Showing papers by "Eugene Demler published in 2010"
TL;DR: In this paper, the authors show that the Floquet operators of periodically driven systems can be divided into topologically distinct (homotopy) classes and give a simple physical interpretation of this classification in terms of the spectra ofFloquet operators.
Abstract: Topological properties of physical systems can lead to robust behaviors that are insensitive to microscopic details. Such topologically robust phenomena are not limited to static systems but can also appear in driven quantum systems. In this paper, we show that the Floquet operators of periodically driven systems can be divided into topologically distinct (homotopy) classes and give a simple physical interpretation of this classification in terms of the spectra of Floquet operators. Using this picture, we provide an intuitive understanding of the well-known phenomenon of quantized adiabatic pumping. Systems whose Floquet operators belong to the trivial class simulate the dynamics generated by time-independent Hamiltonians, which can be topologically classified according to the schemes developed for static systems. We demonstrate these principles through an example of a periodically driven two-dimensional hexagonal lattice tight-binding model which exhibits several topological phases. Remarkably, one of these phases supports chiral edge modes even though the bulk is topologically trivial.
991 citations
TL;DR: In this paper, the decoupling of the nuclear spin from the electronic angular momentum is used to implement many-body systems with an unprecedented degree of symmetry, characterized by the SU(N)group withNaslargeas10.
Abstract: Fermionic alkaline-earth atoms have unique properties that make them attractive candidates for the realization of atomic clocks and degenerate quantum gases. At the same time, they are attracting considerable theoretical attention in the context of quantum information processing. Here we demonstrate that when such atoms are loaded in optical lattices, they can be used as quantum simulators of unique many-body phenomena. In particular, we show that the decoupling of the nuclear spin from the electronic angular momentum can be used to implement many-body systems with an unprecedented degree of symmetry, characterizedbytheSU(N)groupwithNaslargeas10.Moreover,theinterplayofthenuclearspinwiththeelectronicdegreeof freedomprovidedbyastableopticallyexcitedstateshouldenablethestudyofphysicsgovernedbythespin‐orbitalinteraction. Such systems may provide valuable insights into the physics of strongly correlated transition-metal oxides, heavy-fermion materials and spin-liquid phases.
570 citations
TL;DR: In this paper, the authors show that discrete-time quantum walks provide a versatile platform for studying topological phases, which are currently the subject of intense theoretical and experimental investigations, and demonstrate that quantum walks with cold atoms, photons, and ions simulate a nontrivial one-dimensional topological phase.
Abstract: The quantum walk was originally proposed as a quantum-mechanical analog of the classical random walk, and has since become a powerful tool in quantum information science. In this paper, we show that discrete-time quantum walks provide a versatile platform for studying topological phases, which are currently the subject of intense theoretical and experimental investigations. In particular, we demonstrate that recent experimental realizations of quantum walks with cold atoms, photons, and ions simulate a nontrivial one-dimensional topological phase. With simple modifications, the quantum walk can be engineered to realize all of the topological phases, which have been classified in one and two dimensions. We further discuss the existence of robust edge modes at phase boundaries, which provide experimental signatures for the nontrivial topological character of the system.
500 citations
TL;DR: It is shown that ordered states in extended ensembles of Rydberg atoms can be selectively excited by chirped laser pulses, and, via quantum state transfer from atoms to light, be used to create crystalline photonic states.
Abstract: We describe a method for controlling many-body states in extended ensembles of Rydberg atoms, forming crystalline structures during laser excitation of a frozen atomic gas. Specifically, we predict the existence of an excitation-number staircase in laser excitation of atomic ensembles into Rydberg states. It is shown that such ordered states can be selectively excited by chirped laser pulses, and, via quantum state transfer from atoms to light, be used to create crystalline photonic states.
239 citations
TL;DR: It is argued that the dominant mechanism for the relaxation is a simultaneous many-body process involving several single fermions as scattering partners, yielding fair agreement with the data.
Abstract: We investigate the decay of highly excited states of ultracold fermions in a three-dimensional optical lattice. Starting from a repulsive Fermi-Hubbard system near half filling, we generate additional doubly occupied sites (doublons) by lattice modulation. The subsequent relaxation back to thermal equilibrium is monitored over time. The measured absolute doublon lifetime covers 2 orders of magnitude. In units of the tunneling time h/J it is found to depend exponentially on the ratio of on-site interaction energy U to kinetic energy J. We argue that the dominant mechanism for the relaxation is a simultaneous many-body process involving several single fermions as scattering partners. A many-body calculation is carried out using diagrammatic methods, yielding fair agreement with the data.
211 citations
TL;DR: In this article, it was shown that 1/f noise does not preserve the long-range entanglement of quantum critical points in many-body systems, but does preserve the quantum correlations.
Abstract: Quantum critical points in many-body systems are characterized by the appearance of long-range entanglement. These subtle quantum correlations are known to be extremely fragile with respect to thermal noise. But theoretical work now shows that, unexpectedly, another classical disturbance, the ubiquitous 1/f noise, does preserve the critical correlations.
124 citations
TL;DR: In this paper, the authors studied the time evolution of antiferromagnetic order in the Heisenberg chain after a sudden change of the anisotropy parameter, using various numerical and analytical methods.
Abstract: Recent experimental achievements in controlling ultracold gases in optical lattices open a new perspective on quantum many-body physics. In these experimental setups, it is possible to study coherent time evolution of isolated quantum systems. These dynamics reveal new physics beyond the low-energy properties that are usually relevant in solid-state many-body systems. In this paper, we study the time evolution of antiferromagnetic order in the Heisenberg chain after a sudden change of the anisotropy parameter, using various numerical and analytical methods. As a generic result, we find that the order parameter, which can show oscillatory or non-oscillatory dynamics, decays exponentially except for the effectively non-interacting case of the XX limit. For weakly ordered initial states, we also find evidence for an algebraic correction to the exponential law. The study is based on numerical simulations using a numerical matrix product method for infinite system sizes (iMPS), for which we provide a detailed description and an error analysis. Additionally, we investigate in detail the exactly solvable XX limit. These results are compared to approximative analytical approaches including an effective description by the XZ model as well as by mean-field, Luttinger-liquid and sine-Gordon theories. The comparison reveals which aspects of non-equilibrium dynamics can, as in equilibrium, be described by low-energy theories and which are the novel phenomena specific to quantum quench dynamics. The relevance of the energetically high part of the spectrum is illustrated by means of a full numerical diagonalization of the Hamiltonian.
123 citations
TL;DR: In this paper, the authors measured the two-point density correlation function of freely expanding quasicondensates in the weakly interacting quasi-one-dimensional (1D) regime.
Abstract: We measure the two-point density correlation function of freely expanding quasicondensates in the weakly interacting quasi-one-dimensional (1D) regime. While initially suppressed in the trap, density fluctuations emerge gradually during expansion as a result of initial phase fluctuations present in the trapped quasicondensate. Asymptotically, they are governed by the thermal coherence length of the system. Our measurements take place in an intermediate regime where density correlations are related to near-field diffraction effects and anomalous correlations play an important role. Comparison with a recent theoretical approach described by Imambekov et al. yields good agreement with our experimental results and shows that density correlations can be used for thermometry of quasicondensates.
101 citations
TL;DR: In this paper, the decay of artificially created double occupancies in a repulsive Fermi-Hubbard system in the strongly interacting limit using diagrammatic many-body theory and experiments with ultracold fermions in optical lattices was investigated.
Abstract: We investigate the decay of artificially created double occupancies in a repulsive Fermi-Hubbard system in the strongly interacting limit using diagrammatic many-body theory and experiments with ultracold fermions in optical lattices. The lifetime of the doublons is found to scale exponentially with the ratio of the on-site repulsion to the bandwidth. We show that the dominant decay process in presence of background holes is the excitation of a large number of particle-hole pairs to absorb the energy of the doublon. We also show that the strongly interacting nature of the background state is crucial in obtaining the correct estimate of the doublon lifetime in these systems. The theoretical estimates and the experimental data are in agreement.
91 citations
TL;DR: In this article, a broad class of time-dependent interacting systems subject to external linear and parabolic potentials is discussed, for which the many-body Schrodinger equation can be solved using a scaling transformation.
Abstract: Understanding the non-equilibrium quantum dynamics of many-body systems is one of the most challenging problems in modern theoretical physics. While numerous approximate and exact solutions exist for systems in equilibrium, examples of non-equilibrium dynamics of many-body systems that allow reliable theoretical analysis are few and far between. In this paper, we discuss a broad class of time-dependent interacting systems subject to external linear and parabolic potentials, for which the many-body Schrodinger equation can be solved using a scaling transformation. We demonstrate that scaling solutions exist for both local and non-local interactions, and derive appropriate self-consistency equations. We apply this approach to several specific experimentally relevant examples of interacting bosons in one and two dimensions. As an intriguing result, we find that weakly and strongly interacting Bose gases expanding from a parabolic trap can exhibit very similar dynamics.
83 citations
TL;DR: In this article, the authors study the non-equilibrium dynamics of one-dimensional Bose gas from the general perspective of the dynamics of integrable systems and develop a numerical procedure which allows explicit summation over intermediate states and analysis of the time evolution of non-local density correlation functions.
Abstract: In this paper we study the non-equilibrium dynamics of one-dimensional Bose gas from the general perspective of the dynamics of integrable systems. After outlining and critically reviewing methods based on the inverse scattering transform, intertwining operators, q-deformed objects, and extended dynamical conformal symmetry, we focus on the form-factor based approach. Motivated by possible applications in nonlinear quantum optics and experiments with ultracold atoms, we concentrate on the regime of strong repulsive interactions. We consider dynamical evolution starting from two initial states: a condensate of particles in a state with zero momentum and a condensate of particles in a Gaussian wavepacket in real space. Combining the form-factor approach with the method of intertwining operators we develop a numerical procedure which allows explicit summation over intermediate states and analysis of the time evolution of non-local density–density correlation functions. In both cases we observe a tendency toward the formation of crystal-like correlations at intermediate timescales.
TL;DR: An isentropic effect in a spin mixture of attractively interacting fermionic atoms in an optical lattice is reported on, demonstrating the crucial role of the lattice potential in the thermodynamics of the fermionics Hubbard model.
Abstract: The interplay of thermodynamics and quantum correlations can give rise to counterintuitive phenomena in many-body systems. We report on an isentropic effect in a spin mixture of attractively interacting fermionic atoms in an optical lattice. As we adiabatically increase the attraction between the atoms, we observe that the gas expands instead of contracting. This unexpected behavior demonstrates the crucial role of the lattice potential in the thermodynamics of the fermionic Hubbard model.
TL;DR: It is argued that Ramsey interference experiments provide a powerful tool for analyzing strongly correlated nature of 1D interacting systems.
Abstract: We theoretically analyze Ramsey interference experiments in one-dimensional quasicondensates and obtain explicit expressions for the time evolution of full distribution functions of fringe contrast. We show that distribution functions contain unique signatures of the many-body mechanism of decoherence. We argue that Ramsey interference experiments provide a powerful tool for analyzing strongly correlated nature of 1D interacting systems.
TL;DR: An effective Ising-XY lattice model is constructed that describes the interplay between dimerization and superfluid phase fluctuations, and an unusual dimerized "pseudogap" state with only short-range phase coherence is found.
Abstract: We consider a layered system of fermionic molecules with permanent dipole moments aligned perpendicular to the layers by an external field. The dipole interactions between fermions in adjacent layers are attractive and induce interlayer pairing. Because of the competition for pairing among adjacent layers, the mean-field ground state of the layered system is a dimerized superfluid, with pairing only between every other layer. We construct an effective Ising-XY lattice model that describes the interplay between dimerization and superfluid phase fluctuations. In addition to the dimerized superfluid ground state, and high-temperature normal state, at intermediate temperature, we find an unusual dimerized "pseudogap" state with only short-range phase coherence. We propose light-scattering experiments to detect dimerization.
TL;DR: In this paper, a pedagogical review of electrical resistance in superconductors is presented, where the authors introduce the idea of the superconducting order parameter as a condensate wave function and introduce vortices as topological excitations with quantized phase winding.
Abstract: In this pedagogical review, we discuss how electrical resistance can arise in superconductors. Starting with the idea of the superconducting order parameter as a condensate wave function, we introduce vortices as topological excitations with quantized phase winding, and we show how phase slips occur when vortices cross the sample. Superconductors exhibit non-zero electrical resistance under circumstances where phase slips occur at a finite rate. For one-dimensional superconductors or Josephson junctions, phase slips can occur at isolated points in space-time. Phase slip rates may be controlled by thermal activation over a free-energy barrier, or in some circumstances, at low temperatures, by quantum tunneling through a barrier. We present an overview of several phenomena involving vortices that have direct implications for the electrical resistance of superconductors, including the Berezinskii-Kosterlitz-Thouless transition for vortex-proliferation in thin films, and the effects of vortex pinning in bulk type II superconductors on the nonlinear resistivity of these materials in an applied magnetic field. We discuss how quantum fluctuations can cause phase slips and review the non-trivial role of dissipation on such fluctuations. We present a basic picture of the superconductor-to-insulator quantum phase transitions in films, wires, and Josephson junctions. We point out related problems in superfluid helium films and systems of ultra-cold trapped atoms. While our emphasis is on theoretical concepts, we also briefly describe experimental results, and we underline some of the open questions.
TL;DR: In this paper, a technique for the preparation of low-entropy many-body states of atoms in optical lattices based on adiabatic passage was proposed, which allows preparation of strongly correlated states as stable highest energy states of Hamiltonians that have trivial ground states.
Abstract: We analyze a technique for the preparation of low-entropy many-body states of atoms in optical lattices based on adiabatic passage. In particular, we show that this method allows preparation of strongly correlated states as stable highest energy states of Hamiltonians that have trivial ground states. As an example, we analyze the generation of antiferromagnetically ordered states by adiabatic change of a staggered field acting on the spins of bosonic atoms with ferromagnetic interactions.
28 Jul 2010
TL;DR: In this article, a layered system of fermionic molecules with permanent dipole moments aligned perpendicular to the layers by an external field is considered, and an effective Ising-XY lattice model is constructed to describe the interplay between dimerization and superfluid phase fluctuations.
Abstract: We consider a layered system of fermionic molecules with permanent dipole moments aligned perpendicular to the layers by an external field. The dipole interactions between fermions in adjacent layers are attractive and induce interlayer pairing. Because of the competition for pairing among adjacent layers, the mean-field ground state of the layered system is a dimerized superfluid, with pairing only between every other layer. We construct an effective Ising-XY lattice model that describes the interplay between dimerization and superfluid phase fluctuations. In addition to the dimerized superfluid ground state, and high-temperature normal state, at intermediate temperature, we find an unusual dimerized ‘‘pseudogap’’ state with only short-range phase coherence. We propose light-scattering experiments to detect dimerization.
TL;DR: In this article, the phase is acquired by mapping two single photons into atomic excitations with fermionic character and exchanging their positions, while photon storage techniques provide the interface between the photons and the spin waves.
Abstract: We propose a new protocol for implementing the two-qubit photonic phase gate. In our approach, the � phase is acquired by mapping two single photons into atomic excitations with fermionic character and exchanging their positions. The fermionic excitations are realized as spin waves in a spin chain, while photon storage techniques provide the interface between the photons and the spin waves. Possible imperfections and experimental systems suitable for implementing the gate are discussed.
TL;DR: In this paper, a method for controllable preparation and detection of interaction-induced ferromagnetism in ultracold fermionic atoms loaded in optical superlattices is proposed.
Abstract: We propose a method for controllable preparation and detection of interaction-induced ferromagnetism in ultracold fermionic atoms loaded in optical superlattices. First, we discuss how to probe and control Nagaoka ferromagnetism in an array of isolated plaquettes (four lattice sites arranged in a square). Then, we allow for weak interplaquette tunneling. Since ferromagnetism is unstable in the presence of weak interplaquette couplings, we propose to mediate long-range ferromagnetic correlations via double-exchange processes by exciting atoms to the first vibrational band. We calculate the phase diagram of the two-band plaquette array and discuss conditions for the stability and robustness of the ferromagnetic phases in this system. Experimental implementation of the proposed schemes is discussed.
Journal Article•
TL;DR: Two schemes are proposed and discussed based on two-particle interference revealed in atom-atom or atomic density correlations that can be used for relative phase measurements for nontrivial particle-hole order parameters, such as d-density wave order.
TL;DR: In this paper, the repulsive fermionic Hubbard model on square and cubic lattices with spin imbalance and in the presence of a parabolic confinement was analyzed and the magnetic structure was analyzed as a function of repulsive interaction strength and polarization.
Abstract: We analyze the repulsive fermionic Hubbard model on square and cubic lattices with spin imbalance and in the presence of a parabolic confinement. We analyze the magnetic structure as a function of the repulsive interaction strength and polarization. In the first part of the article, we perform unrestricted Hartree-Fock calculations for the two-dimensional (2D) case and find that above a critical interaction strength ${U}_{c}$ the system turns ferromagnetic at the edge of the trap, which is in agreement with the ferromagnetic Stoner instability of a homogeneous system away from half-filling. For $Ul{U}_{c}$, we find a canted antiferromagnetic structure in the Mott region in the center and a partially polarized compressible edge. The antiferromagnetic order in the Mott plateau is perpendicular to the direction of the imbalance. In this regime, the same qualitative behavior is expected for 2D and three-dimensional (3D) systems. In the second part of the article, we give a general discussion of magnetic structures above ${U}_{c}$. We argue that spin conservation leads to nontrivial textures, both in the ferromagnetic polarization at the edge and for the N\'eel order in the Mott plateau. We discuss differences in magnetic structures for 2D and 3D cases.
TL;DR: A functional renormalization group is developed to treat the effects of disorder, and it is demonstrated that the disorder results in the smearing of the superfluid-normal phase transition via the formation of a Griffiths phase.
Abstract: We consider a stack of weakly Josephson coupled superfluid layers with c-axis disorder in the form of random superfluid stiffnesses and vortex fugacities in each layer as well as random interlayer coupling strengths. In the absence of disorder this system has a 3D XY type superfluid-normal phase transition as a function of temperature. We develop a functional renormalization group to treat the effects of disorder, and demonstrate that the disorder results in the smearing of the superfluid-normal phase transition via the formation of a Griffiths phase. Remarkably, in the Griffiths phase, the emergent power-law distribution of the interlayer couplings gives rise to a sliding Griffiths superfluid, with a finite stiffness in the a-b direction along the layers, and a vanishing stiffness perpendicular to it.
TL;DR: A balanced two-component system of ultracold fermions in one dimension with attractive interactions and subject to a spin-dependent optical lattice potential of opposite sign, which is intrinsically stable to phase separation.
Abstract: We study a balanced two-component system of ultracold fermions in one dimension with attractive interactions and subject to a spin-dependent optical lattice potential of opposite sign for the two components. We find states with different types of modulated pairing order parameters which are conceptually similar to $\ensuremath{\pi}$ phases discussed for superconductor-ferromagnet heterostructures. Increasing the lattice depth induces sharp transitions between states of different parity. While the origin of the order parameter oscillations is similar to the Fulde-Ferrel-Larkin-Ovchinnikov phase for paired states with spin imbalance, the current system is intrinsically stable to phase separation. We discuss experimental requirements for creating and probing these novel phases.
Posted Content•
19 May 2010
TL;DR: Schneider et al. as mentioned in this paper proposed Schneider's work in the field of particle physics and showed that Schneider's approach can be applied to the problem of particle beamforming in physics.
Abstract: Ulrich Schneider, 2, ∗ Lucia Hackermuller, Jens Philipp Ronzheimer, 2 Sebastian Will, 2 Simon Braun, 2 Thorsten Best, Immanuel Bloch, 2, 3 Eugene Demler, Stephan Mandt, David Rasch, and Achim Rosch Institut fur Physik, Johannes Gutenberg-Universitat, 55099 Mainz, Germany Fakultat fur Physik, Ludwig-Maximilians-Universitat, 80799 Munchen, Germany Max-Planck-Institut fur Quantenoptik, 85748 Garching, Germany Department of Physics, Harvard University, Cambridge, MA 02138, USA Institut fur Theoretische Physik, Universitat zu Koln, 50937 Cologne, Germany (Dated: May 21, 2010)
01 Mar 2010
TL;DR: In this paper, it was shown that 1/f noise does not preserve the long-range entanglement of quantum critical points in many-body systems, but does preserve the quantum correlations.
Abstract: Quantum critical points in many-body systems are characterized by the appearance of long-range entanglement. These subtle quantum correlations are known to be extremely fragile with respect to thermal noise. But theoretical work now shows that, unexpectedly, another classical disturbance, the ubiquitous 1/f noise, does preserve the critical correlations.
TL;DR: In this paper, the two-electron eigenspectrum of a carbon-nanotube double quantum dot with spin-orbit coupling was studied and exact calculations were combined with a simple model to provide an intuitive and accurate description of singleparticle and interaction effects.
Abstract: We study the two-electron eigenspectrum of a carbon-nanotube double quantum dot with spin-orbit coupling. Exact calculations are combined with a simple model to provide an intuitive and accurate description of single-particle and interaction effects. For symmetric dots and weak magnetic fields, the two-electron ground state is antisymmetric in the spin-valley degree of freedom and is not a pure spin-singlet state. When double occupation of one dot is favored by increasing the detuning between the dots, the Coulomb interaction causes strong correlation effects realized by higher orbital-level mixing. Changes in the double-dot configuration affect the relative strength of the electron-electron interactions and can lead to different ground-state transitions. In particular, they can favor a ferromagnetic ground state both in spin and valley degrees of freedom. The strong suppression of the energy gap can cause the disappearance of the Pauli blockade in transport experiments and thereby can also limit the stability of spin qubits in quantum information proposals. Our analysis is generalized to an array of coupled dots which is expected to exhibit rich many-body behavior.
TL;DR: In this article, a pedagogical review of electrical resistance in superconductors is presented, where the authors introduce the idea of the superconducting order parameter as a condensate wave function and introduce vortices as topological excitations with quantized phase winding.
Abstract: In this pedagogical review, we discuss how electrical resistance can arise in superconductors. Starting with the idea of the superconducting order parameter as a condensate wave function, we introduce vortices as topological excitations with quantized phase winding, and we show how phase slips occur when vortices cross the sample. Superconductors exhibit non-zero electrical resistance under circumstances where phase slips occur at a finite rate. For one-dimensional superconductors or Josephson junctions, phase slips can occur at isolated points in space-time. Phase slip rates may be controlled by thermal activation over a free-energy barrier, or in some circumstances, at low temperatures, by quantum tunneling through a barrier. We present an overview of several phenomena involving vortices that have direct implications for the electrical resistance of superconductors, including the Berezinskii-Kosterlitz-Thouless transition for vortex-proliferation in thin films, and the effects of vortex pinning in bulk type II superconductors on the non-linear resistivity of these materials in an applied magnetic field. We discuss how quantum fluctuations can cause phase slips and review the non-trivial role of dissipation on such fluctuations. We present a basic picture of the superconductor-to-insulator quantum phase transitions in films, wires, and Josephson junctions. We point out related problems in superfluid helium films and systems of ultra-cold trapped atoms. While our emphasis is on theoretical concepts, we also briefly describe experimental results, and we underline some of the open questions.
Journal Article•
TL;DR: In this article, the relative strength of the electron-electron interactions and can lead to dierent ground state transitions are investigated and shown to favor a ferromagnetic ground state both in spin and valley degrees of freedom.
Abstract: aect the relative strength of the electron-electron interactions and can lead to dierent ground state transitions. In particular, they can favor a ferromagnetic ground state both in spin and valley degrees of freedom. The strong suppression of the energy gap to a ferromagnetic state can cause the disappearance of the Pauli blockade in transport experiments and thereby also limit the stability of spin-qubits in double dots based quantum information proposal. Our analysis is generalized to an array of coupled dots which is expected to exhibit rich many-body behavior. PACS numbers: 73.63.Fg,73.23.-b,73.22.-f