Secure quantum remote state preparation of squeezed microwave states
NatureCommunications
, Volume 10(2604)
June
2019
Abstract: 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. DOI: 10.1063/1.5052414

Avoided quasiparticle decay from strong quantum interactions
Nature Physics
May
2019
Abstract: 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. 
Classifying snapshots of the doped Hubbard model with machine learning
Nature Physics
January
2019
Abstract: 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. 
Dynamical Quantum Phase Transitions in Spin Chains with LongRange Interactions: Merging Different Concepts of Nonequilibrium Criticality
Physical Review Letters
, Volume 120, page: 130601
March
2018
Abstract: 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. 
Almost conserved operators in nearly manybody localized systems
Physical Review B
, Volume 97, page: 094206
March
2018
Abstract: 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. 
Angleresolved photoemission spectroscopy with quantum gas microscopes
Physical Review B
, Volume 97(12), page: 125117
March
2018
Abstract: 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. 
Ultrafast quantum control of ionization dynamics in krypton
Nature Communications
, Volume 9(719)
February
2018
Abstract: 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. 
PhotonMediated Quantum Gate between Two Neutral Atoms in an Optical Cavity
Physics Review X
, Volume 8, page: 011018
February
2018
Abstract: 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. 
Spin Hall photoconductance in a threedimensional topological insulator at room temperature
Nature Communications
, Volume 9(331)
January
2018
Abstract: 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. 
Universal manybody response of heavy impurities coupled to a Fermi sea: a review of recent progress
Document number: 2
January
2018
Abstract: 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. 
Exploring 4D quantum Hall physics with a 2D topological charge pump
Nature
, Volume 553, page: 5558
January
2018
Abstract: 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. DOI: 10.1038/nature25000

Sizedriven quantum phase transitions
Proceedings of the National Academy of Sciences
, Volume 115(1), page: 1923
January
2018
Abstract: 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. 
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