Showing papers by "Eugene Demler published in 2013"
TL;DR: In this article, the topological properties of Bloch bands in one-dimensional optical lattices were investigated using Bloch oscillations and Ramsey interferometry, and the Zak phase obtained by cold atoms moving across the Brillouin zone was extracted.
Abstract: Geometric phases that characterize the topological properties of Bloch bands play a fundamental role in the band theory of solids. Here we report on the measurement of the geometric phase acquired by cold atoms moving in one-dimensional optical lattices. Using a combination of Bloch oscillations and Ramsey interferometry, we extract the Zak phase—the Berry phase gained during the adiabatic motion of a particle across the Brillouin zone—which can be viewed as an invariant characterizing the topological properties of the band. For a dimerized lattice, which models polyacetylene, we measure a difference of the Zak phase’ Zak D 0:97(2) for the two possible polyacetylene phases with different dimerization. The two dimerized phases therefore belong to different topological classes, such that for a filled band, domain walls have fractional quantum numbers. Our work establishes a new general approach for probing the topological structure of Bloch bands in optical lattices.
881 citations
TL;DR: The existence of a collective atomic dark state, decoupled from the radiation field, is demonstrated and it is shown that such dark states can be deterministically prepared via dissipative means, thus turning dissipation into a resource for entanglement.
Abstract: We present and analyze a new approach for the generation of atomic spin-squeezed states. Our method involves the collective coupling of an atomic ensemble to a decaying mode of an open optical cavity. We demonstrate the existence of a collective atomic dark state, decoupled from the radiation field. By explicitly constructing this state we find that it can feature spin squeezing bounded only by the Heisenberg limit. We show that such dark states can be deterministically prepared via dissipative means, thus turning dissipation into a resource for entanglement. The scaling of the phase sensitivity taking realistic imperfections into account is discussed.
121 citations
TL;DR: In this paper, a real-space renormalization group method for excited state (RSRG-X) was developed to characterize a finite-temperature transition between two localized phases, characterized by nonanalyticities of the dynamic spin correlation function and the low frequency heat conductivity.
Abstract: We consider a new class of unconventional critical phenomena that is characterized by singularities only in dynamical quantities and has no thermodynamic signatures. A possible example is the recently proposed many-body localization transition, in which transport coefficients vanish at a critical temperature. Describing this unconventional quantum criticality has been technically challenging as understanding the finite-temperature dynamics requires the knowledge of a large number of many-body eigenstates. Here we develop a real-space renormalization group method for excited state (RSRG-X), that allow us to overcome this challenge, and establish the existence and universal properties of such temperature-tuned dynamical phase transitions. We characterize a specific example: the 1D disordered transverse field Ising model with interactions. Using RSRG-X, we find a finite-temperature transition, between two localized phases, characterized by non-analyticities of the dynamic spin correlation function and the low frequency heat conductivity.
116 citations
TL;DR: In this article, the authors proposed to use Ramsey interferometry and single-site addressability to measure real-space and time-resolved spin correlation functions, which directly probe the excitations of the system, and contain valuable information about phase transitions where they exhibit scale invariance.
Abstract: We propose to use Ramsey interferometry and single-site addressability, available in synthetic matter such as cold atoms or trapped ions, to measure real-space and time-resolved spin correlation functions. These correlation functions directly probe the excitations of the system, which makes it possible to characterize the underlying many-body states. Moreover, they contain valuable information about phase transitions where they exhibit scale invariance. We also discuss experimental imperfections and show that a spin-echo protocol can be used to cancel slow fluctuations in the magnetic field. We explicitly consider examples of the two-dimensional, antiferromagnetic Heisenberg model and the one-dimensional, long-range transverse field Ising model to illustrate the technique.
102 citations
TL;DR: This Letter proposes an interferometric method to use a combination of Ramsey interference and Bloch oscillations to measure Zak phases, i.e., Berry's phases for closed trajectories corresponding to reciprocal lattice vectors.
Abstract: Ludwig-Maximilians-Universita ¨t, 80799 Munchen, Germany(Received 8 December 2012; published 18 April 2013)Recently, optical lattices with nonzero Berry’s phases of Bloch bands have been realized. Newapproaches for measuring Berry’s phases and topological properties of bands with experimental toolsappropriate forultracold atoms need to be developed. In this Letter, we propose an interferometric methodfor measuring Berry’s phases of two-dimensional Bloch bands. The key idea is to use a combination ofRamsey interference and Bloch oscillations to measure Zak phases, i.e., Berry’s phases for closedtrajectories corresponding to reciprocal lattice vectors. We demonstrate that this technique can be usedto measure the Berry curvature of Bloch bands, the Berry’s phase of Dirac points, and the first Chernnumber of topological bands. We discuss several experimentally feasible realizations of this technique,which make it robust against low-frequency magnetic noise.
102 citations
TL;DR: In this article, the authors study the dynamics of a rapidly and coherently split one-dimensional Bose gas and show that the system dephases to a prethermalized state rather than undergoing thermalization toward a final thermal equilibrium state.
Abstract: We detail the experimental observation of the non-equilibrium many-body phenomenon prethermalization. We study the dynamics of a rapidly and coherently split one-dimensional Bose gas. An analysis based on the use of full quantum mechanical probability distributions of matter wave interference contrast reveals that the system evolves toward a quasi-steady state. This state, which can be characterized by an effective temperature, is not the final thermal equilibrium state. We compare the evolution of the system to an integrable Tomonaga-Luttinger liquid model, and show that the system dephases to a prethermalized state rather than undergoing thermalization toward afinal thermal equilibrium state.
90 citations
TL;DR: This work considers a quantum quench in which two initially independent condensates are suddenly coupled and study the subsequent "rephasing" dynamics, and compute universal time-dependent expectation values, allowing a quantitative comparison with future experiments.
Abstract: We consider a quantum quench in which two initially independent condensates are suddenly coupled and study the subsequent ``rephasing'' dynamics. For weak tunneling couplings, the time evolution of physical observables is predicted to follow universal scaling laws, connecting the short-time dynamics to the long-time nonperturbative regime. We first present a two-mode model valid in two and three dimensions and then move to one dimension, where the problem is described by a gapped sine-Gordon theory. Combining analytical and numerical methods, we compute universal time-dependent expectation values, allowing a quantitative comparison with future experiments.
73 citations
TL;DR: In this article, it was shown that spin-orbit coupled superconductors can be characterized as having subgap states that are bound to localized nonmagnetic impurities, referred to as the Shiba states.
Abstract: Spin-orbit coupled superconductors are interesting candidates for realizing topological and potentially non-Abelian states with Majorana fermions. We argue that time-reversal broken spin-orbit coupled superconductors generically can be characterized as having subgap states that are bound to localized nonmagnetic impurities. Such bound states, which are referred to as the Shiba states, can be detected as sharp resonances in the tunneling spectrum of the spin-orbit coupled superconductors. The Shiba state resonance can be tuned using a gate voltage or a magnetic field from being at the edge of the gap at zero magnetic fields to crossing zero energy when the Zeeman splitting is tuned into the topological superconducting regime. The zero-crossing signifies a fermion parity changing first-order quantum phase transition, which is characterized by a Pfaffian topological invariant. These zero crossings of the impurity level can be used to locally characterize the topological superconducting state in topological nanowires from tunneling experiments.
71 citations
TL;DR: The nonequilibrium dynamics of a coherently split one-dimensional Bose gas is studied by measuring the full probability distribution functions of matter-wave interference and revealing two distinct length-scale-dependent regimes of relaxation.
Abstract: We study the nonequilibrium dynamics of a coherently split one-dimensional Bose gas by measuring the full probability distribution functions of matter-wave interference. Observing the system on different length scales allows us to probe the dynamics of excitations on different energy scales, revealing two distinct length-scale-dependent regimes of relaxation. We measure the crossover length scale separating these two regimes and identify it with the prethermalized phase-correlation length of the system. Our approach enables a direct observation of the multimode dynamics characterizing one-dimensional quantum systems.
68 citations
TL;DR: This work demonstrates how the coupling obtained using the Majorana qubit-controlled 4π Josephson effect to flux bias the fluxonium qubit can be used to sensitively identify topological superconductivity, to measure the state of the MajorANA qubit, to construct 2-qubit operations, and to implement quantum memories with topological protection.
Abstract: We propose to use an ancilla fluxonium qubit to interact with a Majorana qubit hosted by a topological one-dimensional wire. The coupling is obtained using the Majorana qubit-controlled 4π Josephson effect to flux bias the fluxonium qubit. We demonstrate how this coupling can be used to sensitively identify topological superconductivity, to measure the state of the Majorana qubit, to construct 2-qubit operations, and to implement quantum memories with topological protection.
60 citations
TL;DR: It is shown that the degenerate dispersion minimum due to the spin-orbit coupling, combined with the long-range dipolar interaction, can stabilize a number of quantum crystalline and quasicrystalline ground states.
Abstract: We study quasi-two-dimensional dipolar Bose gases in which the bosons experience a Rashba spin-orbit coupling. We show that the degenerate dispersion minimum due to the spin-orbit coupling, combined with the long-range dipolar interaction, can stabilize a number of quantum crystalline and quasicrystalline ground states. Coupling the bosons to a fermionic species can further stabilize these phases. We estimate that the crystalline and quasicrystalline phases should be detectable in realistic dipolar condensates, e.g., dysprosium, and discuss their symmetries and excitations.
TL;DR: In this paper, a novel realization of Kondo physics with ultracold atomic gases is proposed, based on a Fermi sea of two different hyperfine states of one atom species forming bound states with a different species, which is spatially confined in a trapping potential.
Abstract: We propose a novel realization of Kondo physics with ultracold atomic gases. It is based on a Fermi sea of two different hyperfine states of one atom species forming bound states with a different species, which is spatially confined in a trapping potential. We show that different situations displaying Kondo physics can be realized when Feshbach resonances between the species are tuned by a magnetic field and the trapping frequency is varied. We illustrate that a mixture of 40K and 23Na atoms can be used to generate a Kondo-correlated state and that momentum resolved radio frequency spectroscopy can provide unambiguous signatures of the formation of Kondo resonances at the Fermi energy. We discuss how tools of atomic physics can be used to investigate open questions for Kondo physics, such as the extension of the Kondo screening cloud.
TL;DR: In this article, a model of polaron-dressed electronic TLSs was proposed to explain the elastic strain on TLS properties and the dephasing of TLSs in amorphous solids.
Abstract: While two-level systems (TLSs) are ubiqitous in solid state systems, microscopic understanding of their nature remains an outstanding problem. Conflicting phenomenological models are used to describe TLSs in seemingly similar materials when probed with different experimental techniques. Specifically, bulk measurements in amorphous solids have been interpreted using the model of a tunneling atom or group of atoms, whereas TLSs observed in the insulating barriers of Josephson junction qubits have been understood in terms of tunneling of individual electrons. Motivated by recent experiments studying TLSs in Josephson junctions, especially the effects of elastic strain on TLS properties, we analyze the interaction of the electronic TLS with phonons. We demonstrate that strong polaronic effects lead to dramatic changes in TLS properties. Our model gives a quantitative understanding of the TLS relaxation and dephasing as probed in Josephson junction qubits, while providing an alternative interpretation of bulk experiments. We demonstrate that a model of polaron dressed electronic TLS leads to estimates for the density and distribution of parameters of TLSs consistent with bulk experiments in amorphous solids. This model explains such surprising observations of recent experiments as the existence of minima in the energy of some TLSs as a function of strain and makes concrete predictions for the character of TLS dephasing near such minima. We argue that better understanding of the microscopic nature of TLSs can be used to improve properties of quantum devices, from an enhancement of relaxation time of TLSs to creating new types of strongly interacting optomechanical systems.
Journal Article•
TL;DR: In this paper, the topological properties of Bloch bands in one-dimensional optical lattices were investigated using Bloch oscillations and Ramsey interferometry, and the Zak phase obtained by cold atoms moving across the Brillouin zone was extracted.
Abstract: Geometric phases that characterize the topological properties of Bloch bands play a fundamental role in the band theory of solids. Here we report on the measurement of the geometric phase acquired by cold atoms moving in one-dimensional optical lattices. Using a combination of Bloch oscillations and Ramsey interferometry, we extract the Zak phase—the Berry phase gained during the adiabatic motion of a particle across the Brillouin zone—which can be viewed as an invariant characterizing the topological properties of the band. For a dimerized lattice, which models polyacetylene, we measure a difference of the Zak phase’ Zak D 0:97(2) for the two possible polyacetylene phases with different dimerization. The two dimerized phases therefore belong to different topological classes, such that for a filled band, domain walls have fractional quantum numbers. Our work establishes a new general approach for probing the topological structure of Bloch bands in optical lattices.
TL;DR: In this paper, the authors show that the optimal protocol for maximizing any figure of merit in a given time is bang-bang, i.e., the coupling alternates between only two values through a sequence of sudden quenches.
Abstract: Nonadiabatic unitary evolution with tailored time-dependent Hamiltonians can prepare systems of cold-atomic gases with various desired properties such as low excess energies. For a system of two one-dimensional quasicondensates coupled with a time-varying tunneling amplitude, we show that the optimal protocol, for maximizing any figure of merit in a given time, is bang-bang, i.e., the coupling alternates between only two values through a sequence of sudden quenches. Minimizing the energy of one of the quasicondensates with such a nonadiabatic protocol, and then decoupling it at the end of the process, can result in effective cooling beyond the current state of the art. Our cooling method can be potentially applied to arbitrary systems through an integration of the experiment with simulated annealing computations.
TL;DR: In this paper, a minimal phenomenological model was developed to quantitatively describe scanning tunneling microscopy (STM) and resonant elastic X-ray scattering (REXS) experiments.
Abstract: One of the key challenges in the field of high-temperature superconductivity is understanding the nature of fermionic quasiparticles. Experiments consistently demonstrate the existence of a second energy scale, distinct from the d-wave superconducting gap, that persists above the transition temperature into the "pseudogap" phase. One common class of models relates this energy scale to the quasiparticle gap due to a competing order, such as the incommensurate "checkerboard" order observed in scanning tunneling microscopy (STM) and resonant elastic X-ray scattering (REXS). In this paper we show that these experiments are better described by identifying the second energy scale with the inverse lifetime of quasiparticles. We develop a minimal phenomenological model that allows us to quantitatively describe STM and REXS experiments and discuss their relation with photoemission spectroscopy. Our study refocuses questions about the nature of the pseudogap phase to the study of the origin of inelastic scattering.
TL;DR: The functional determinant method builds upon earlier theoretical work in mesoscopic physics and accounts for excitonic effects as well as the orthogonality catastrophe arising from interaction between the core hole and the valence band electrons.
Abstract: We present a microscopic theory of resonant soft-x-ray scattering that accounts for the delocalized character of valence electrons. Unlike past approaches based on local form factors, our functional determinant method treats realistic band structures. This method builds upon earlier theoretical work in mesoscopic physics and accounts for excitonic effects as well as the orthogonality catastrophe arising from interaction between the core hole and the valence band electrons. We show that the two-peak structure observed near the O K edge of stripe-ordered La1.875Ba0.125CuO4 is due to dynamical nesting within the canonical cuprate band structure. Our results provide evidence for reasonably well-defined, high-energy quasiparticles in cuprates and establish resonant soft-x-ray scattering as a bulk-sensitive probe of the electron quasiparticles.
TL;DR: The dynamics and the steady state of a driven quantum spin coupled to a bath of fermions, which can be realized with a strongly imbalanced mixture of ultracold atoms, is explored using currently available experimental tools.
Abstract: We explore the dynamics and the steady state of a driven quantum spin coupled to a bath of fermions, which can be realized with a strongly imbalanced mixture of ultracold atoms using currently available experimental tools. Radio-frequency driving can be used to induce tunneling between the spin states. The Rabi oscillations are modified due to the coupling of the quantum spin to the environment, which causes frequency renormalization and damping. The spin-bath coupling can be widely tuned by adjusting the scattering length through a Feshbach resonance. When the scattering potential creates a bound state, by tuning the driving frequency it is possible to populate either the ground state, in which the bound state is filled, or a metastable state in which the bound state is empty. In the latter case, we predict an emergent inversion of the steady-state magnetization. Our work shows that different regimes of dissipative dynamics can be explored with a quantum spin coupled to a bath of ultracold fermions.
TL;DR: In this paper, a meta-analysis of the available Quantum Monte Carlo (QMC) results is presented, showing that two-dimensional fermions with repulsive interactions exhibit universal behavior in strongly correlated regime, and their freezing transition can be described using a quantum generalization of the classical Hansen-Verlet freezing criterion.
Abstract: We show by a meta-analysis of the available Quantum Monte Carlo (QMC) results that two-dimensional fermions with repulsive interactions exhibit universal behavior in the strongly correlated regime, and that their freezing transition can be described using a quantum generalization of the classical Hansen-Verlet freezing criterion. We calculate the liquid-state energy and the freezing point of the 2D dipolar Fermi gas (2DDFG) using a variational method by taking ground-state wave functions of 2D electron gas (2DEG) as trial states. A comparison with the recent fixed-node diffusion Monte Carlo analysis of the 2DDFG shows that our simple variational technique captures more than of the correlation energy, and predicts the freezing transition within the uncertainty bounds of QMC. Finally, we utilize the ground-state wave functions of 2DDFG as trial states and provide a variational account of the effects of finite 2D confinement width. Our results indicate significant beyond mean-field effects. We calculate the frequency of collective monopole oscillations of the quasi-2D dipolar gas as an experimental demonstration of correlation effects.
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Journal Article•
TL;DR: Yao et al. as discussed by the authors proposed a method to solve the problem of quantum information and showed that quantum information can be expressed as a set of equations, where each equation can be represented as a graph.
Abstract: N. Y. Yao1†∗, A. V. Gorshkov2†, C. R. Laumann1,3†, A. M. Lauchli, J. Ye, M. D. Lukin Physics Department, Harvard University, Cambridge, MA 02138, U.S.A. Institute for Quantum Information and Matter, California Institute of Technology, Pasadena, CA 91125, U.S.A. ITAMP, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, U.S.A. Institute for Theoretical Physics, University of Innsbruck, A-6020 Innsbruck, Austria JILA, National Institute of Standards and Technology and University of Colorado, Department of Physics, University of Colorado, Boulder, Colorado 80309, USA †These authors contributed equally to this work and ∗e-mail: nyao@fas.harvard.edu
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Posted Content•
TL;DR: In this paper, the dynamics of an impurity of finite mass injected into a one-dimensional quantum liquid at zero temperature, either at finite velocity or at zero velocity with a force driving the impurity, were studied.
Abstract: We study the dynamics of an impurity of finite mass injected into a one-dimensional quantum liquid at zero temperature, either at finite velocity or at zero velocity with a force driving the impurity. We obtain accurate results using numerical simulations based on matrix product states, and find that in both cases, the impurity undergoes oscillations, however the physical mechanism is different: the driven impurity undergoes Bloch oscillations by following the ground state branch while continuously emitting phonons, whereas the undriven impurity undergoes coherent quantum oscillations at an emergent energy scale, called quantum flutter in previous work, whose amplitude grows with increasing initial velocity. We find these results to be independent of whether the system is integrable or not, and robust to changes in the microscopics of the model, suggesting that they are universal.
Journal Article•
TL;DR: A.A. Lauchli, E. M. Laumann, 5 E. R. Stoudenmire, K. A. Hazzard, S. D. Manmana, 4 C. V. Demler, P. Zoller, J. Y. Ye, M. D Lukin, and A.
Abstract: A. V. Gorshkov, N. Y. Yao, S. R. Manmana, 4 C. R. Laumann, 5 E. M. Stoudenmire, K. R. A. Hazzard, S. D. Bennett, A. M. Lauchli, E. Demler, P. Zoller, J. Ye, M. D. Lukin, and A. M. Rey IQIM, California Institute of Technology, Pasadena, CA 91125, USA Physics Department, Harvard University, Cambridge, MA 02138, USA Institute for Theoretical Physics, University of Gottingen, D-37077 Gottingen, Germany JILA, NIST & Department of Physics, University of Colorado, Boulder, CO 80309, USA ITAMP, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA Institut fur Theoretische Physik, Universitat Innsbruck, A-6020 Innsbruck, Austria IQOQI of the Austrian Academy of Sciences, A-6020 Innsbruck, Austria
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TL;DR: In this article, a microscopic theory of resonant soft-x-ray scattering that accounts for the delocalized character of valence electrons is presented. But unlike previous approaches based on local form factors, their functional determinant method treats realistic band structures.
Abstract: We present a microscopic theory of resonant soft-x-ray scattering that accounts for the delocalized character of valence electrons. Unlike past approaches based on local form factors, our functional determinant method treats realistic band structures. This method builds upon earlier theoretical work in mesoscopic physics and accounts for excitonic effects as well as the orthogonality catastrophe arising from interaction between the core hole and the valence band electrons. We show that the two-peak structure observed near the O K edge of stripe-ordered La1.875Ba0.125CuO4 is due to dynamical nesting within the canonical cuprate band structure. Our results provide evidence for reasonably well-defined, high-energy quasiparticles in cuprates and establish resonant soft-x-ray scattering as a bulk-sensitive probe of the electron quasiparticles.
Posted Content•
TL;DR: In this paper, a scheme for realizing quantum quasicrystals using spin-orbit coupled dipolar bosons was proposed, in which the Gapless modes are enumerated and whether the additional modes are ''phasons'' is a matter of definition.
Abstract: In a recent Letter [Phys. Rev. Lett. 111, 185304 (2013)], we proposed a scheme for realizing quantum quasicrystals using spin-orbit coupled dipolar bosons. We remarked that these quantum quasicrystals have additional ``phason''-like modes compared with their classical counterparts. A recent comment by Lifshitz [arXiv:1312.1388] contests this claim. We argue here that our enumeration of gapless modes is indeed the physically relevant one; whether the additional modes are ``phasons'' is, however, a matter of definition.
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14 Mar 2013
TL;DR: In this article, the authors investigate the motion of an impurity particle injected with finite velocity into an interacting one-dimensional quantum gas and observe and quantitatively analyze long-lived oscillations of the impurity momentum around a non-zero saturation value, called quantum flutter.
Abstract: We investigate the motion of an impurity particle injected with finite velocity into an interacting one-dimensional quantum gas. Using large-scale numerical simulations based on matrix product states, we observe and quantitatively analyze long-lived oscillations of the impurity momentum around a non-zero saturation value, called quantum flutter. We show that the quantum flutter frequency is equal to the energy difference between two branches of collective excitations of the model. We propose an explanation of the finite saturation momentum of the impurity based on the properties of the edge of the excitation spectrum. Our results indicate that quantum flutter exists away from integrability, and provide parameter regions in which it could be observed in experiments with ultracold atoms using currently available technology.
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TL;DR: In this paper, a formalism for calculating resonant inelastic x-ray scattering (RIXS) spectra in systems of itinerant electrons with arbitrary band structures was developed, accounting for the effect of the positively-charged core hole exactly.
Abstract: We develop a formalism for calculating resonant inelastic x-ray scattering (RIXS) spectra in systems of itinerant electrons with arbitrary band structures, accounting for the effect of the positively-charged core hole exactly. We apply this formalism to the cuprate superconductors and obtain quantitative agreement with experimental data over a wide range of dopings. We reproduce the dispersing peaks and non-trivial polarization dependence found in RIXS experiments on several materials. Thus we demonstrate that features previously attributed to collective magnetic modes can be explained by band structure alone.