end, the development of a robust twoqubit gate that allows to link distant computational nodes is "a pressing challenge". Here we demonstrate such a quantum gate between the spin state of a single trapped atom and the polarization state of an optical photon contained in a faint laser pulse. The presented gate mechanism is deterministic, robust and expected to be applicable to almost any matter qubit. It is based on reflecting the photonic qubit from a cavity that provides strong lightmatter coupling. To demonstrate its versatility, we use the quantum gate to create atomphoton, atomphotonphoton, and photonphoton entangled states from separable input states. We expect our experiment to break ground for various applications, including the generation of atomic and photonic cluster states, Schrödingercat states, deterministic photonic Bellstate measurements, and quantum communication using a redundant quantum parity code.
AndreasReiserer
NorbertKalb
GerhardRempe
StephanRitter
article
Reiserer2014
Cavitybased quantum networks with single atoms and optical photons
2014
The implementation of a largescale quantum network is a key challenge for quantum science. Such network consists of stationary quantum nodes that can store and process quantum information locally. The nodes are connected by quantum channels for flying information carriers, i.e. photons. These channels serve both to directly exchange quantum information between nodes as well as to distribute entanglement over the whole network. In order to scale such network to many particles and long distances, an efficient interface between the nodes and the channels is required. This article describes the cavitybased approach to this goal, with an emphasis on experimental systems in which single atoms are trapped in and coupled to optical resonators. Besides being conceptually appealing, this approach is promising for quantum networks on larger scales, as it gives access to long qubit coherence times and high lightmatter coupling efficiencies. Thus, it allows one to generate entangled photons on the push of a button, to reversibly map the quantum state of a photon onto an atom, to transfer and teleport quantum states between remote atoms, to entangle distant atoms, to detect optical photons nondestructively, to perform entangling quantum gates between an atom and one or several photons, and even provides a route towards efficient heralded quantum memories for future repeaters. The presented general protocols and the identification of key parameters are applicable to other experimental systems.
AndreasReiserer
GerhardRempe
article
Schwemmer2014
Experimental multipartite entanglement without multipartite correlations
2014
Nonclassical correlations between measurement results make entanglement the essence of quantum physics and the main resource for quantum information applications. Surprisingly, there are nparticle states which do not exhibit npartite correlations at all but still are genuinely npartite entangled. We introduce a general construction principle for such states, implement them in a multiphoton experiment and analyze their properties in detail. Remarkably, even without npartite correlations, these states do violate Bell inequalities showing that there is no local realistic model describing their properties.
ChristianSchwemmer
LukasKnips
MinhCong Tran
Annade Rosier
WieslawLaskowski
TomaszPaterek
HaraldWeinfurter
article
Schwemmer2014a
Experimental Comparison of Efficient Tomography Schemes for a SixQubit State
Phys. Rev. Lett.
2014
113,
040503
Quantum state tomography suffers from the measurement effort increasing exponentially with the number of qubits. Here, we demonstrate permutationally invariant tomography for which, contrary to conventional tomography, all resources scale polynomially with the number of qubits both in terms of the measurement effort as well as the computational power needed to process and store the recorded data. We demonstrate the benefits of combining permutationally invariant tomography with compressed sensing by studying the influence of the pump power on the noise present in a sixqubit symmetric Dicke state, a case where full tomography is possible only for very high pump powers.
ChristianSchwemmer
GezaToth
AlexanderNiggebaum
TobiasMoroder
DavidGross
OtfriedG
HaraldWeinfurter
article
Tiarks2014
SinglePhoton Transistor Using a Förster Resonance
Phys. Rev. Lett.
2014
113,
053602
An alloptical transistor is a device in which a gate light pulse switches the transmission of a target light pulse with a gain above unity. The gain quantifies the change of the transmitted target photon number per incoming gate photon. We study the quantum limit of one incoming gate photon and observe a gain of 20. The gate pulse is stored as a Rydberg excitation in an ultracold gas. The transmission of the subsequent target pulse is suppressed by Rydberg blockade which is enhanced by a Förster resonance. The detected target photons reveal in a single shot with a fidelity above 0.86 whether a Rydberg excitation was created during the gate pulse. The gain offers the possibility to distribute the transistor output to the inputs of many transistors, thus making complex computational tasks possible.
DanielTiarks
SimonBaur
KatharinaSchneider
StephanD
GerhardRempe
article
GebertKM2014
Anderson's orthogonality catastrophe
Commun. Math. Phys.
2014
329
979998
We give an upper bound on the modulus of the groundstate overlap of two noninteracting fermionic quantum systems with N particles in a large but finite volume Ld of ddimensional Euclidean space. The underlying oneparticle Hamiltonians of the two systems are standard Schrödinger operators that differ by a nonnegative compactly supported scalar potential. In the thermodynamic limit, the bound exhibits an asymptotic powerlaw decay in the system size L, showing that the groundstate overlap vanishes for macroscopic systems. The decay exponent can be interpreted in terms of the total scattering cross section averaged over all incident directions. The result confirms and generalises P. W. Anderson's informal computation [Phys. Rev. Lett. 18, 10491051 (1967)].
10.1007/s0022001419143
MartinGebert
HeinrichKüttler
PeterMüller
article
piazza2013
BoseEinstein Condensation versus DickeHeppLieb Transition in an Optical Cavity
Annals of Physics
2013
12
339,
135
We provide an exact solution for the interplay between BoseEinstein condensation and the DickeHeppLieb selforganization transition of an ideal Bose gas trapped inside a singlemode optical cavity and subject to a transverse laser drive. Based on an effective action approach, we determine the full phase diagram at arbitrary temperature, which features a bicritical point where the transitions cross. We calculate the dynamically generated band structure of the atoms and the associated supression of the critical temperature for BoseEinstein condensation in the phase with a spontaneous periodic density modulation. Moreover, we determine the evolution of the polariton spectrum due to the coupling of the cavity photons and the atomic field near the selforganization transition, which is quite different above or below the BoseEinstein condensation temperature. At low temperatures, the critical value of the DickeHeppLieb transition decreases with temperature and thus thermal fluctuations can enhance the tendency to a periodic arrangement of the atoms.
https://www.sciencedirect.com/science/article/pii/S0003491613001905?via%3Dihub
10.1016/j.aop.2013.08.015
FrancescoPiazza
PhilippStrack
WilhelmZwerger
article
Rath20130
Fieldtheoretical study of the Bose polaron
Phys. Rev. A
2013
11
26
88,
053632
We study the properties of the Bose polaron, an impurity strongly interacting with a BoseEinstein condensate, using a fieldtheoretic approach and make predictions for the spectral function and various quasiparticle properties that can be tested in experiment. We find that most of the spectral weight is contained in a coherent attractive and a metastable repulsive polaron branch. We show that the qualitative behavior of the Bose polaron is well described by a nonselfconsistent Tmatrix approximation by comparing analytical results to numerical data obtained from a fully selfconsistent Tmatrix approach. The latter takes into account an infinite number of bosons excited from the condensate.
https://journals.aps.org/pra/abstract/10.1103/PhysRevA.88.053632
10.1103/PhysRevA.88.053632
SteffenPatrick Rath
RichardSchmidt
article
rath2013
Nonlocal order in Mott insulators, Duality and Wilson Loops
Annals of Physics (N.Y.)
2013
7
Annals
334,256(2013)
It is shown that the Mott insulating and superfluid phases of bosons in an optical lattice may be distinguished by a nonlocal 'parity order parameter' which is directly accessible via single site resolution imaging. In one dimension, the lattice Bose model is dual to a classical interface roughening problem. We use known exact results from the latter to prove that the parity order parameter exhibits long range order in the Mott insulating phase, consistent with recent experiments by Endres et al. [Science 334, 200 (2011)]. In two spatial dimensions, the parity order parameter can be expressed in terms of an equal time Wilson loop of a nontrivial U(1) gauge theory in 2+1 dimensions which exhibits a transition between a Coulomb and a confining phase. The negative logarithm of the parity order parameter obeys a perimeter law in the Mott insulator and is enhanced by a logarithmic factor in the superfluid.
https://www.sciencedirect.com/science/article/pii/S0003491613000717?via%3Dihub
10.1016/j.aop.2013.04.006
SteffenPatrick Rath
WolfgangSimeth
ManuelEndres
WilhelmZwerger
article
handrek2012
On the Maximal Excess Charge of the ChandrasekharCoulomb Hamiltonian in Two Dimensions
Letters in Mathematical Physics
2013
3
3
103
8
843–849
We show that for the straightforward quantized relativistic Coulomb Hamiltonian of a twodimensional atom  or the corresponding magnetic quantum dot  the maximal number of electrons does not exceed twice the nuclear charge. It result is then generalized to the presence of external magnetic fields and atomic Hamiltonians. This is based on the positivity of \bx T(\bp) + T(\bp) \bx which  in two dimensions  is false for the nonrelativistic case T(\bp) = \bp^2, but is proven in this paper for T(\bp) = \bp, i.e., the ultrarelativistic kinetic energy.
https://link.springer.com/article/10.1007%2Fs1100501306185
10.1007/s1100501306185
MichaelHandrek
HeinzSiedentop
article
aizenman2011
Resonant delocalization for random Schrödinger operators on tree graphs
Journal of the European Mathematical Society (JEMS)
2013
11671222 (2013)
JournaloftheEuropeanMathematicalSociety(JEMS),15(4), 11671222(2013)
We analyse the spectral phase diagram of Schrödinger operators T +λ V on regular tree graphs, with T the graph adjacency operator and V a random potential given by iid random variables. The main result is a criterion for the emergence of absolutely continuous (ac) spectrum due to fluctuationenabled resonances between distant sites. Using it we prove that for unbounded random potentials ac spectrum appears at arbitrarily weak disorder (λ &lring; 1) in an energy regime which extends beyond the spectrum of T. Incorporating considerations of the Green function's large deviations we obtain an extension of the criterion which indicates that, under a yet unproven regularity condition of the large deviations' 'free energy function', the regime of pure ac spectrum is complementary to that of previously proven localization. For bounded potentials we disprove the existence at weak disorder of a mobility edge beyond which the spectrum is localized.
http://arxiv.org/abs/1104.0969
10.4171/JEMS/389
MichaelAizenman
SimoneWarzel
article
bauer2013
Microscopic origin of the 0.7anomaly in quantum point contacts
Nature
2013
501
7378
Quantum point contacts are narrow, onedimensional constrictions usually patterned in a twodimensional electron system, for example by applying voltages to local gates. The linear conductance of a point contact, when measured as function of its channel width, is quantized (1, 2, 3) in units of GQ = 2e2/h, where e is the electron charge and h is Planck’s constant. However, the conductance also has an unexpected shoulder at ~0.7GQ, known as the ‘0.7anomaly’ (4, 5, 6, 7, 8, 9, 10, 11, 12), whose origin is still subject to debate (11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21). Proposed theoretical explanations have invoked spontaneous spin polarization (4, 17), ferromagnetic spin coupling (19), the formation of a quasibound state leading to the Kondo effect (13, 14), Wigner crystallization (16, 20) and various treatments of inelastic scattering18, 21. However, explicit calculations that fully reproduce the various experimental observations in the regime of the 0.7anomaly, including the zerobias peak that typically accompanies it6, 9, 10, 11, are still lacking. Here we offer a detailed microscopic explanation for both the 0.7anomaly and the zerobias peak: their common origin is a smeared van Hove singularity in the local density of states at the bottom of the lowest onedimensional subband of the point contact, which causes an anomalous enhancement in the Hartree potential barrier, the magnetic spin susceptibility and the inelastic scattering rate. We find good qualitative agreement between theoretical calculations and experimental results on the dependence of the conductance on gate voltage, magnetic field, temperature, source–drain voltage (including the zerobias peak) and interaction strength. We also clarify how the lowenergy scale governing the 0.7anomaly depends on gate voltage and interactions. For low energies, we predict and observe Fermiliquid behaviour similar to that associated with the Kondo effect in quantum dots (22). At high energies, however, the similarities between the 0.7anomaly and the Kondo effect end.
10.1038/nature12421
F.Bauer
E.Heyder
Schubert
D.Borowsky
D.Taubert
B.Bruognolo
S.Schuh
W.Wegscheider
J.von Delft
S.Ludwig
article
chen2013
Positivity of p^aq^b+q^bp^a
2013
We show that J<sub>a,b,n</sub>:=\frac12(p^aq^b+q^bp^a) is positive, if n\geq b+a. (Here q is the multiplication by x and p:= \mathrm<prt>i</prt><sup>1</sup>\nabla.) Furthermore we show that it generalizes the generalized Hardy inequalities for the fractional Laplacians.
http://arxiv.org/abs/1301.1524
LiChen
HeinzSiedentop
article
dolde2013
High fidelity spin entanglement using optimal control
Nat. Commun.
2013
5,
3371
Precise control of quantum systems is of fundamental importance for quantum device engineering, such as is needed in the fields of quantum information processing, highresolution spectroscopy and quantum metrology. When scaling up the quantum registers in such devices, several challenges arise: individual addressing of qubits in a dense spectrum while suppressing crosstalk, creation of entanglement between distant nodes, and decoupling from unwanted interactions. The experimental implementation of optimal control is a prerequisite to meeting these challenges. Using engineered microwave pulses, we experimentally demonstrate optimal control of a prototype solid state spin qubit system comprising thirty six energy levels. The spin qubits are associated with proximal nitrogenvacancy (NV) centers in diamond. We demonstrate precise singleelectron spin qubit operations with an unprecedented fidelity F ≈ 0.99 in combination with highefficiency storage of electron spin states in a nuclear spin quantum memory. Matching singleelectron spin operations with spinecho techniques, we further realize highquality entangled states (F > 0.82) between two electron spins on demand. After exploiting optimal control, the fidelity is mostly limited by the coherence time and imperfect initialization. Errors from crosstalk in a crowded spectrum of 8 lines as well as detrimental effects from active dipolar couplings have been simultaneously eliminated to unprecedented extent. Finally, by entanglement swapping to nuclear spins, nuclear spin entanglement over a length scale of 25 nm is demonstrated. This experiment underlines the importance of optimal control for scalable room temperature spinbased quantum information devices.
10.1038/ncomms4371
FlorianDolde
VilleBergholm
YaWang
IngmarJakobi
SebastienPezzagna
JanMeijer
PhilippNeumann
T.SchulteHerbrueggen
JacobBiamonte
JörgWrachtrup
article
forster2013
Characterization of Qubit Dephasing by LandauZener Interferometry
Phys. Rev. Lett.
2013
112,
116803
Controlling coherent interaction at avoided crossings is at the heart of quantum information processing. The regime between sudden switches and adiabatic transitions is characterized by quantum superpositions that enable interference experiments. Here, we implement periodic passages at intermediate speed in a GaAsbased twoelectron charge qubit and observe LandauZenerStückelbergMajorana (LZSM) quantum interference of the resulting superposition state. We demonstrate that LZSM interferometry is a viable and very general tool to not only study qubit properties but beyond to decipher decoherence caused by complex environmental influences. Our scheme is based on straightforward steady state experiments. The coherence time of our twoelectron charge qubit is limited by electronphonon interaction. It is much longer than previously reported for similar structures.
http://arxiv.org/abs/1309.5907
10.1103/PhysRevLett.112.116803
F.Forster
G.Petersen
S.Manus
P.Hänggi
D.Schuh
W.Wegscheider
S.Kohler
S.Ludwig
article
kendirlik2013
Anomalous resistance overshoot in the integer quantum Hall effect
Nature Sci. Rep
2013
3
3133
In this work we report experiments on defined by shallow etching narrow Hall bars. The magnetotransport properties of intermediate mobility twodimensional electron systems are investigated and analyzed within the screening theory of the integer quantized Hall effect. We observe a nonmonotonic increase of Hall resistance at the low magnetic field ends of the quantized plateaus, known as the overshoot effect. Unexpectedly, for Hall bars that are defined by shallow chemical etching the overshoot effect becomes more pronounced at elevated temperatures. We observe the overshoot effect at odd and even integer plateaus, which favor a spin independent explanation, in contrast to discussion in the literature. In a second set of the experiments, we investigate the overshoot effect in gate defined Hall bar and explicitly show that the amplitude of the overshoot effect can be directly controlled by gate voltages. We offer a comprehensive explanation based on scattering between evanescent incompressible channels.
10.1038/srep03133
E.M. Kendirlik
S.Sirt
S.B. Kalkan
W.Dietsche
W.Wegscheider
S.Ludwig
A.Siddiki
article
liebermeister2013
Tapered fiber coupling of single photons emitted by a deterministically positioned single nitrogen vacancy center
Appl. Phys. Lett.
2013
104,
031101
A diamond nanocrystal hosting a single nitrogen vacancy (NV) center is optically selected with a confocal scanning microscope and positioned deterministically onto the subwavelengthdiameter waist of a tapered optical fiber (TOF) with the help of an atomic force microscope. Based on this nanomanipulation technique we experimentally demonstrate the evanescent coupling of single fluorescence photons emitted by a single NVcenter to the guided mode of the TOF. By comparing photon count rates of the fiberguided and the freespace modes and with the help of numerical FDTD simulations we determine a lower and upper bound for the coupling efficiency of (9.5+/0.6)% and (10.4+/0.7)%, respectively. Our results are a promising starting point for future integration of single photon sources into photonic quantum networks and applications in quantum information science.
http://dx.doi.org/10.1063/1.4862207
LarsLiebermeister
FabianPetersen
Asmusv. Münchow
DanielBurchardt
JulianeHermelbracht
ToshiyukiTashima
AndreasW. Schell
OliverBenson
ThomasMeinhardt
AnkeKrueger
ArianeStiebeiner
ArnoRauschenbeutel
HaraldWeinfurter
MarkusWeber
article
PhysRevLett.110.127403
Confinement and Interaction of Single Indirect Excitons in a VoltageControlled Trap Formed Inside Double InGaAs Quantum Wells
Phys. Rev. Lett.
2013
110
127403
Voltagetunable quantum traps confining individual spatially indirect and longliving excitons are realized by providing a coupled double quantum well with nanoscale gates. This enables us to study the transition from confined multiexcitons down to a single, electrostatically trapped indirect exciton. In the few exciton regime, we observe discrete emission lines identified as resulting from a single dipolar exciton, a biexciton, and a triexciton, respectively. Their energetic splitting is well described by Wignerlike molecular structures reflecting the interplay of dipolar interexcitonic repulsion and spatial quantization.
http://link.aps.org/doi/10.1103/PhysRevLett.110.127403
American Physical Society
10.1103/PhysRevLett.110.127403
G. J.Schinner
J.Repp
E.Schubert
A. K.Rai
D.Reuter
A. D.Wieck
A. O.Govorov
A. W.Holleitner
J. P.Kotthaus
article
HofmannGNBDH2013
Bright, longlived and coherent excitons in carbon nanotube quantum dots
Nat Nano
2013
8
7
502505
Carbon nanotubes exhibit a wealth of unique physical properties. By virtue of their exceptionally low mass and extreme stiffness they provide ultrahighquality mechanical resonances (1), promise long electron spin coherence times in a nuclearspin free lattice (2, 3) for quantum information processing and spintronics, and feature unprecedented tunability of optical transitions (4, 5) for optoelectronic applications (6). Excitons in semiconducting singlewalled carbon nanotubes (7, 8) could facilitate the upconversion of spin (9), mechanical (10) or hybrid spin–mechanical (11) degrees of freedom to optical frequencies for efficient manipulation and detection. However, successful implementation of such schemes with carbon nanotubes has been impeded by rapid exciton decoherence at nonradiative quenching sites (12), environmental dephasing (13) and emission intermittence (14). Here we demonstrate that these limitations may be overcome by exciton localization in suspended carbon nanotubes. For excitons localized in nanotube quantum dots we found narrow optical lines free of spectral wandering, radiative exciton lifetimes (15, 16, 17) and effectively suppressed blinking. Our findings identify the great potential of localized excitons for efficient and spectrally precise interfacing of photons, phonons and spins in novel carbon nanotubebased quantum devices.
http://dx.doi.org/10.1038/nnano.2013.119
10.1038/nnano.2013.119
Matthias S.Hofmann
Jan T.Gluckert
JonathanNoe
ChristianBourjau
RaphaelDehmel
AlexanderHögele
article
bermejovega2012
Classical simulations of Abeliangroup normalizer circuits with intermediate measurements
2012
Quantum normalizer circuits were recently introduced as generalizations of Clifford circuits [arXiv:1201.4867]: a normalizer circuit over a finite Abelian group G is composed of the quantum Fourier transform (QFT) over G, together with gates which compute quadratic functions and automorphisms. In [arXiv:1201.4867] it was shown that every normalizer circuit can be simulated efficiently classically. This result provides a nontrivial example of a family of quantum circuits that cannot yield exponential speedups in spite of usage of the QFT, the latter being a central quantum algorithmic primitive. Here we extend the aforementioned result in several ways. Most importantly, we show that normalizer circuits supplemented with intermediate measurements can also be simulated efficiently classically, even when the computation proceeds adaptively. This yields a generalization of the GottesmanKnill theorem (valid for nqubit Clifford operations [quantph/9705052, quantph/9807006] to quantum circuits described by arbitrary finite Abelian groups. Moreover, our simulations are twofold: we present efficient classical algorithms to sample the measurement probability distribution of any adaptivenormalizer computation, as well as to compute the amplitudes of the state vector in every step of it. Finally we develop a generalization of the stabilizer formalism [quantph/9705052, quantph/9807006] relative to arbitrary finite Abelian groups: for example we characterize how to update stabilizers under generalized Pauli measurements and provide a normal form of the amplitudes of generalized stabilizer states using quadratic functions and subgroup cosets.
http://arxiv.org/abs/1210.3637
JuanBermejoVega
MaartenVan den Nest
article
barthel2012
Scaling of the thermal spectral function for quantum critical bosons in one dimension
2012
We present an improved scheme for the precise evaluation of finitetemperature response functions of strongly correlated systems in the framework of the timedependent density matrix renormalization group. The maximum times that we can reach at finite temperatures T are typically increased by a factor of two, when compared against the earlier approaches. This novel scheme, complemented with linear prediction, allows us now to evaluate dynamic correlators for interacting bosons in one dimension. We demonstrate that the considered spectral function in the quantum critical regime with dynamic critical exponent z=2 is captured by the universal scaling form S(k,omega)=(1/T)*Phi(k/sqrt(T),omega/T) and calculate the scaling function precisely.
ThomasBarthel
UlrichSchollw
SubirSachdev
article
Urbaszek2012
Nuclear spin physics in quantum dots: an optical investigation
Reviews of Modern Physics
2012
85,
79
The mesoscopic spin system formed by the 10E410E6 nuclear spins in a semiconductor quantum dot offers a unique setting for the study of manybody spin physics in the condensed matter. The dynamics of this system and its coupling to electron spins is fundamentally different from its bulk counterpart as well as that of atoms due to increased fluctuations that result from reduced dimensions. In recent years, the interest in studying quantum dot nuclear spin systems and their coupling to confined electron spins has been fueled by its direct implication for possible applications of such systems in quantum information processing as well as by the fascinating nonlinear (quantum)dynamics of the coupled electronnuclear spin system. In this article, we review experimental work performed over the last decades in studying this mesoscopic,coupled electronnuclear spin system and discuss how optical addressing of electron spins can be exploited to manipulate and readout quantum dot nuclei. We discuss how such techniques have been applied in quantum dots to efficiently establish a nonzero mean nuclear spin polarization and, most recently, were used to reduce fluctuations of the average quantum dot nuclear spin orientation. Both results in turn have important implications for the preservation of electron spin coherence in quantum dots, which we discuss. We conclude by speculating how this recently gained understanding of the quantum dot nuclear spin system could in the future enable experimental observation of quantummechanical signatures or possible collective behavior of mesoscopic nuclear spin ensembles.
http://arxiv.org/abs/1202.4637
http://dx.doi.org/10.1103/RevModPhys.85.79
BernhardUrbaszek
XavierMarie
ThierryAmand
OlivierKrebs
PaulVoisin
PatrickMaletinsky
AlexanderHögele
AtacImamoglu
article
dallAcqua2011
Real analyticity away from the nucleus of pseudorelativistic HartreeFock orbitals
Analysis & PDE
2011
5
,no.3,657691
We prove that the HartreeFock orbitals of pseudorelativistic atoms, that is, atoms where the kinetic energy of the electrons is given by the pseudorelativistic operator sqrt<prt>Delta+1</prt>1, are real analytic away from the origin. As a consequence, the quantum mechanical ground state of such atoms is never a HartreeFock state. Our proof is inspired by the classical proof of analyticity by nested balls of Morrey and Nirenberg. However, the technique has to be adapted to take care of the nonlocal pseudodifferential operator, the singularity of the potential at the origin, and the nonlinear terms in the equation.
http://arxivweb3.library.cornell.edu/abs/1103.5026
AnnaDall'Acqua
SørenFournais
ThomasØstergaard Sørensen
EdgardoStockmeyer
article
barthel2011
Quasilocality and efficient simulation of Markovian quantum dynamics
Phys. Rev. Lett.
2011
108,
230504
We consider open manybody systems governed by a timedependent quantum master equation with shortrange interactions. With a generalized LiebRobinson bound, we show that the evolution in this very generic framework is quasilocal, i.e., the evolution of observables can be approximated by implementing the dynamics only in a vicinity of the observables' support. The precision increases exponentially with the diameter of the considered subsystem. Hence, the timeevolution can be simulated on classical computers with a cost that is independent of the system size. Providing error bounds for Trotter decompositions, we conclude that the simulation on a quantum computer is additionally efficient in time. For experiments and simulations, our result can be used to rigorously bound finitesize effects.
ThomasBarthel
MartinKliesch
article
barthel2009
Spectral functions in onedimensional quantum systems at T>0
Phys. Rev. B
2009
79,
245101
We present for the first time timedependent densitymatrix renormalizationgroup simulations (tDMRG) at finite temperatures. It is demonstrated how a combination of finitetemperature tDMRG and timeseries prediction allows for an easy and very accurate calculation of spectral functions in onedimensional quantum systems, irrespective of their statistics, for arbitrary temperatures. This is illustrated with spin structure factors of XX and XXX spin1/2 chains. For the XX model we can compare against an exact solution and for the XXX model (Heisenberg antiferromagnet) against a Bethe Ansatz solution and quantum Monte Carlo data.
ThomasBarthel
UlrichSchollwöck
StevenR. White
article
bloch2007
ManyBody Physics with Ultracold Gases
Rev. Mod. Phys.
2007
80,
885
This article reviews recent experimental and theoretical progress on manybody phenomena in dilute, ultracold gases. Its focus are effects beyond standard weakcoupling descriptions, like the MottHubbardtransition in optical lattices, strongly interacting gases in one and two dimensions or lowest Landau level physics in quasi twodimensional gases in fast rotation. Strong correlations in fermionic gases are discussed in optical lattices or near Feshbach resonances in the BCSBEC crossover.
I.Bloch
J.Dalibard
R.Zwerger