Showing papers by "Eugene Demler published in 2008"
TL;DR: By dynamically modifying the potential bias between neighboring lattice sites, the magnitude and sign of the superexchange interaction can be controlled, thus allowing the system to be switched between antiferromagnetic and ferromagnetic spin interactions.
Abstract: Quantum mechanical superexchange interactions form the basis of quantum magnetism in strongly correlated electronic media. We report on the direct measurement of superexchange interactions with ultracold atoms in optical lattices. After preparing a spin-mixture of ultracold atoms in an antiferromagnetically ordered state, we measured coherent superexchange-mediated spin dynamics with coupling energies from 5 hertz up to 1 kilohertz. By dynamically modifying the potential bias between neighboring lattice sites, the magnitude and sign of the superexchange interaction can be controlled, thus allowing the system to be switched between antiferromagnetic and ferromagnetic spin interactions. We compare our findings to predictions of a two-site Bose-Hubbard model and find very good agreement, but are also able to identify corrections that can be explained by the inclusion of direct nearest-neighbor interactions.
573 citations
TL;DR: In this paper, a complete experimental analysis of the shot-to-shot variations of interference-fringe contrast for pairs of independently created one-dimensional Bose condensates is provided.
Abstract: The probabilistic character of the measurement process is one of the most puzzling and fascinating aspects of quantum mechanics. In many-body systems quantum-mechanical noise reveals non-local correlations of the underlying many-body states. Here, we provide a complete experimental analysis of the shot-to-shot variations of interference-fringe contrast for pairs of independently created one-dimensional Bose condensates. Analysing different system sizes, we observe the crossover from thermal to quantum noise, reflected in a characteristic change in the distribution functions from poissonian to Gumbel type, in excellent agreement with theoretical predictions on the basis of the Luttinger-liquid formalism. We present the first experimental observation of quasi-long-range order in one-dimensional atomic condensates, which is a hallmark of quantum fluctuations in one-dimensional systems. Furthermore, our experiments constitute the first analysis of the full distribution of quantum noise in an interacting many-body system. The analysis of the interference fringes generated by initially independent one-dimensional Bose condensates reveals contributions of both quantum noise and thermal noise, advancing our fundamental understanding of quantum states in interacting many-body systems.
233 citations
TL;DR: In this article, the authors describe a technique that enables the creation of a strongly correlated quantum gas of photons using one-dimensional optical systems with tight field confinement and coherent photon trapping techniques, which enables the generation of large, tunable optical nonlinearities via the interaction of photons with a nearby cold atomic gas.
Abstract: Understanding strongly correlated quantum systems is a central problem in many areas of physics. The collective behaviour of interacting particles gives rise to diverse fundamental phenomena such as confinement in quantum chromodynamics, electron fractionalization in the quantum Hall regime and phase transitions in unconventional superconductors and quantum magnets. Such systems typically involve massive particles, but optical photons can also interact with one another in a nonlinear medium. In practice, however, such interactions are often very weak. Here we describe a technique that enables the creation of a strongly correlated quantum gas of photons using one-dimensional optical systems with tight field confinement and coherent photon trapping techniques. The confinement enables the generation of large, tunable optical nonlinearities via the interaction of photons with a nearby cold atomic gas. In its extreme, we show that a quantum light field can undergo fermionization in such one-dimensional media, which can be probed via standard photon correlation measurements. Interactions between photons are typically extremely weak. But when light pulses are confined to an optical waveguide and manipulated with nearby cold atoms, strongly interacting photons can be created that may even undergo crystallization, as is now shown theoretically.
195 citations
TL;DR: By controlling the nonlinear atomic interactions close to a Feshbach resonance the authors are able to induce a phase diffusive many-body spin dynamics of the relative phase between the two components, which allows us to probe the nonequilibrium evolution of one-dimensional many- body quantum systems.
Abstract: We report on the observation of many-body spin dynamics of interacting, one-dimensional (1D) ultracold bosonic gases with two spin states. By controlling the nonlinear atomic interactions close to a Feshbach resonance we are able to induce a phase diffusive many-body spin dynamics of the relative phase between the two components. We monitor this dynamical evolution by Ramsey interferometry, supplemented by a novel, many-body echo technique, which unveils the role of quantum fluctuations in 1D. We find that the time evolution of the system is well described by a Luttinger liquid initially prepared in a multimode squeezed state. Our approach allows us to probe the nonequilibrium evolution of one-dimensional many-body quantum systems.
103 citations
TL;DR: In this article, the authors describe how to reliably read and write topologically protected quantum memory using an atomic or photonic qubit, which can be used to probe statistics and dynamics of anyonic excitations.
Abstract: Strongly correlated quantum systems can exhibit exotic behaviour called topological order which is characterized by non-local correlations that depend on the system topology. Such systems can exhibit remarkable phenomena such as quasiparticles with anyonic statistics and have been proposed as candidates for naturally error-free quantum computation. However, anyons have never been observed in nature directly. Here, we describe how to unambiguously detect and characterize such states in recently proposed spin–lattice realizations using ultracold atoms or molecules trapped in an optical lattice. We propose an experimentally feasible technique to access non-local degrees of freedom by carrying out global operations on trapped spins mediated by an optical cavity mode. We show how to reliably read and write topologically protected quantum memory using an atomic or photonic qubit. Furthermore, our technique can be used to probe statistics and dynamics of anyonic excitations. A proposal describes how to detect topologically ordered states of ultracold matter in an optical lattice, and shows how these exotic states, which strongly correlated quantum systems can exhibit, could be harnessed for practical applications, such as robust quantum computation.
93 citations
TL;DR: In this article, a method for controllable generation of nonlocal entangled pairs using spinor atoms loaded in an optical superlattice was proposed, which iteratively increases the distance between entangled atoms by controlling the coupling between the double wells.
Abstract: We propose a method for controllable generation of nonlocal entangled pairs using spinor atoms loaded in an optical superlattice. Our scheme iteratively increases the distance between entangled atoms by controlling the coupling between the double wells. When implemented in a finite linear chain of $2N$ atoms, it creates a triplet valence bond state with large persistency of entanglement (of the order of $N$). We also study the nonequilibrium dynamics of the one-dimensional ferromagnetic Heisenberg Hamiltonian and show that the time evolution of a state of decoupled triplets on each double well leads to the formation of a highly entangled state where short-distance antiferromagnetic correlations coexist with longer-distance ferromagnetic ones. We present methods for detection and characterization of the various dynamically generated states. These ideas are a step forward toward the use of atoms trapped by light as quantum-information processors and quantum simulators.
58 citations
TL;DR: In this article, a method to achieve decoherence resistant entanglement generation in strongly interacting ensembles of two-level spin systems is discussed, where designed gapped Hamiltonians are used to create a protected manifold of multidegenerate levels which is robust against local decoverherence processes.
Abstract: We discuss a method to achieve decoherence resistant entanglement generation in strongly interacting ensembles of two-level spin systems. Our method uses designed gapped Hamiltonians to create a protected manifold of multidegenerate levels which is robust against local decoherence processes. We apply the protected evolution to achieve decoherence resistant generation of many-particle Greenberger-Horne-Zeilinger (GHZ) states in two specific physical systems, trapped ions and neutral atoms in optical lattices, and discuss how to engineer the desired many-body protected manifold with them. We analyze the fidelity of GHZ generation and show our method can significantly increase the sensitivity in frequency spectroscopy.
57 citations
TL;DR: In a ring-interferometer-type configuration, the transport is completely insensitive to the (effective) flux contained in the ring, in contrast with the Aharonov-Bohm effect of a single particle in the same geometry.
Abstract: We study one-dimensional Bose liquids of interacting ultracold atoms in the Y-shaped potential when each branch is filled with atoms. We find that the excitation packet incident on a single Y junction should experience a negative density reflection analogous to the Andreev reflection at normal-superconductor interfaces, although the present system does not contain fermions. In a ring-interferometer-type configuration, we find that the transport is completely insensitive to the (effective) flux contained in the ring, in contrast with the Aharonov-Bohm effect of a single particle in the same geometry.
50 citations
TL;DR: In this paper, the effect of magnetic Mn ions on the two-band superconductor (MgB) was studied and the total and spin-resolved scanning tunneling spectrum in the vicinity of the magnetic impurity was computed.
Abstract: We study the effect of magnetic Mn ions on the two-band superconductor ${\text{MgB}}_{2}$, and compute both the total and spin-resolved scanning tunneling spectrum in the vicinity of the magnetic impurity. We show that when the internal structure of the Mn ion's $d$-shell is taken into account, multiple Shiba states appear in the spectrum. The presence of these multiplets could alter significantly the overall interpretation of local tunneling spectra for a wide range of superconducting hosts and magnetic impurities.
27 citations
TL;DR: In this paper, the spectrum of collective excitations of the XY spiral state prepared adiabatically or suddenly from a uniform ferromagnetic F = 1 condensate was calculated.
Abstract: We calculate the spectrum of collective excitations of the XY spiral state prepared adiabatically or suddenly from a uniform ferromagnetic F=1 condensate. For spiral wave vectors past a critical value, spin wave excitation energies become imaginary indicating a dynamical instability. We construct phase diagrams as functions of spiral wave vector and quadratic Zeeman energy.
27 citations
TL;DR: In this article, a nonperturbative approach for calculating partition functions of sine-Gordon models was proposed, which relies on mapping them to statistical properties of random surfaces, and the amplitude of interference fringes in experiments with two independent low-dimensional Bose gases with nonzero temperatures.
Abstract: We introduce a new class of sine-Gordon models, for which the interaction term is present in a region different from the domain over which the quadratic part is defined. We develop a nonperturbative approach for calculating partition functions of such models, which relies on mapping them to statistical properties of random surfaces. As a specific application of our method, we consider the problem of calculating the amplitude of interference fringes in experiments with two independent low dimensional Bose gases. We calculate full distribution functions of interference amplitude for one-dimensional and two-dimensional gases with nonzero temperatures.
TL;DR: In this paper, the authors proposed a controlled method to create and detect d-wave superfluidity with ultracold fermionic atoms loaded in two-dimensional optical superlattices.
Abstract: We propose a controlled method to create and detect d-wave superfluidity with ultracold fermionic atoms loaded in two-dimensional optical superlattices. Our scheme consists in preparing an array of nearest-neighbor coupled square plaquettes or ``superplaquettes'' and using them as building blocks to construct a d-wave superfluid state. We describe how to use the coherent dynamical evolution in such a system to experimentally probe the pairing mechanism. We also derive the zero temperature phase diagram of the fermions in a checkerboard lattice (many weakly coupled plaquettes) and show that by tuning the inter-plaquette tunneling spin-dependently or varying the filling factor one can drive the system into a d-wave superfluid phase or a Cooper pair density wave phase. We discuss the use of noise correlation measurements to experimentally probe these phases.
TL;DR: In this paper, the ground state of strongly interacting systems on a lattice is characterized using the Chern number, which indicates the existence of a topological order in the degenerate ground-state manifold.
Abstract: We study Chern numbers to characterize the ground state of strongly interacting systems on a lattice. This method allows us to perform a numerical characterization of bosonic fractional quantum Hall (FQH) states on a lattice where the conventional overlap calculation with the known continuum case such as the Laughlin state, breaks down due to the lattice structure or dipole-dipole interaction. The non-vanishing Chern number indicates the existence of a topological order in the degenerate ground-state manifold.
TL;DR: In this article, a method for detecting paired states in either bosonic or fermionic systems using interference experiments with independent or weakly coupled low-dimensional systems is proposed, which can be used to detect both the Fulde, Ferrel, Larkin, Ovchinnikov and the d-wave paired states of fermions, as well as quasicondensates of singlet pairs for polar F=1 atoms in two-dimensional system.
Abstract: We propose a method for detecting paired states in either bosonic or fermionic systems using interference experiments with independent or weakly coupled low-dimensional systems. We demonstrate that our method can be used to detect both the Fulde, Ferrel, Larkin, Ovchinnikov, and the d-wave paired states of fermions, as well as quasicondensates of singlet pairs for polar F=1 atoms in two-dimensional systems. We discuss how this method can be used to perform phase-sensitive determination of the symmetry of the pairing amplitude.
Posted Content•
TL;DR: In this article, the collective mode dispersion in multi-layer stacks of two-dimensional dipolar condensates was analyzed and a strong enhancement of the roton instability was found.
Abstract: We analyze theoretically the collective mode dispersion in multi-layer stacks of two dimensional dipolar condensates and find a strong enhancement of the roton instability. We discuss the interplay between the dynamical instability and roton softening for moving condensates. We use our results to analyze the decoherence rate of Bloch oscillations for systems in which the s-wave scattering length is tuned close to zero using Feshbach resonance. Our results are in qualitative agreement with recent experiments of Fattori {\it et al.} on $^{39}$K atoms.
Journal Article•
TL;DR: In this article, the authors derived the self-consistent mean-field transition line, and account for both the static and dynamic screening effects of the fermions, and analyzed this effect for various mixture parameters and temperatures, and considered possible signatures of the orthogonality catastrophe effect in other measurables of the mixture.
Abstract: The superfluid-insulator transition of bosons is strongly modified by the presence of fermions. Through an imaginary-time path-integral approach, we derive the self-consistent mean-field transition line, and account for both the static and dynamic screening effects of the fermions. We find that an effect akin to the fermionic orthogonality catastrophe, arising from the fermionic screening fluctuations, suppresses superfluidity. We analyze this effect for various mixture parameters and temperatures, and consider possible signatures of the orthogonality-catastrophe effect in other measurables of the mixture.
TL;DR: In this paper, the authors derived the self-consistent mean-field transition line, and account for both the static and dynamic screening effects of the fermions, and analyzed this effect for various mixture parameters and temperatures, and considered possible signatures of the orthogonality catastrophe effect in other measurables of the mixture.
Abstract: The superfluid-insulator transition of bosons is strongly modified by the presence of fermions. Through an imaginary-time path-integral approach, we derive the self-consistent mean-field transition line, and account for both the static and dynamic screening effects of the fermions. We find that an effect akin to the fermionic orthogonality catastrophe, arising from the fermionic screening fluctuations, suppresses superfluidity. We analyze this effect for various mixture parameters and temperatures, and consider possible signatures of the orthogonality-catastrophe effect in other measurables of the mixture.
Journal Article•
TL;DR: In this paper, the authors describe how to reliably read and write topologically protected quantum memory using an atomic or photonic qubit using an optical cavity mode, which can be used to probe statistics and dynamics of anyonic excitations.
Abstract: Strongly correlated quantum systems can exhibit exotic behaviour called topological order which is characterized by non-local correlations that depend on the system topology Such systems can exhibit remarkable phenomena such as quasiparticles with anyonic statistics and have been proposed as candidates for naturally error-free quantum computation However, anyons have never been observed in nature directly Here, we describe how to unambiguously detect and characterize such states in recently proposed spin–lattice realizations using ultracold atoms or molecules trapped in an optical lattice We propose an experimentally feasible technique to access non-local degrees of freedom by carrying out global operations on trapped spins mediated by an optical cavity mode We show how to reliably read and write topologically protected quantum memory using an atomic or photonic qubit Furthermore, our technique can be used to probe statistics and dynamics of anyonic excitations A proposal describes how to detect topologically ordered states of ultracold matter in an optical lattice, and shows how these exotic states, which strongly correlated quantum systems can exhibit, could be harnessed for practical applications, such as robust quantum computation
Posted Content•
TL;DR: In this article, it was shown that roton softening occurs in 87Rb spinor condensates once dipolar interactions and spin dynamics are taken into account, and a dynamical instability develops in the collective mode spectrum at finite wavevectors.
Abstract: Superfluids with a tendency towards periodic crystalline order have both a phonon and roton like spectrum of collective modes. The softening of the roton spectrum provides one route to a supersolid. We show that roton softening occurs in 87Rb spinor condensates once dipolar interactions and spin dynamics are taken into account. By including the effects of a quasi-two-dimensional geometry and rapid Larmor precession, we show a dynamical instability develops in the collective mode spectrum at finite wavevectors. We construct phase diagrams showing a variety of instabilities as a function of the direction of the magnetic field and strength of the quadratic Zeeman shift. Our results provide a possible explanation of current experiments in the Berkeley group Phys. Rev. Lett. 100:170403 (2008).
TL;DR: The zero-energy bound states at the edges or vortex cores of chiral p-wave superconductors are expected to behave like Majorana fermions as mentioned in this paper, and the tunnelling process when electrons are injected into such states using a non-equilibrium Green function formalism.
Abstract: The zero-energy bound states at the edges or vortex cores of chiral p-wave superconductors are expected to behave like Majorana fermions We introduce a model Hamiltonian that describes the tunnelling process when electrons are injected into such states Using a non-equilibrium Green function formalism, we find exact analytic expressions for the tunnelling current and noise and identify experimental signatures of the Majorana nature of the bound states to be found in the shot noise We discuss the results in the context of different candidate materials that are believed to support triplet superconductivity
01 Mar 2008
TL;DR: The effect of quantum and thermal fluctuations on the phase diagram of spin-2 BECs is examined in this paper, where a continuous transition of the Ising type from uniaxial to square BEC is predicted on raising the magnetic field.
Abstract: Submitted for the MAR08 Meeting of The American Physical Society Nematic order by disorder in spin-2 BECs RYAN BARNETT, Caltech, ARI TURNER, EUGENE DEMLER, Harvard, ASHVIN VISHWANATH, Berkeley — The effect of quantum and thermal fluctuations on the phase diagram of spin-2 BECs is examined. They are found to play an important role in the nematic part of the phase diagram, where a mean-field treatment of two-body interactions is unable to lift the accidental degeneracy between nematic states. Quantum and thermal fluctuations resolve this degeneracy, selecting the uniaxial nematic state, for scattering lengths a4 greater than a2, and the square biaxial nematic state for a4 less than a2. Paradoxically, the fluctuation induced order is stronger at higher temperatures, for a range of temperatures below Tc. For the experimentally relevant cases of spin-2 87Rb and 23Na, we argue that such fluctuations could successfully compete against other effects like the quadratic Zeeman field, and stabilize the uniaxial phase for experimentally realistic conditions. A continuous transition of the Ising type from uniaxial to square biaxial order is predicted on raising the magnetic field. These systems present a promising experimental opportunity to realize the ‘order by disorder’ phenomenon. Ryan Barnett Caltech Date submitted: 27 Nov 2007 Electronic form version 1.4
Journal Article•
01 Jan 2008
Posted Content•
TL;DR: In this article, a scheme that makes use of interactions between spins to protect certain correlated many-body states from decoherence was proposed and applied to achieved coherent generation of many particle GHZ states.
Abstract: Fachbereich Physik, Technische Universita¨t Kaiserslautern, D-67663 Kaiserslautern, Germany(Dated: February 6, 2008)We propose and analyze a scheme that makes use of interactions between spins to protect certain corre-lated many-body states from decoherence. The method exploits the finite energy gap of properly designedHamiltonians to generate a manifold insensitive to local noise fluctuations. We apply the scheme to achievedecoherence-resistant generation of many particle GHZ states and show that it can improve the sensitivity inprecision spectroscopy with trapped ions. Finally we also show that cold atoms in optical lattices interacting viashort range interactions can be utilized to engineer the required long range interactions for a robust generationof entangled states.