Mar 25 - Mar 29, Bo Liu
Mar 29
1. arXiv:1303.7052 [pdf, ps, other]
Wall-vortex composite solitons in two-component Bose-Einstein condensates
Kenichi Kasamatsu, Hiromitsu Takeuchi, Makoto Tsubota, Muneto Nitta
We study composite solitons, consisting of domain walls and vortex lines attaching to the walls in two-component Bose-Einstein condensates. When the total density of two components is homogeneous, the system can be mapped to the O(3) nonlinear sigma model for the pseudospin representing the two-component order parameter and the analytical solutions of the composite solitons can be obtained. Based on the analytical solutions, we discuss the detailed structure of the composite solitons in two-component condensates by employing the generalized nonlinear sigma model, where all degrees of freedom of the original Gross-Pitaevskii theory are active. The density inhomogeneity results in reduction of the domain wall tension from that in the sigma model limit. We find that the domain wall pulled by a vortex is logarithmically bent as a membrane pulled by a pin, and it bends more flexibly than not only the domain wall in the sigma model but also the expectation from the reduced tension. Finally, we study the composite soliton structure for actual experimental situations with trapped immiscible condensates under rotation through numerical simulations of the coupled Gross-Pitaevskii equations.
2. arXiv:1303.7169 [pdf, ps, other]
Loschmidt echo and the many-body orthogonality catastrophe in a qubit-coupled Luttinger liquid
B. Dóra, F. Pollmann, G. Zaránd
We investigate the many-body generalization of the orthogonality catastrophe by studying the Loschmidt echo of Luttinger liquids (LLs) after a global interaction quench or ramp. It decays exponentially with system size and exhibits universal behaviour: the steady state exponent after quenching back and forth n-times between 2 LLs (bang-bang protocol) is 2n-times bigger than that of the adiabatic overlap, and depends only on the initial and final LL parameters. These are corroborated numerically by matrix-product state based methods of the XXZ Heisenberg model. An experimental setup consisting of a hybrid system containing cold atoms and a flux qubit is proposed to measure the Loschmidt echo using rf spectroscopy or Ramsey interferometry.
Mar 28
1.arXiv:1303.6903 [pdf, ps, other]
Superstripes and the excitation spectrum of a spin-orbit-coupled Bose-Einstein condensate
Yun Li, Giovanni I. Martone, Lev P. Pitaevskii, Sandro Stringari
Using Bogoliubov theory we calculate the excitation spectrum of a spinor Bose-Einstein condensed gas with equal Rashba and Dresselhaus spin-orbit coupling in the stripe phase. The emergence of a double gapless band is pointed out as a key signature of Bose-Einstein condensation and of the spontaneous breaking of translational invariance symmetry. In the long wavelength limit the lower and upper branches exhibit, respectively, a clear spin and density nature. For wave vectors close to the first Brillouin zone the lower branch acquires an important density character responsible for the divergent behavior of the structure factor and of the static response function. The sound velocities are calculated as functions of the Raman coupling for excitations propagating orthogonal and parallel to the stripes. Our predictions provide new perspectives for the identification of supersolid phenomena in ultracold atomic gases.
2. arXiv:1303.6670 [pdf, other]
Photon bubbles in ultra-cold matter
J.T. Mendonca, R. Kaiser
We show that static and oscillating photon bubbles can be excited by diffused light in the laser cooled matter confined in a magneto-optical trap (MOT). The bubble instability is due to the coupling between the radiation field and the mean field oscillations of the ultra-cold gas, and it can provide a source for low frequency turbulence. We consider a diffusion dominated regime, which can be described by a radiation transport equation, coupled with the mean field equations for the cold atom gas. A perturbative analysis shows the occurrence of two different regimes with either oscillating or purely growing bubbles. This work could also be useful to understand similar processes in astrophysics.
Mar 27
1. arXiv:1303.6297 [pdf, ps, other]
Tetramerization in a SU(4)-Heisenberg model on the honeycomb lattice
Miklos Lajko, Karlo Penc
The SU(4) Heisenberg model can serve as a low energy model of the Mott insulating state in materials where the spins and orbitals are highly symmetric, or in systems of alkaline-earth atoms on optical lattice. Recently, it has been argued that on the honeycomb lattice the model exhibits a unique spin-orbital liquid phase with an algebraic decay of correlations [P. Corboz et al., Phys. Rev. X 2, 041013 (2012)]. Here we study the instability of the algebraic spin-orbital liquid toward spontaneous formation of SU(4) singlet plaquettes (tetramerization). Using a variational Monte Carlo approach to evaluate the projected wave-function of fermions with $\pi$-flux state, we find that the algebraic liquid is robust, and that a finite value of the next nearest exchange is needed to induce tetramerization. We also studied the phase diagram of a model which interpolates between the nearest neighbor Heisenberg model and a Hamiltonian for which the singlet-plaquette product state is an exact ground state.
2. arXiv:1303.5564[pdf, other]
Synchronization transition in the quantum dynamics of two spins
Y. Liu, F. Piechon, J.N. Fuchs
Motivated by the spin self-rephasing recently observed in an atomic clock, we introduce a simple dynamical model to study the competition between dephasing and synchronization. Two spins $S$ are taken to be initially parallel and in the plane perpendicular to an inhomogeneous magnetic field $\Delta$ that tends to dephase them. In addition, the spins are coupled by exchange interaction $J$ that tries to keep them locked. The analytical solution of the classical dynamics shows that, there is a phase transition to a synchronized regime for sufficiently large exchange interaction $J>\Delta$ compared to the inhomogeneity. The quantum dynamics is solved analytically in four limits -- large/small $J/\Delta$ and large/small $S$ -- and numerically in between. In sharp contrast to the classical case, the quantum solution features very rich $S$-dependent multiscale dynamics. For any finite $S$, there is no synchronization but a crossover around $J=\Delta$ between two regimes. The synchronization transition is only recovered when $S\to \infty$, approaching the classical solution in a non-trivial way.
Mar 26
1.arXiv:1303.6245 [pdf, other]
Contact and Momentum Distribution of the Unitary Fermi Gas by Bold Diagrammatic Monte Carlo
K. Van Houcke, F. Werner, E. Kozik, N. Prokof'ev, B. Svistunov
A key observable in strongly interacting resonant Fermi gases is the contact parameter C, which governs both the pair correlation function at short distances and the momentum distribution at large momenta. The temperature dependence of C was recently measured at unitarity, where existing theoretical predictions differ substantially. We report accurate data for the contact and the momentum distribution in the normal phase of the unitary gas, obtained by Bold Diagrammatic Monte Carlo. In our scheme, C is extracted from the pair correlation function, while the C/k^4 tail of the momentum distribution, being built in at the analytical level, is free of k-dependent noise.
2.arXiv:1303.6004 [pdf, other]
Synthetic Gauge Field with Highly Magnetic Lanthanide Atoms
Xiaoling Cui, Biao Lian, Tin-Lun Ho, Benjamin L. Lev, Hui Zhai
We present a scheme for generating a synthetic magnetic field and spin-orbit coupling via Raman coupling in highly magnetic lanthanide atoms such as dysprosium. Employing these atoms offer several advantages for realizing strongly correlated states and exotic spinor phases. The large spin and narrow optical transitions of these atoms allow the generation of synthetic magnetic fields an order of magnitude larger than those in the alkalis, but with considerable reduction of the heating rate for equal Raman coupling. The effective hamiltonian of these systems differs from that of the alkalis' by an additional nematic coupling term, which leads to a phase transition in the dressed states as detuning varies. For \text{high-spin} condensates, spin-orbit coupling leads to a spatially periodic structure, which is described in Majorana representation by a set of points moving periodically on a unit sphere. We name this a "Majorana spinor helix" in analogy to the persistent spin-1/2 helix observed in electronic systems.
3. arXiv:1303.6243 [pdf, ps, other]
Quantum Monte Carlo Calculations with Chiral Effective Field Theory Interactions
A. Gezerlis, I. Tews, E. Epelbaum, S. Gandolfi, K. Hebeler, A. Nogga, A. Schwenk
We present the first Quantum Monte Carlo (QMC) calculations with chiral Effective Field Theory (EFT) interactions. To achieve this, we remove all sources of nonlocality, which hamper the inclusion in QMC, in nuclear forces to next-to-next-to-leading order (N2LO). We perform Auxiliary-Field Diffusion Monte Carlo (AFDMC) calculations for the neutron matter energy up to saturation density based on local leading-order, next-to-leading order, and N2LO nucleon-nucleon interactions. Our results exhibit a systematic order-by-order convergence in chiral EFT and provide nonperturbative benchmarks with theoretical uncertainties. For the softer interactions, perturbative calculations are in excellent agreement with the AFDMC results. This work paves the way for QMC calculations with systematic chiral EFT interactions for nuclei and nuclear matter, for testing the perturbativeness of different orders, and allows for matching to lattice QCD results by varying the pion mass.
Mar 25
1. arXiv:1303.5539 [pdf, ps, other]
Role of single-particle and pair condensates in Bose systems with arbitrary intensity of interaction
A. S. Peletminskii, S. V. Peletminskii, Yu. M. Poluektov
We study a superfluid Bose system with single-particle and pair condensates on the basis of a half-phenomenological theory of a Bose liquid not involving the weakness of interparticle interaction. The coupled equations describing the equilibrium state of such system are derived from the variational principle for entropy. These equations are analyzed at zero temperature both analytically and numerically. It is shown that the fraction of particles in the single-particle and pair condensates essentially depends on the total density of the system. At densities attainable in condensates of alkali-metal atoms, almost all particles are in the single-particle condensate. The pair condensate fraction grows with an increasing total density and becomes dominant. It is shown that at density of liquid helium, the single-particle condensate fraction is less than 10%, which agrees with experimental data on inelastic neutron scattering, Monte Carlo calculations and other theoretical predictions. The ground state energy, pressure, and compressibility are found for the system under consideration. The spectrum of single-particle excitations is also analyzed.
2.arXiv:1303.5493 [pdf, ps, other]
Equivalence between fractional exclusion statistics and Fermi liquid theory in interacting particle systems
Dragos-Victor Anghel, George Alexandru Nemnes
We develop a method based on fractional exclusion statistics (FES) to describe s-dimensional systems of interacting particles. The particles -- which are described in the quasiclassical approximation -- are in an external potential and experience a generic particle-particle interaction. We define the FES quasiparticle energies, we calculate the FES parameters of the system and we deduce the equations for the equilibrium particle populations. The FES gas is "ideal", in the sense that the quasiparticle energies do not depend on the other quasiparticle levels populations and the sum of the quasiparticle energies is equal to the total energy of the system. We prove that the FES formalism is equivalent to the standard Landau's Fermi liquid theory (FLT) approach and the FES quasiparticle populations may be calculated from the FLT populations by making the correspondence between the FES and the FLT quasiparticle energies. The FES provides a natural ideal gas description of the interacting particle gas in a generic external potential and in any number of dimensions.
Mar 18 - Mar 22, Xiaopeng Li
Mar 22
1. arXiv:1303.5209 [pdf]
Anisotropic quantum transport in monolayer graphene in the presence of Rashba spin-orbit coupling
Kobra Hasanirokh, Hakimeh Mohammadpour, Arash Phirouznia
We have studied spin-depend electron tunnelling through the Rashba barrier in a monolayer graphene lattice. The transfer matrix method, have been employed to obtain the spin dependent transport properties of the chiral particles. It is shown that graphene sheets in presence of Rashba spin-orbit barrier will act as an electron spin- inverter.
2.arXiv:1303.5232 [pdf, ps, other]
Spin dynamics of two bosons in an optical lattice site: a role of anharmonicity and anisotropy of the trapping potential
Joanna Pietraszewicz, Tomasz Sowiński, Miros\law Brewczyk, Maciej Lewenstein, Mariusz Gajda
We study a spin dynamics of two magnetic Chromium atoms trapped in a single site of a deep optical lattice in a resonant magnetic field. Dipole-dipole interactions couple spin degrees of freedom of the two particles to their motion in the site. The motion is quantized, therefore a trap geometry combined with two-body contact s-wave interactions influence a spin dynamics through the energy spectrum of the two atom system. Anharmonicity and anisotropy of the site results in a `fine' structure of two body eigenenergies. The structure can be easily resolved by a weak magnetic dipole-dipole interactions. As an example we examine the effect of anharmonicity and anisotropy of the binding potential on the Einstein-de Haas effect. We show that the weak dipolar interactions provide a perfect tool for a precision spectroscopy of the energy spectrum of the interacting few particle system.
3.arXiv:1303.5352 [pdf]
LaAlO3 stoichiometry found key to electron liquid formation at LaAlO3/SrTiO3 interfaces
M. P. Warusawithana, C. Richter, J. A. Mundy, P. Roy, J. Ludwig, S. Paetel, T. Heeg, A. A. Pawlicki, L. F. Kourkoutis, M. Zheng, M. Lee, B. Mulcahy, W. Zander, Y. Zhu, J. Schubert, J. N. Eckstein, D. A. Muller, C. Stephen Hellberg, J. Mannhart, D. G. Schlom
Emergent phenomena, including superconductivity and magnetism, found in the two-dimensional electron liquid (2-DEL) at the interface between the insulators LaAlO3 and SrTiO3 distinguish this rich system from conventional two-dimensional electron gases at compound semiconductor interfaces. The origin of this 2-DEL, however, is highly debated with focus on the role of defects in the SrTiO3 while the LaAlO3 has been assumed perfect. Our experiments and first principles calculations show that the cation stoichiometry of the nominal LaAlO3 layer is key to 2-DEL formation: only Al-rich LaAlO3 results in a 2-DEL. While extrinsic defects including oxygen deficiency are known to render LaAlO3/SrTiO3 samples conducting, our results show that in the absence of such extrinsic defects, an interface 2-DEL can form. Its origin is consistent with an intrinsic electronic reconstruction occurring to counteract a polarization catastrophe. This work provides a roadmap for identifying other interfaces where emergent behaviors await discovery.
Mar 21
1.arXiv:1303.4909 [pdf, ps, other]
Dynamics of Majorana States in a Topological Josephson Junction
Manuel Houzet, Julia S. Meyer, Leonid I. Glazman
Topological Josephson junctions carry 4pi-periodic bound states. A finite bias applied to the junction limits the lifetime of the bound state by dynamically coupling it to the continuum. That lifetime may be shorter or longer than the phase adjustment time for a given bound state, depending on the resistance of the circuit "seen" by the junction. We show that, in the former case, the presence of a 4pi-periodic bound state manifests itself in a peak in the current noise spectrum at half the Josephson frequency, when a constant bias is applied to the junction. In the opposite case of fast phase adjustment, the 4pi-periodicity is evidenced by an even-odd effect in Shapiro steps. We specify the conditions necessary for observing the manifestations of 4pi-periodicity in the noise spectrum and Shapiro step measurements.
2. arXiv:1303.4745 [pdf, other]
Fluctuation-induced pair density wave in itinerant ferromagnets
G.J. Conduit, C.J. Pedder, A.G. Green
Magnetic fluctuations near to quantum criticality can have profound effects. They lead to characteristic scaling at high temperature which may ultimately give way to a reconstruction of the phase diagram and the formation of new phases at low temperatures. The ferromagnet UGe2 is unstable to p-wave superconducting order -- an effect presaged by the superfluidity in He3 -- whereas in CeFePO fluctuations drive the formation of spiral magnetic order. Here we develop a general quantum order-by-disorder description of these systems that encompasses both of these instabilities within a unified framework. This allows us to demonstrate that in fact these instabilities intertwine to form a new phase, a pair density wave.
3.arXiv:1303.5040 (cross-list from quant-ph) [pdf, other]
Quantum simulations of gauge theories with ultracold atoms: local gauge invariance from angular momentum conservation
Erez Zohar, J. Ignacio Cirac, Benni Reznik
Quantum simulations of High Energy Physics, and especially of gauge theories, is an emerging and exciting direction in quantum simulations. However, simulations of such theories, compared to simulations of condensed matter physics, must satisfy extra restrictions, such as local gauge and Lorentz invariance. In this paper we discuss these special requirements, and present a new method for quantum simulation of lattice gauge theories using ultracold atoms. This method allows to include local gauge invariance as a \emph{fundamental} symmetry of the atomic Hamiltonian, arising from natural atomic interactions and conservation laws (and not as a property of a low energy sector). This allows us to implement elementary gauge invariant interactions for three lattice gauge theories: compact QED (U(1)), SU(N) and Z_N, which can be used to build quantum simulators in 1+1 dimensions. We also present a new loop method, which uses the elementary interactions as building blocks in the effective construction of quantum simulations for d+1 dimensional lattice gauge theories (d>1), without having to use Gauss's law as a constraint, as in previous proposals. We discuss in detail the quantum simulation of 2+1 dimensional compact QED and provide a numerical proof of principle. The simplicity of the already gauge invariant elementary interactions of this model suggests it may be useful for future experimental realizations.
Mar 20
1.arXiv:1303.4716 [pdf, ps, other]
Entanglement subspaces, trial wavefunctions, and special Hamiltonians in the fractional quantum Hall effect
T.S. Jackson, N. Read, S.H. Simon
We consider systems of trial wavefunctions in the fractional quantum Hall effect that can be obtained in various ways. In one way, the functions are obtained by analyzing the entanglement of the ground state wavefunction, partitioned into two parts. In another, functions are defined by the way in which they vanish as several coordinates approach the same value, or by a projection operator Hamiltonian that enforces those conditions. In a third way, the functions are given by conformal blocks from a conformal field theory (CFT). These different sets are closely related. The use of CFT methods permits an algebraic formulation to be given for all of them. In some cases, we can prove that all of these spaces are the same (a finite-size bulk-edge correspondence), thus answering several questions and conjectures. We can also use the analysis of functions to produce a projection operator Hamiltonian that produces them as zero-energy states. For a model related to the N=1 superconformal algebra, the corresponding Hamiltonian imposes vanishing properties involving only three particles; for this we determine all the wavefunctions explicitly. We do the same for a sequence of models involving the M(3,p) Virasoro minimal models that has been considered previously, using results from the literature. We exhibit the Hamiltonians for the first few cases of these.
================the following paper seems very interesting, and maybe easily adapted to cold atoms
2. arXiv:1303.4643 [pdf, other]
How to Measure the Quantum Geometry of Bloch Bands
Titus Neupert, Claudio Chamon, Christopher Mudry
Single-particle states in electronic Bloch bands form a Riemannian manifold whose geometric properties are described by two gauge invariant tensors, one being symmetric the other being antisymmetric, that can be combined into the so-called Fubini-Study metric tensor of the projective Hilbert space. The latter directly controls the Hall conductivity. Here we show that the symmetric part of the Fubini-Study metric tensor also has measurable consequences by demonstrating that it enters the current noise spectrum. In particular, we show that a non-vanishing equilibrium current noise spectrum at zero temperature is unavoidable whenever Wannier states have non-zero minimum spread, the latter being quantifiable by the symmetric part of the Fubini-Study metric tensor. We illustrate our results by three examples: (1) atomic layers of hexagonal boron nitride, (2) graphene, and (3) the surface states of three-dimensional topological insulators when gaped by magnetic dopants.
Mar 19
1.arXiv:1303.4369 [pdf, ps, other]
Floquet-Bloch theory and topology in periodically driven lattices
Alvaro Gomez-Leon, Gloria Platero
We propose a general framework to solve tight binding models in D dimensional lattices driven by ac electric fields. Our method is valid for arbitrary driving regimes and allows to obtain effective Hamiltonians for different external fields configurations. We establish an equivalence with time independent lattices in D+1 dimensions, and analyze their topological properties. Further, we demonstrate that non-adiabaticity drives a transition from topological invariants defined in D+1 to D dimensions. Our approach provides a theoretical framework to analyze ac driven systems, with potential applications in topological states of matter, and non-adiabatic topological quantum computation, predicting novel outcomes for future experiments.
2. arXiv:1303.4144 [pdf, other]
Topological Mirror Superconductivity
Fan Zhang, C. L. Kane, E. J. Mele
We demonstrate the existence of topological superconductors (SC) protected by mirror and time reversal (TR) symmetries. D-dimensional (D=1,2,3) crystalline SCs are characterized by 2^(D-1) independent integer topological invariants, which take the form of mirror Berry phases. These invariants determine the distribution of Majorana modes on a mirror symmetric boundary. The parity of total mirror Berry phase is the Z_2 index of a class DIII SC, implying that a DIII topological SC with a mirror line must also be a topological mirror SC but not vice versa, and that a DIII SC with a mirror plane is always TR trivial but can be mirror topological. We introduce representative models and suggest experimental signatures in feasible systems. Advances in quantum computing, the case for class D, and topological SCs protected by rotational symmetries are pointed out.
3. arXiv:1303.4379 (cross-list from quant-ph) [pdf, other]
Flux-controlled quantum computation with Majorana fermions
T. Hyart, B. van Heck, I. C. Fulga, M. Burrello, A. R. Akhmerov, C. W. J. Beenakker
Majorana fermions hold promise for quantum computation, because their non-Abelian braiding statistics allows for topologically protected operations on quantum information. Topological qubits can be constructed from pairs of well-separated Majoranas in networks of nanowires. The coupling to a superconducting charge qubit in a transmission line resonator (transmon) permits braiding of Majoranas by external variation of magnetic fluxes. We show that readout operations can also be fully flux-controlled, without requiring microscopic control over tunnel couplings. We identify the minimal circuit that can perform the initialization--braiding--measurement steps required to demonstrate non-Abelian statistics. We introduce the Random Access Majorana Memory, a scalable circuit that can perform a joint parity measurement on Majoranas belonging to any selection of topological qubits. Such multi-qubit measurements allow for the efficient creation of highly entangled states and simplify quantum error correction protocols by avoiding the need for ancilla qubits.
Mar 18
1. arXiv:1303.3583 [pdf, other]
Quantum flutter versus Bloch oscillations in one-dimensional quantum liquids out of equilibrium
Michael Knap, Charles J. M. Mathy, Mikhail B. Zvonarev, Eugene Demler
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.
Mar 11 - Mar 15, Saubhik Sarkar
Mar 15
1.arXiv:1303.3558 [pdf, other]
Spin-orbit interactions in a helical Luttinger liquid with a Kondo impurity
Erik Eriksson
The combined effect of Rashba and Dresselhaus spin-orbit interactions on the physics of a helical Luttinger liquid coupled to a Kondo impurity is studied. A Rashba coupling can potentially destroy the Kondo singlet formation in certain parameter regimes [Phys. Rev. B 86, 161103(R) (2012)]. This effect is here shown to vanish for sufficiently strong Dresselhaus interaction. The transport properties of the system are investigated by calculating electrical conductance, current and current fluctuations, and thermal conductance.
Mar 14
1.arXiv:1303.3006 [pdf, other]
Monopoles in 2+1-dimensional conformal field theories with global U(1) symmetry
Silviu S. Pufu, Subir Sachdev
In 2+1-dimensional conformal field theories with a global U(1) symmetry, monopoles can be introduced through a background gauge field that couples to the U(1) conserved current. We use the state-operator correspondence to calculate scaling dimensions of such monopoles. We obtain the next-to-leading term in the 1/N_b expansion of the Wilson-Fisher fixed point in the theory of N_b complex bosons.
2.arXiv:1303.3168 [pdf, ps, other]
Charge Order in a Two-Dimensional Kondo Lattice Model
Takahiro Misawa, Junki Yoshitake, Yukitoshi Motome
The possibility of charge order is theoretically examined for the Kondo lattice model in two dimensions, which does not include bare repulsive interactions. Using two complementary numerical methods, we find that charge order appears at quarter filling in an intermediate Kondo coupling region. The charge ordered ground state is an insulator exhibiting an antiferromagnetic order at charge-poor sites, while the paramagnetic charge-ordered state at finite temperatures is metallic with pseudogap behavior. We confirm that the stability of charge order is closely related with the local Kondo-singlet formation at charge-rich sites. Our results settle the controversy on charge order in the Kondo lattice model in realistic spatial dimensions.
Mar 13
1.arXiv:1111.3375 (replaced) [pdf, other]
Quantum integrability in systems with finite number of levels
Emil A. Yuzbashyan, B. Sriram Shastry
We consider the problem of defining quantum integrability in systems with finite number of energy levels starting from commuting matrices and construct new general classes of such matrix models with a given number of commuting partners. We argue that if the matrices depend on a (real) parameter, one can define quantum integrability from this feature alone, leading to specific results such as exact solvability, Poissonian energy level statistics and to level crossings.
Mar 12
1.arXiv:1303.2114 [pdf, other]
Charge ordering in metals with antiferromagnetic spin correlations
Subir Sachdev, Rolando La Placa
Metals with antiferromagnetic spin correlations have an instability to the superconductivity of spin-singlet Cooper pairs with d symmetry (for the Fermi surface of the cuprates). Metlitski et al. (arXiv:1005.1288) noted that in two dimensions, in the low energy continuum theory, this superconductivity is degenerate with a charge density wave ordering which has a local d symmetry under rotations about the lattice points. We present a momentum-space Hartree-Fock computation on a simple lattice model, and find that the d symmetry is dominant for a range of small ordering wavevectors including those observed in recent experiments. We propose a charge order parameter for the underdoped cuprates.
2.arXiv:1303.2408 [pdf, other]
Exotic Ising dynamics in a Bose-Hubbard model
Luis Seabra, Frank Pollmann
We explore the dynamical properties of a one-dimensional Bose-Hubbard model, where two different bosonic species interact via Feshbach resonance. We focus on the region in the phase diagram which is described by an effective, low-energy ferromagnetic Ising model in both transverse and longitudinal fields. In this regime, we numerically calculate the dynamical structure factor of the Bose-Hubbard model using the Time-Evolving Block Decimation method. In the ferromagnetic phase, we observe both the continuum of excitations and the bound states in the presence of a longitudinal field. Near the Ising critical point, we observe the celebrated E8 mass spectrum in the excited states. We also point out possible measurements which could be used to detect these excitations in an optical lattice experiment.
Mar 11
1.arXiv:1209.2051 (replaced) [pdf, other]
Identifying topological edge states in 2D optical lattices using light scattering
Nathan Goldman, Jerome Beugnon, Fabrice Gerbier
We recently proposed in a Letter [Physical Review Letters 108 255303] a novel scheme to detect topological edge states in an optical lattice, based on a generalization of Bragg spectroscopy. The scope of the present article is to provide a more detailed and pedagogical description of the system - the Hofstadter optical lattice - and probing method. We first show the existence of topological edge states, in an ultra-cold gas trapped in a 2D optical lattice and subjected to a synthetic magnetic field. The remarkable robustness of the edge states is verified for a variety of external confining potentials. Then, we describe a specific laser probe, made from two lasers in Laguerre-Gaussian modes, which captures unambiguous signatures of these edge states. In particular, the resulting Bragg spectra provide the dispersion relation of the edge states, establishing their chiral nature. In order to make the Bragg signal experimentally detectable, we introduce a "shelving method", which simultaneously transfers angular momentum and changes the internal atomic state. This scheme allows to directly visualize the selected edge states on a dark background, offering an instructive view on topological insulating phases, not accessible in solid-state experiments.
Mar 4 - Mar 8, Johannes Schachenmayer
Mar 8
arXiv:1303.1666 [pdf, other]
Persistence of equilibrium states in an oscillating double-well potential
H. Jiang, H. Susanto, T.M. Benson, K.A. Cliffe
We investigate numerically parametrically driven coupled nonlinear Schrodinger equations modelling the dynamics of coupled wavefields in a periodically oscillating double-well potential. The equations describe among other things two coupled periodically-curved optical waveguides with Kerr nonlinearity or horizontally shaken Bose-Einstein condensates in a double-well magnetic trap. In particular, we study the persistence of equilibrium states of the undriven system due to the presence of the parametric drive. Using numerical continuations of periodic orbits and calculating the corresponding Floquet multipliers, we find that the drive can (de)stabilize a continuation of an equilibrium state indicated by the change of the (in)stability of the orbit. Hence, we show that parametric drives can provide a powerful control to nonlinear (optical or matter wave) field tunneling. Analytical approximations based on an averaging method are presented. Using perturbation theory the influence of the drive on the symmetry breaking bifurcation point is discussed.
arXiv:1303.1616 [pdf, ps, other]
Universal damping behavior of dipole oscillations of one-dimensional ultracold gases induced by quantum phase slips
Ippei Danshita
We study superflow decay via quantum phase slips in trapped one-dimensional (1D) quantum gases through dipole oscillations induced by sudden displacement of the trapping potential. We find the relation between the damping rate of the dipole oscillation $G$ and the phase-slip nucleation rate $\Gamma$ as $G\propto \Gamma/v$, where $v$ is the flow velocity. This relation allows us to show that damping of 1D Bose gases in optical lattices, which has been extensively studied in experiment, is due to quantum phase slips. It is also found that the damping rate versus the flow velocity obeys the scaling formula for an impurity potential even in the absence of an explicit impurity. We suggest that the damping rate at a finite temperature exhibits a universal crossover behavior upon changing the flow velocity.
arXiv:1303.1546 [pdf, ps, other]
Finite temperature stability and dimensional crossover of exotic superfluidity in lattices
M. O. J. Heikkinen, D.-H. Kim, P. Törmä
We investigate exotic paired states of spin-imbalanced Fermi gases in anisotropic lattices, tuning the dimension between one and three. We calculate the finite temperature phase diagram of the system using real-space dynamical mean-field theory in combination with the quantum Monte Carlo method. We find that regardless of the intermediate dimensions examined, the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state survives to reach about one third of the BCS critical temperature of the spin-density balanced case. We show how the gapless nature of the state found is reflected in the local spectral function. While the FFLO state is found at a wide range of polarizations at low temperatures across the dimensional crossover, with increasing temperature we find out strongly dimensionality-dependent melting characteristics of shell structures related to harmonic confinement. Moreover, we show that intermediate dimension can help to stabilize an extremely uniform finite temperature FFLO state despite the presence of harmonic confinement.
arXiv:1303.1711 [pdf, other]
Controlled ripple texturing of suspended graphene membrane due to coupling with ultracold atoms
Sofia Ribeiro, Stefan Scheel
We discuss the possibility to create hybrid quantum systems that combine ultracold atoms with graphene membranes. We investigate a setup in which a cold atom cloud is placed close to a free-standing sheet of graphene at distances on the order of a few hundred nanometers. The atoms then couple to the graphene membrane via Casimir-Polder forces. Temporal changes in the atomic state of the atomic cloud changes the Casimir-Polder interaction, thereby leading to the creation of a backaction force in the graphene sheet. This setup provides a controllable way to engineer ripples in a graphene sheet.
Mar 7
arXiv:1303.1388 [pdf, other]
Collision dynamics and entanglement generation of two initially independent and indistinguishable boson pairs in one-dimensional harmonic confinement
David I. H. Holdaway, Christoph Weiss, Simon A. Gardiner
We investigate finite number effects in collisions between two states of an initially well known number of identical bosons with contact interactions, oscillating in the presence of harmonic confinement in one dimension. We investigate two N/2 (interacting) ground states, which are initially displaced from the trap center, and the effects of varying interaction strength. The numerics focus on the simplest case of N=4. In the non-interacting case, such a system would display periodic oscillation with a half harmonic oscillator period (due to the left-right symmetry). With the addition contact interactions between the bosons, collisions generate entanglement between each of the states and distribute energy into other modes of the oscillator. We study the system numerically via an exact diagonalization of the Hamiltonian with a finite basis, investigating left/right number uncertainty as our primary measure of entanglement. Additionally we study the time-evolution and equilibration of the single-body von Neumann entropy for both the attractive and repulsive cases. We identify parameter regimes for which attractive interactions create qualitatively different behavior to repulsive interactions, due to the presence of bound states (quantum solitons) and explain the processes behind this.
arXiv:1303.1230 [pdf, other]
Confinement modifies interactions of ultracold dipolar gases on optical lattices
M. L. Wall, L. D. Carr
We study the effective dipole-dipole interactions in ultracold quantum gases on optical lattices as a function of imbalance in confinement along the principal axes of the lattice. We demonstrate that the effective dipole-dipole interaction in the lattice decays exponentially with the inter-particle separation at short to medium distance on the lattice scale and has a long-range power-law tail, in contrast to the pure power-law behavior of the dipole-dipole interaction in free space. The effect can be sizable; we identify differences of up to 36% from the free-space interaction at the nearest-neighbor distance in quasi-1D arrangements. The modified interaction arises from quantum fluctuations induced by heavy tails of the localized single-particle probability distributions, and also relies crucially on imbalance in confinement, due to the d-wave anisotropy of the dipole-dipole interaction. Using matrix product state simulations, we demonstrate that use of the correct lattice dipolar interaction leads to significant deviations from many-body predictions using the free-space interaction in the lattice. Our results are relevant to up and coming experiments with heteronuclear molecules, Rydberg atoms, and strongly magnetic atoms in optical lattices.
arXiv:1303.1313 (cross-list from quant-ph) [pdf, other]
Quantum metrology with a scanning probe atom interferometer
Caspar F. Ockeloen, Roman Schmied, Max F. Riedel, Philipp Treutlein
We use a small atomic Bose-Einstein condensate as an interferometric scanning probe to map out a microwave field near a chip surface with a few micrometers resolution. Using entanglement between the atoms we overcome the standard quantum limit of interferometry by 4 dB and maintain enhanced performance for interrogation times up to 20 ms. This demonstrates the usefulness of quantum metrology with entangled states when the particle number is limited due to the small probe size. Extending atom interferometry to micrometer spatial resolution enables new applications in electromagnetic field sensing, surface science, and the search for fundamental short-range interactions.
Mar 6
arXiv:1303.1137 [pdf, ps, other]
Observation of Transient Momentum-Space Interference During Scattering of a Condensate From an Optical Barrier
Rockson Chang, Shreyas Potnis, Christopher W. Ellenor, Mirco Siercke, Alex Hayat, Aephraim M. Steinberg
Scattering theory traditionally deals with the asymptotic behaviour of a system far removed from the actual scattering event. Here we present an experimental study of the one-dimensional scattering of a non-interacting condensate of 87-Rb atoms from a potential barrier in the non-asymptotic regime, for which the collision dynamics are still ongoing. We show that for near-transparent barriers, there are two distinct transient scattering effects that arise and dramatically change the momentum distribution during the collision: a deceleration of wavepacket components in mid-collision, and an interference between incident and transmitted portions of the wavepacket. Both effects lead to the re-distribution of momenta giving rise to a rich interference pattern that can be used to perform reconstruction of the single-particle phase profile.
arXiv:1303.1061 [pdf, other]
Direct measurement of topological invariants in optical lattices
Lei Wang, Alexey A. Soluyanov, Matthias Troyer
We propose an experimental technique for classifying the topology of band structures realized in optical lattices, based on a generalization of topological charge pumping in quantum Hall systems to cold atom in optical lattices. Time-of-flight measurement along one spatial direction combined with in situ detection along the transverse direction provide a direct measure of the system's Chern number, as we illustrate by calculations for the Hofstadter lattice. Based on an analogy with Wannier functions techniques of topological band theory, the method is very general and also allows the measurement of other topological invariants, such as the $Z_2$ topological invariant of time-reversal symmetric insulators.
arXiv:1303.1030 [pdf, other]
Integrated Mach-Zehnder interferometer for Bose-Einstein condensates
Tarik Berrada, Sandrine van Frank, Robert Bücker, Thorsten Schumm, Jean-François Schaff, Jörg Schmiedmayer
Quantum mechanical particle-wave duality enables the construction of interferometers for matter waves, which may complement lasers in precision measurement devices such as gravimeters or gyroscopes. This requires the development of atom-optics analogs to beam splitters, phase shifters, and recombiners. Realizing and integrating these elements into a transportable device has been a long-standing goal. Here we demonstrate the realization of a full Mach-Zehnder sequence with trapped atomic Bose-Einstein condensates (BEC) confined on an atom chip. Particle interactions in our BEC matter waves lead to an intrinsic non-linearity, absent in photon optics. We exploit these interactions to generate a non-classical state with reduced number fluctuations as an input to the interferometer. Making use of spatially separated wave packets, a controlled phase shift is applied and read out by a diabatic matter-wave recombiner. We demonstrate coherence times a factor of three beyond what is expected for a classical coherent state, highlighting the potential of entanglement as a resource for metrology. Our results pave the way towards integrated quantum-enhanced matter-wave sensors.
arXiv:1303.1027 [pdf, other]
Dymamical Casimir emission from polariton condensates
Selma Koghee, Michiel Wouters
We study theoretically the dynamical Casimir effect in an exciton-polariton condensate that is suddenly created by an ultrashort laser pulse at normal incidence. As a consequence of the abrupt change of the quantum vacuum, Bogoliubov excitations are generated. The subsequent evolution, governed by polariton interactions and losses, is studied within a linearized truncted Wigner approximation. We focus in particular on the momentum distribution and spatial coherence. The limiting behavior at large and small momenta is determined analytically. A simple scaling relation for the final condensate depletion as a function of the system parameters is found and the correlation length is shown to depend linearly on the condensate depletion.
arXiv:1303.0914 [pdf, other]
Direct observation of Zitterbewegung in a Bose-Einstein condensate
L. J. LeBlanc, M. C. Beeler, K. Jimenez-Garcia, A. R. Perry, S. Sugawa, R. A. Williams, I. B. Spielman
Zitterbewegung, a force-free trembling motion first predicted for relativistic fermions like electrons, was an unexpected consequence of the Dirac equation's unification of quantum mechanics and special relativity. Though the oscillatory motion's large frequency and small amplitude have precluded its measurement with electrons, zitterbewegung is observable via quantum simulation. We engineered an environment for 87Rb Bose-Einstein condensates where the constituent atoms behaved like relativistic particles subject to the one-dimensional Dirac equation. With direct imaging, we observed the sub-micrometer trembling motion of these clouds, demonstrating the utility of neutral ultracold quantum gases for simulating Dirac particles.
Mar 5
1. arXiv:1303.0747 [pdf, ps, other]
Hall response of interacting bosonic atoms in strong gauge fields: from condensed to FQH states
H. Pino, E. Alba, J. Taron, J. J. Garcia-Ripoll, N. Barberan
Interacting bosonic atoms under strong gauge fields undergo a series of phase transitions that take the cloud from a simple Bose-Einstein condensate all the way to a family of fractional quantum Hall states [M. Popp et al, Phys. Rev. A 70, 053612 (2004)]. In this work we demonstrate that the Hall response of the atoms can be used to locate the phase transitions and characterize the ground state of the many-body state. Moreover, the same response function reveals within some regions of the parameter space, the structure of the spectrum and the allowed transitions to excited states. We verify numerically these ideas using exact diagonalization for a small number of atoms, and provide an experimental protocol to implement the gauge fields and probe the linear response using a periodically driven optical lattice. Finally, we discuss our theoretical results in relation to recent experiments with condensates in artificial magnetic fields [L. J. LeBlanc et al, Proc. Natl. Acad. Sci. USA 109,10811 (2012)] and in agreement to their results verify that the existence of vortex states decreases the Hall response.
2. arXiv:1303.0640 [pdf, other]
Topological stirring of two-dimensional atomic Bose-Einstein condensates
Angela White, Nick Proukakis, Carlo Barenghi
We stir vortices into a trapped quasi two-dimensional atomic Bose-Einstein condensate by moving three laser stirrers. We apply stirring protocols introduced by Boyland et. al. (2000) that efficiently build in topological chaos in classical fluids and are classified as Pseudo-Anosov stirring protocols. These are compared to their inefficient mixing counterparts, finite-order stirring protocols. We investigate if inefficient stirring protocols result in a more clustered distribution of vortices. The efficiency with which vortices are `mixed' or distributed in a condensate is important for investigating dynamics of continuously forced quantum turbulence and the existence of the inverse cascade in turbulent two-dimensional superfluids.
3. arXiv:1303.0516 [pdf, ps, other]
Critical correlations in an ultracold Bose gas revealed by means of a temporal Talbot-Lau interferometer
Wei Xiong, Xiaoji Zhou, Xuguang Yue, Xuzong Chen, Biao Wu, Hongwei Xiong
We study experimentally the critical correlation in an ultra-cold Bose gas with a temporal Talbot-Lau (TL) interferometer. Near the critical temperature, we observe a bi-modal density distribution in an ultra-cold Bose gas after the application of the TL interferometer. The measured fraction of the narrower peak in the density distribution displays a clear peak within the critical regime. The peak position agrees with the critical temperature calculated with the finite-size and interaction corrections. The critical exponents are extracted from the peak and they agree with the critical exponents for the correlation length.
4. arXiv:1303.0782 [pdf, other]
Statistics of the work done by splitting a one-dimensional condensate
Spyros Sotiriadis, Andrea Gambassi, Alessandro Silva
Motivated by experiments on splitting one-dimensional condensates, we study the statistics of the work done by a quantum quench in a bosonic system. We discuss the general features of the probability distribution of the work and focus on its behaviour at the lowest energy threshold, which develops an edge singularity. A formal connection between this probability distribution and the critical Casimir effect in thin classical films shows that certain features of the edge singularity are universal as the post-quench gap tends to zero. Our results are quantitatively illustrated by an exact calculation for non-interacting bosonic systems. The effects of finite system size, dimensionality, and non-zero initial temperature are discussed in detail.
5. arXiv:1303.0299 [pdf, ps, other]
Limit of spin squeezing in trapped Bose-Einstein condensates
Alice Sinatra, Yvan Castin, Emilia Witkowska
The evolution of an interacting two-component Bose-Einstein condensate from an initial phase state leads to a spin squeezed state that may be used in atomic clocks to increase the signal-to-noise ratio, opening the way to quantum metrology. The efficiency of spin squeezing is limited by the finite temperature of the gas, as was shown theoretically in a spatially homogeneous system. Here we determine the limit of spin squeezing in the realistic trapped case, with classical field simulations, and with a completely analytical treatment that includes the quantum case.
Mar 4
1. arXiv:1303.0129 [pdf, ps, other]
Modeling the transport of interacting matter-waves in disorder by a non-linear diffusion equation
Eleonora Lucioni, Luca Tanzi, Chiara D'Errico, Marco Moratti, Massimo Inguscio, Giovanni Modugno
We model the expansion of an interacting atomic Bose-Einstein condensate in a disordered lattice with a nonlinear diffusion equation normally used for a variety of classical systems. We find approximate solutions of the diffusion equation that well reproduce the experimental observations for both short and asymptotic expansion times. Our study establishes a connection between the peculiar shape of the expanding density profiles and the microscopic nonlinear diffusion coefficients.
