Quantum sensing
Rev. Mod. Phys.
, Volume 89(3)
July
2017
Abstract: “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. 
MultipleQuantum Transitions and ChargeInduced Decoherence of Donor Nuclear Spins in Silicon
PRL
, Volume 118
June
2017
Abstract: 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. 
Rare region effects and dynamics near the manybody localization transition.
Annalen der Physik, Special issue on ManyBody Localization
January
2017
Abstract: 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. 
Floquet prethermalization and regimes of heating in a periodically driven, interacting quantum system.
Sci. Rep.
, Volume 7
2017
Abstract: 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. DOI: 10.1038/srep45382

Dynamical Cooper pairing in nonequilibrium electronphonon systems.
Phys. Rev. B
, Volume 94
December
2016
Abstract: 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. 
Finitetemperature scaling close to Isingnematic quantum critical points in twodimensional metals
Phys. Rev. B
, Volume 94(195113)
November
2016
Abstract: 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. 
Ultrafast manybody interferometry of impurities coupled to a Fermi sea
Science
, Volume 354(6308), page: 9699
October
2016
Abstract: 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. 
Adiabatic Quantum Search in Open Systems
Phys. Rev. Lett.
, Volume 117(150501)
October
2016
Abstract: 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. 
Regimes of heating and dynamical response in driven manybody localized systems
Phys. Rev. B
, Volume 94(094201)
September
2016
Abstract: 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. 
Spin structure factors of chiral quantum spin liquids on the kagome lattice
Phys. Rev. B
, Volume 94(104413)
September
2016
Abstract: 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. 
Coulomb potentials and Taylor expansions in timedependent densityfunctional theory
Phys. Rev. A
, Volume 92(062510)
June
2016
Abstract: 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. 
Ubiquity of Exciton Localization in Cryogenic Carbon Nanotubes
Nano Lett.
, Volume 16(5), page: 2958–2962
April
2016
Abstract: 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. 
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