Quantum sensing of weak radiofrequency signals by pulsed Mollow absorption spectroscopy
Nat. Commun.
, Volume 8, page: 964
October
2017
Abstract: 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. 
Noiseinduced subdiffusion in strongly localized quantum systems
Phys. Rev. Lett.
, Volume 119, page: 046601
July
2017
Abstract: 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. 
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. 
Theory of parametrically amplified electronphonon superconductivity
Phys. Rev. B
, Volume 96, page: 014512
July
2017
Abstract: 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. Note: Editors' Suggestion 
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. 
Scrambling and thermalization in a diffusive quantum manybody system
Abstract: 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. 
Bloch oscillations in the absence of a lattice.
Science
, Volume 356, page: 945
June
2017
Abstract: 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. 
Floquet prethermalization and regimes of heating in a periodically driven, interacting quantum system.
Sci. Rep.
, Volume 7, page: 45382
April
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

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. 
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. 
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