Aug 26-Aug 30, Saubhik Sarkar
Aug 26
1.arXiv:1308.4975 [pdf, ps, other]
Driven dipole oscillations and the lowest energy excitations of lattice bosons in a harmonic trap
Kai He, Jennifer Brown, Stephan Haas, Marcos Rigol
We show that the analysis of the time evolution of the occupation of site and momentum modes of harmonically trapped lattice hard-core bosons, under driven dipole oscillations, allows one to determine the energy of the lowest one-particle excitations of the system in equilibrium. The analytic solution of a single particle in the absence of a lattice is used to identify which function of those time-dependent observables is best fit for the analysis, as well as to relate the dynamic response of the system to its single-particle spectrum. In the presence of the lattice and of multiple particles, a much richer and informative dynamical response is observed under the drive.
2.arXiv:1301.1342 [pdf, ps, other]
Quantum simulation of many-body spin interactions with ultracold polar molecules
Hendrik Weimer
We present an architecture for the quantum simulation of many-body spin interactions based on ultracold polar molecules trapped in optical lattices. Our approach employs digital quantum simulation, i.e., the dynamics of the simulated system is reproduced by the quantum simulator in a stroboscopic pattern, and allows to simulate both coherent and dissipative dynamics. We discuss the realization of Kitaev's toric code Hamiltonian, a paradigmatic model involving four-body interactions, and we analyze the requirements for an experimental implementation.
Aug 27
1.arXiv:1308.5387 [pdf, other]
Entanglement of spin-orbit qubits induced by Coulomb interaction
Y. N. Fang, Yusuf Turek, J. Q. You, C. P. Sun
Spin-orbit qubit (SOQ) is the dressed spin by the orbital degree of freedom through a strong spin-orbit coupling. We show that Coulomb interaction between two electrons in quantum dots located separately in two nanowires can efficiently induce quantum entanglement between two SOQs. The physical mechanism to achieve such quantum entanglement is based on the feasibility of the SOQ responding to the external electric field via an intrinsic electric dipole spin resonance.
2.arXiv:1306.2519 [pdf, ps, other]
Effects of non-equilibrium noise on a quantum memory encoded in Majorana zero modes
François Konschelle, Fabian Hassler
In order to increase the coherence time of topological quantum memories in systems with Majorana zero modes, it has recently been proposed to encode the logical qubit states in non-local Majorana operators which are immune to localized excitations involving the unpaired Majorana modes. In this encoding, a logical error only happens when the quasi-particles, subsequent to their excitation, travel a distance of the order of the spacing between the Majorana modes. Here, we study the decay time of a quantum memory encoded in a clean topological nanowire interacting with an environment with a particular emphasis on the propagation of the quasi-particles above the gap. We show that the non-local encoding does not provide a significantly longer coherence time than the local encoding. In particular, the characteristic speed of propagation is of the order of the Fermi velocity of the nanowire and not given by the much slower group velocity of quasi-particles which are excited just above the gap.
Aug 28
1.arXiv:1308.5680 [pdf, other]
Sudden and slow quenches into the antiferromagnetic phase of ultracold fermions
M. Ojekhile, R. Höppner, H. Moritz, L. Mathey
We propose a method to reach the antiferromagnetic state of two-dimensional Fermi gases trapped in optical lattices: Independent subsystems are prepared in suitable initial states and then connected by a sudden or slow quench of the tunneling between the subsystems. Examples of suitable low-entropy subsystems are double wells or plaquettes, which can be experimentally realized in Mott insulating shells using optical super-lattices. We estimate the effective temperature T* of the system after the quench by calculating the distribution of excitations created using the spin wave approximation in a Heisenberg model. We investigate the effect of an initial staggered magnetic field and find that for an optimal polarization of the initial state the effective temperature can be significantly reduced from T* approximately T_c at zero polarization to T*<0.65 T_c, where T_c is the crossover temperature to the antiferromagnetic state. The temperature can be further reduced by using a finite quench time. We also show that T* decreases logarithmically with the linear size of the subsystem.
2.arXiv:1308.5909 [pdf, other]
Quantum phases of 1D Hubbard models with three- and four-body couplings
Fabrizio Dolcini, Arianna Montorsi
The experimental advances in cold atomic and molecular gases stimulate the investigation of lattice correlated systems beyond the conventional on-site Hubbard approximation, by possibly including multi-particle processes. We study fermionic extended Hubbard models in a one dimensional lattice with different types of particle couplings, including also three- and four-body interaction up to nearest neighboring sites. By using the Bosonization technique, we investigate the low-energy regime and determine the conditions for the appearance of ordered phases, for arbitrary particle filling. We find that three- and four-body couplings may significantly modify the phase diagram. In particular, diagonal three-body terms that directly couple the local particle densities have qualitatively different effects from off-diagonal three-body couplings originating from correlated hopping, and favor the appearance of a Luther-Emery phase even when two-body terms are repulsive. Furthermore, the four-body coupling gives rise to a rich phase diagram and may lead to the realization of the Haldane insulator phase at half-filling.
Aug 29
1.arXiv:1308.6211 [pdf, other]
Phases of correlated spinless fermions on the honeycomb lattice
Maria Daghofer, Martin Hohenadler
We use exact diagonalization and cluster perturbation theory to address the role of strong interactions and quantum fluctuations for spinless fermions on the honeycomb lattice. We find quantum fluctuations to be very pronounced both at weak and strong interactions. A weak second-neighbor Coulomb repulsion $V_2$ induces a tendency toward an interaction-generated quantum anomalous Hall phase, as borne out in mean-field theory. However, quantum fluctuations prevent the formation of a stable quantum Hall phase before the onset of the charge-modulated phase predicted at large $V_2$ by mean-field theory. Consequently, the system undergoes a direct transition from the semimetal to the charge-modulated phase. For the latter, charge fluctuations also play a key role. While the phase, which is related to pinball liquids, is stabilized by the repulsion $V_2$, the energy of its low-lying charge excitations scales with the kinetic energy $t$, as in a band insulator.
2.arXiv:1306.5515 [pdf, ps, other]
Interacting spin-1 bosons in a two-dimensional optical lattice
L. de Forges de Parny, F. Hébert, V.G. Rousseau, G.G. Batrouni
We study, using quantum Monte Carlo (QMC) simulations, the ground state properties of spin-1 bosons trapped in a square optical lattice. The phase diagram is characterized by the mobility of the particles (Mott insulating or superfluid phase) and by their magnetic properties. For ferromagnetic on-site interactions, the whole phase diagram is ferromagnetic and the Mott insulators-superfluid phase transitions are second order. For antiferromagnetic on-site interactions, spin nematic order is found in the odd Mott lobes and in the superfluid phase. Furthermore, the superfluid-insulator phase transition is first or second order depending on whether the density in the Mott is even or odd. Inside the even Mott lobes, we observe a singlet-to-nematic transition for certain values of the interactions. This transition appears to be first order.
Aug 30
1.arXiv:1308.6349 [pdf, other]
Spin-orbit coupling and spin Hall effect for neutral atoms without spin-flips
Colin J. Kennedy, Georgios A. Siviloglou, Hirokazu Miyake, William Cody Burton, Wolfgang Ketterle
We propose a scheme which realizes spin-orbit coupling and the spin Hall effect for neutral atoms in optical lattices without relying on near resonant laser light to couple different spin states. The spin-orbit coupling is created by modifying the motion of atoms in a spin-dependent way by laser recoil. The spin selectivity is provided by Zeeman shifts created with a magnetic field gradient. Alternatively, a quantum spin Hamiltonian can be created by all-optical means using a period- tripling, spin-dependent superlattice.
2.arXiv:1206.4254 [pdf, other]
Matrix Product States with long-range Localizable Entanglement
Thorsten B. Wahl, David Perez-Garcia, J. Ignacio Cirac
We derive a criterion to determine when a translationally invariant matrix product state (MPS) has long-range localizable entanglement, where that quantity remains finite in the thermodynamic limit. We give examples fulfilling this criterion and eventually use it to obtain all such MPS with bond dimension 2 and 3.
Aug 19-Aug 23, Li-Jun Lang
Aug 19
1. arXiv:1308.3626 [pdf, other]
Tunable critical supercurrent and spin-asymmetric Josephson effect in superlattices
J. M. Kreula, M. O. J. Heikkinen, F. Massel, P. Törmä
Combining the Josephson effect with magnetism, or spin-dependence in general, creates intriguing, novel physical phenomena. Examples include pi-junctions, spin-triplet Cooper pair current, and the predicted enhancement, or tunability, of the critical direct current (DC) in ferromagnetic Josephson junctions. Another, striking prediction concerning the Josephson effect in the presence of spin-dependence is the existence of a spin-asymmetric Josephson effect. In this phenomenon, a spin-dependent potential is applied across a Josephson junction, inducing a spin-polarized Josephson current. Here, we propose a new approach to experimentally realize this yet-unobserved effect with spin-dependent superlattices, e.g. in ultracold atomic gas systems. We demonstrate that the observation of the spin-asymmetric Josephson effect is feasible with existing experimental techniques by studying numerically the quantum dynamics of the system in one dimension (1D). We further explain the physical origin of the tunable DC supercurrent in superconductor-ferromagnet structures by the spin-asymmetric Josephson effect.
2. arXiv:1308.3696 [pdf, ps, other]
Universal dynamics of a degenerate unitary Bose gas
Philip Makotyn, Catherine E. Klauss, David L. Goldberger, Eric. A. Cornell, Deborah S. Jin
Understanding the rich behavior that emerges from systems of interacting quantum particles, such as electrons in materials, nucleons in nuclei or neutron stars, the quark-gluon plasma, and superfluid liquid helium, requires investigation of systems that are clean, accessible, and have tunable parameters. Ultracold quantum gases offer tremendous promise for this application largely due to an unprecedented control over interactions. Specifically, $a$, the two-body scattering length that characterizes the interaction strength, can be tuned to any value. This offers prospects for experimental access to regimes where the behavior is not well understood because interactions are strong, atom-atom correlations are important, mean-field theory is inadequate, and equilibrium may not be reached or perhaps does not even exist. Of particular interest is the unitary gas, where $a$ is infinite, and where many aspects of the system are universal in that they depend only on the particle density and quantum statistics. While the unitary Fermi gas has been the subject of intense experimental and theoretical investigation, the degenerate unitary Bose gas has generally been deemed experimentally inaccessible because of three-body loss rates that increase dramatically with increasing $a$. Here, we investigate dynamics of a unitary Bose gas for timescales that are short compared to the loss. We find that the momentum distribution of the unitary Bose gas evolves on timescales fast compared to losses, and that both the timescale for this evolution and the limiting shape of the momentum distribution are consistent with universal scaling with density. This work demonstrates that a unitary Bose gas can be created and probed dynamically, and thus opens the door for further exploration of this novel strongly interacting quantum liquid.
Aug 20
1. arXiv:1308.3969 [pdf, other]
Topological superconducting phase in helical Shiba chains
Falko Pientka, Leonid Glazman, Felix von Oppen
Comments: 14 pages, 9 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Superconductivity (cond-mat.supr-con)
Recently, it has been suggested that topological superconductivity and Majorana end states can be realized in a chain of magnetic impurities on the surface of an s-wave superconductor when the magnetic moments form a spin helix as a result of the RKKY interaction mediated by the superconducting substrate. Here, we investigate this scenario theoretically by developing a tight-binding Bogoliubov-de Gennes description starting from the Shiba bound states induced by the individual magnetic impurities. While the resulting model Hamiltonian has similarities with the Kitaev model for one-dimensional spinless p-wave superconductors, there are also important differences, most notably the long-range nature of hopping and pairing as well as the complex hopping amplitudes. We use both analytical and numerical approaches to explore the consequences of these differences for the phase diagram and the localization properties of the Majorana end states when the Shiba chain is in a topological superconducting phase.
2. arXiv:1308.4066 [pdf, other]
Supermetallic and Trapped States in Periodically Kicked Lattices
Indubala I Satija, Bala Sundaram
Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
A periodically driven lattice with two commensurate spatial periodicities is found to exhibit super metallic states characterized by enhancements in wave packet spreading and entropy. These resonances occur at critical values of parameters where multi-band dispersion curves reduce to a universal function that is topologically a circle and the effective quantum dynamics describes free propagation. Sandwiching every resonant state are a pair of anti-resonant {\it trapped states} distinguished by dips in entropy where the transport, as seen in the spreading rate, is only somewhat inhibited. Existing in gapless phases fo the spectrum, a sequence of these peaks and dips are interspersed by gapped phases assocated with flat band states where both the wave packet spreading as well as the entropy exhibit local minima.
3. arXiv:1308.3812 [pdf, ps, other]
Normal mass density of a superfluid Fermi gas at unitarity
Gordon Baym, C.J. Pethick
Comments: 6 pages, 2 figuresSubjects: Quantum Gases (cond-mat.quant-gas)
We calculate the normal mass density of a paired Fermi gas at unitarity. The dominant contribution near the superfluid transition is from fermionic quasiparticle excitations, and is thus sensitive to the pairing gap. A comparison with the recent experiment of Sidorenkov et al. suggests that the superfluid gap near the transition temperature is larger than the BCS value, but the data do not permit a quantitative inference of the gap. Calculations of the quenched moment of inertia of a BCS superfluid in a harmonic trap are in reasonable agreement with the earlier experiment of Riedl et al.
Aug 21
1. arXiv:1308.4382 [pdf, other]
Three-dimensional dynamics of a fermionic Mott wedding-cake in clean and disordered optical lattices
A. Kartsev, D. Karlsson, A. Privitera, C. Verdozzi
Comments: 13 pages, 3 figures, to appear in Sci. RepSubjects: Quantum Gases (cond-mat.quant-gas); Disordered Systems and Neural Networks (cond-mat.dis-nn); Strongly Correlated Electrons (cond-mat.str-el)
Non-equilibrium quantum phenomena are ubiquitous in nature. Yet, theoretical predictions on the real-time dynamics of many-body quantum systems remain formidably challenging, especially for high dimensions, strong interactions or disordered samples. Here we consider a notable paradigm of strongly correlated Fermi systems, the Mott phase of the Hubbard model, in a setup resembling ultracold-gases experiments. We study the three-dimensional expansion of a cloud into an optical lattice after removing the confining potential. We use time-dependent density-functional theory combined with dynamical mean-field theory, considering interactions below and above the Mott threshold, as well as disorder effects. At strong coupling, we observe multiple timescales in the melting of the Mott wedding-cake structure, as the Mott plateau persist orders of magnitude longer than the band insulating core. We also show that disorder destabilises the Mott plateau and that, compared to a clean setup, localisation can decrease, creating an interesting dynamic crossover during the expansion.
2. arXiv:1308.4336 [pdf, ps, other]
Dynamics of a two-dimensional quantum spin liquid: signatures of emergent Majorana fermions and fluxes
J. Knolle, D. L. Kovrizhin, J. T. Chalker, R. Moessner
Comments: 7 pages, 3 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)
Topological states of matter present a wide variety of striking new phenomena. Prominent among these is the fractionalisation of electrons into unusual particles: Majorana fermions [1], Laughlin quasiparticles [2] or magnetic monopoles [3]. Their detection, however, is fundamentally complicated by the lack of any local order, such as, for example, the magnetisation in a ferromagnet. While there are now several instances of candidate topological spin liquids [4], their identification remains challenging [5]. Here, we provide a complete and exact theoretical study of the dynamical structure factor of a two-dimensional quantum spin liquid in gapless and gapped phases. We show that there are direct signatures - qualitative and quantitative - of the Majorana fermions and gauge fluxes emerging in Kitaev's honeycomb model. These include counterintuitive manifestations of quantum number fractionalisation, such as a neutron scattering response with a gap even in the presence of gapless excitations, and a sharp component despite the fractionalisation of electron spin. Our analysis identifies new varieties of the venerable X-ray edge problem and explores connections to the physics of quantum quenches.
3. arXiv:1308.4266 [pdf, other]
Floquet Weyl semimetal induced by topological phase transitions
Rui Wang, Baigeng Wang, Rui Shen, L.Sheng, D.Y. Xing, Sergey Y. Savrasov
Comments: 5 pages, 4 figuresSubjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Materials Science (cond-mat.mtrl-sci)
We investigate the effect of a time--periodical electromagnetic field on magnetic 3D topological insulators by using Floquet theory. It is found that the external field can change the topology of the undriven system by renormalizing its Dirac mass, leading to a new Weyl semimetal phase. A diamond lattice model is further studied to confirm the above results, where topological phase transitions between quantum anomalous Hall and Weyl semimetal phases take place due to the periodical perturbation.
Aug 22
1. arXiv:1308.4401 [pdf, other]
Artificial graphene with tunable interactions
Thomas Uehlinger, Gregor Jotzu, Michael Messer, Daniel Greif, Walter Hofstetter, Ulf Bissbort, Tilman Esslinger
Comments: 11 pages, 10 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Strongly Correlated Electrons (cond-mat.str-el)
We create an artificial graphene system with tunable interactions and study the crossover from metallic to Mott insulating regimes, both in isolated and coupled two-dimensional honeycomb layers. The artificial graphene consists of a two-component spin mixture of an ultracold atomic Fermi gas loaded into a hexagonal optical lattice. For strong repulsive interactions we observe a suppression of double occupancy and measure a gapped excitation spectrum. We present a quantitative comparison between our measurements and theory, making use of a novel numerical method to obtain Wannier functions for complex lattice structures. Extending our studies to time-resolved measurements, we investigate the equilibration of the double occupancy as a function of lattice loading time.
2. arXiv:1308.4449 [pdf, other]
Formation and decay of Bose-Einstein condensates in an excited band of a double-well optical lattice
Saurabh Paul, Eite Tiesinga
Comments: 12 pages, 11 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph)
We study the formation and collision-aided decay of an ultra-cold atomic Bose-Einstein condensate in the first excited band of a double-well 2D-optical lattice with weak harmonic confinement in the perpendicular $z$ direction. This lattice geometry is based on an experiment by Wirth et al. The double well is asymmetric, with the local ground state in the shallow well nearly degenerate with the first excited state of the adjacent deep well. We compare the band structure obtained from a tight-binding (TB) model with that obtained numerically using a plane wave basis. We find the TB model to be in quantitative agreement for the lowest two bands, qualitative for next two bands, and inadequate for even higher bands. The band widths of the excited bands are much larger than the harmonic oscillator energy spacing in the $z$ direction. We then study the thermodynamics of a non-interacting Bose gas in the first excited band. We estimate the condensate fraction and critical temperature, $T_c$, as functions of lattice parameters. For typical atom numbers, the critical energy $k_BT_c$, with $k_B$ the Boltzmann constant, is larger than the excited band widths and harmonic oscillator energy. Using conservation of total energy and atom number, we show that the temperature increases after the lattice transformation. Finally, we estimate the time scale for a two-body collision-aided decay of the condensate as a function of lattice parameters. The decay involves two processes, the dominant one in which both colliding atoms decay to the ground band, and the second involving excitation of one atom to a higher band. For this estimate, we have used TB wave functions for the lowest four bands, and numerical estimates for higher bands. The decay rate rapidly increases with lattice depth, but stays smaller than the tunneling rate between the $s$ and $p$ orbitals in adjacent wells.
Aug 23
1. arXiv:1308.4699 [pdf, ps, other]
Correlation dynamics during a slow interaction quench in a one-dimensional Bose gas
Jean-Sebastien Bernier, Roberta Citro, Corinna Kollath, Edmond Orignac
Comments: 5 pages, 2 figures, + supplementary material
Subjects: Quantum Gases (cond-mat.quant-gas)
We investigate the response of a one-dimensional Bose gas to a slow increase of its interaction strength. We focus on the rich dynamics of equal-time single-particle correlations treating the Lieb-Liniger model within a bosonization approach and the Bose-Hubbard model using the time-dependent density-matrix renormalization group method. For short distances, correlations follow a power-law with distance with an exponent given by the adiabatic approximation. In contrast, for long distances, correlations decay algebraically with an exponent understood within the sudden quench approximation. This long distance regime is separated from an intermediate distance one by a generalized Lieb-Robinson criterion. At long times, in this intermediate regime, bosonization predicts that single-particle correlations decay following a stretched exponential. This latter regime is unconventional as, for one-dimensional interacting systems, the decay of single-particle correlations is usually algebraic within the Luttinger liquid picture. Our results may serve as a benchmark for future experimental studies of breakdowns of conventional many-body dynamics in one-dimensional systems.
2. arXiv:1308.4876 [pdf, other]
Variational study of polarons and bipolarons in a 1D Bose lattice gas in both superfluid and Mott regimes
Shovan Dutta, Erich J. Mueller
Comments: 9 pages, 7 figures (13 subfigures)
Subjects: Quantum Gases (cond-mat.quant-gas)
We use variational methods to study a spin impurity in a 1D Bose lattice gas. Both in the strongly interacting superfluid regime and the Mott regime we find that the impurity binds with a hole, forming a polaron. Our calculations for the dispersion of the polaron are consistent with recent experiments by Fukuhara et. al. [Nature Phys. 9, 235 (2013)] and give a better understanding of their numerical simulations. We find that for sufficiently weak interactions there are ranges of momentum for which the polaron is unstable. We propose experimentally studying the stability of the polaron by measuring the correlation between the impurity and holes. We also study two interacting impurities, finding stable bipolarons for sufficiently strong interactions
3. arXiv:1308.4900 [pdf, ps, other]
Topological Invariants and Ground States Wavefunctions of Topological Insulators on a Torus
Zhong Wang, Shou-Cheng Zhang
Comments: 8 pages
Subjects: Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th); Mathematical Physics (math-ph)
We define topological invariants in terms of the ground states wavefunctions on a torus. This approach leads to precisely defined formulas for the Hall conductance in four dimensions and the topological magneto-electric $\theta$ term in three dimensions, and their generalizations in higher dimensions. They are valid in the presence of arbitrary many-body interaction and disorder. These topological invariants systematically generalize the two-dimensional Niu-Thouless-Wu formula, and will be useful in numerical calculations of disordered topological insulators and strongly correlated topological insulators including fractional topological insulators.
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Aug 12-Aug 16, Jin-Long Yu
Aug 16
1. arXiv:1308.3463[pdf, other]
Field-induced superfluids and Bose liquids in Projected Entangled Pair States
Didier Poilblanc, Norbert Schuch, J. Ignacio Cirac
In two-dimensional incompressible quantum spin liquids, a large enough magnetic field generically induces "doping" of polarized S=1 triplons or S=1/2 spinons. We review a number of cases such as spin-3/2 AKLT or spin-1/2 Resonating Valence Bond (RVB) liquids where the Projected Entangled Pair States (PEPS) framework provides very simple and comprehensive pictures. On the bipartite honeycomb lattice, simple PEPS can describe Bose condensed triplons (AKLT) or spinons (RVB) superfluids with transverse staggered (Neel) magnetic order. On the Kagome lattice, doping the RVB state with deconfined spinons or triplons (i.e. spinon bound pairs) yields uncondensed Bose liquids preserving U(1) spin-rotation symmetry. We find that spinon (triplon) doping destroys (preserves) the topological Z_2 symmetry of the underlying RVB state. We also find that the boundary Hamiltonian of the spinon liquid is long-range, suggesting the existence of (additive) log-corrections to the entanglement entropy.
2. arXiv:1308.3237 [pdf, other]
Chiral Bosonic Mott Insulator on the Frustrated Triangular Lattice
Michael P. Zaletel, S. A. Parameswaran, Andreas Rüegg, Ehud Altman
We study the superfluid and insulating phases of interacting bosons on the triangular lattice with an inverted dispersion, corresponding to frustrated hopping between sites. The resulting single-particle dispersion has multiple minima at nonzero wavevectors in momentum space, in contrast to the unique zero-wavevector minimum of the unfrustrated problem. As a consequence, the superfluid phase is unstable against developing additonal chiral order that breaks time reversal (T) and parity (P) symmetries by forming a condensate at nonzero wavevector. We demonstrate that the loss of superfluidity can lead to an even more exotic phase, the chiral Mott insulator, with nontrivial current order that breaks T, P. These results are obtained via variational estimates, as well as a combination of bosonization and DMRG of triangular ladders, which taken together permit a fairly complete characterization of the phase diagram. We discuss the relevance of these phases to optical lattice experiments, as well as signatures of chiral symmetry breaking in time-of-flight images.
Aug 15
1. arXiv:1308.2994 [pdf, ps, other]
Inelastic Collisions of Solitary Waves in Anisotropic Bose-Einstein Condensates: Sling-Shot Events and Expanding Collision Bubbles
C. Becker, K. Sengstock, P. Schmelcher, P.G. Kevrekidis, R. Carretero-Gonzalez
We study experimentally and theoretically the dynamics of apparent dark soliton stripes in an elongated Bose-Einstein condensate. We show that for the trapping strengths corresponding to our experimental setup, the transverse confinement along one of the tight directions is not strong enough to arrest the formation of solitonic vortices or vortex rings. These solitonic vortices and vortex rings, when integrated along the transverse direction, appear as dark soliton stripes along the longitudinal direction thereby hiding their true character. The latter significantly modifies the interaction dynamics during collision events and can lead to apparent examples of inelasticity and what may appear experimentally even as a merger of two dark soliton stripes. We explain this feature by means of the interaction of two solitonic vortices leading to a sling shot event with one of the solitonic vortices being ejected at a relatively large speed. Furthermore we observe expanding collision bubbles which consist of repeated inelastic collisions of a dark soliton stripe pair with an {\it increasing} time interval between collisions.
2. arXiv:1308.3208 [pdf, ps, other]
SU(3) orbital Kondo effect with ultracold atoms
Yusuke Nishida
We propose a simple but novel scheme to realize the Kondo effect with ultracold atoms. Our system consists of a Fermi sea of spinless fermions interacting with an impurity atom of different species which is confined by an isotropic potential. The interspecies attraction can be tuned with an s-wave Feshbach resonance so that the impurity atom and a spinless fermion form a bound dimer that occupies a threefold-degenerate p-orbital of the confinement potential. Many-body scatterings of this dimer and surrounding spinless fermions occur with exchanging their angular momenta and thus exhibit the SU(3) orbital Kondo effect. The associated Kondo temperature has a universal leading exponent given by T_K\propto\exp[-\pi/(3a_p*kF^3)] that depends only through an effective p-wave scattering volume a_p and a Fermi wave vector kF. We also elucidate a Kondo singlet formation at zero temperature and an anisotropic interdimer interaction mediated by surrounding spinless fermions. The Kondo effect thus realized in ultracold atom experiments may be observed as an increasing atom loss by lowering the temperature or with the radio-frequency spectroscopy. Our scheme and its extension to a dense Kondo lattice will be useful to develop new insights into yet unresolved aspects of the Kondo physics.
Aug 14
1. arXiv:1308.2684 [pdf, other]
Strongly correlated bosons and fermions in optical lattices
Antoine Georges, Thierry Giamarchi
These lectures are an introduction to the physics of strongly correlated fermions and bosons. They are specially targeted for the experimental realizations that have been provided by cold atomic gases in optical lattices.
2. arXiv:1308.2735 [pdf, ps, other]
Precision Measurements with Quantum Gases
Ugo Marzolino, Daniel Braun
We investigate the sensitivity with which the temperature and the chemical potential characterizing quantum gases can be measured. We calculate the corresponding quantum Fisher information matrices for both fermionic and bosonic gases. For the latter, particular attention is devoted to the situation close to the Bose-Einstein condensation transition, which we examine not only for the standard scenario in three dimensions, but also for generalized condensation in lower dimensions, where the bosons condense in a subspace of Hilbert space instead of a unique ground state, as well as condensation at fixed volume or fixed pressure. We show that Bose Einstein condensation can lead to sub-shot noise sensitivity for the measurement of the chemical potential. We also examine the influence of interactions on the sensitivity in three different models, and show that mean-field and contact interactions deteriorate the sensitivity but only slightly for experimentally accessible weak interactions.
Aug 13
1. arXiv:1308.2322 [pdf, ps, other]
Dynamical Casimir Effect in Superradiant Light Scattering by Bose-Einstein Condensate in an Optomechanical Cavity
Sonam Mahajan, Neha Aggarwal, Aranya B Bhattacherjee, ManMohan
We investigate the effects of dynamical Casimir effect in superradiant light scattering by Bose-Einstein condensate in an optomechanical cavity. The system is studied using both classical and quantized mirror motions. The cavity frequency is harmonically modulated in time for both the cases. The main quantity of interest is the number of intracavity scattered photons. The system has been investigated under the weak and strong modulation. It has been observed that the amplitude of the scattered photons is more for the classical mirror motion than the quantized mirror motion. Also, initially, the amplitude of scattered photons is high for lower modulation amplitude than higher modulation amplitude. We also found that the behaviour of the plots are similar under strong and weak modulation for the quantized mirror motion.
2. arXiv:1308.2229 [pdf, ps, other]
Ground state phase diagram of the 2d Bose-Hubbard model with anisotropic hopping
Janik Schönmeier-Kromer, Lode Pollet
We compute the ground state phase diagram of the 2d Bose-Hubbard model with anisotropic hopping using quantum Monte Carlo simulations, connecting the 1d to the 2d system. We find that the tip of the lobe lies on a curve controlled by the 1d limit over the full anisotropy range while the universality class is always the same as in the isotropic 2d system. This behavior can be derived analytically from the lowest RG equations and has a form typical for the underlying Kosterlitz-Thouless transition in 1d. We also compute the phase boundary of the Mott lobe for strong anisotropy and compare it to the 1d system. Our calculations shed light on recent cold gas experiments monitoring the dynamics of an expanding cloud.
Aug 12
1. arXiv:1308.1972 [pdf, other]
Edge Instabilities of Bosons in a Double-Well Optical Lattice
Ryan Barnett
In this work, we consider the dynamics of bosons in bands with non-trivial topological structure. In particular, we focus on the case where bosons are prepared in a higher-energy band and allowed to evolve. The Bogoliubov theory about the initial state can have a dynamical instability, and we show that it is possible to achieve the interesting situation where the topological edge modes are unstable while all bulk modes are stable. Thus, after the initial preparation, the edge modes will become rapidly populated. We illustrate this with the Su-Schrieffer-Heeger model which can be realized with a double-well optical lattice and is perhaps the simplest model with topological edge states. This work provides a direct physical consequence of topological bands whose properties are often not of immediate relevance for bosonic systems.
2. arXiv:1308.1961 [pdf, other]
Ferromagnetism of the Repulsive Atomic Fermi Gas: three-body recombination and domain formation
Ilia Zintchenko, Lei Wang, Matthias Troyer
The simplest model for itinerant ferromagnetism, the Stoner model, has so far eluded experimental observation in repulsive ultracold fermions due to rapid three-body recombination at large scattering lengths. Here we show that a ferromagnetic phase can be stabilised by imposing a moderate optical lattice. The reduced kinetic energy drop upon formation of a polarized phase in an optical lattice extends the ferromagnetic phase to smaller scattering lengths where three-body recombination is small enough to permit experimental detection of the phase. We also show, using time dependent density functional theory, that in such a setup ferromagnetic domains emerge rapidly from a paramagnetic initial state.
Aug 5- Aug 9, Stephan Langer
Aug 5
1. arXiv:1308.0343 [pdf, other]
Topological Flat Band Models and Fractional Chern Insulators
Emil J. Bergholtz, Zhao Liu
Topological insulators are accompanied by exotic edge states that are protected by a bulk single-particle band gap once the filled bands are characterized by non-trivial topological invariants. Interactions can have profound effects and lead to entirely new insulating phases, with an altogether much richer and less explored phenomenology, as is particularly clear in the case of partial filling of weakly dispersive bands. Most saliently, lattice generalizations of fractional quantum Hall states, dubbed fractional Chern insulators, have recently been predicted to be stabilized by interactions within nearly dispersionless bands with non-zero Chern number, $C$. Contrary to their continuum analogues, these states do not require an external magnetic field and may potentially persist at high temperatures, which make these systems very interesting in the context of applications such as topological quantum computation. This review recapitulates the basics of tight-binding models hosting nearly flat bands with non-trivial topology, $C\neq 0$, and summarizes the present understanding of interactions and strongly correlated phases within these bands. Emphasis is put on the analogy with continuum Landau level physics, as well as qualitatively new, lattice specific, aspects including Berry curvature fluctuations, competing instabilities as well as novel collective states of matter emerging in bands with $|C|>1$. Possible experimental realizations, including oxide interfaces and cold atom implementations as well as generalizations to flat bands characterized by other topological invariants are also discussed.
2. arXiv:1308.0528 [pdf, other]
Constrained dynamics via the Zeno effect in quantum simulation: Implementing non-Abelian lattice gauge theories with cold atoms
K. Stannigel, P. Hauke, D. Marcos, M. Hafezi, S. Diehl, M. Dalmonte, P. Zoller
We show how engineered classical noise can be used to generate constrained Hamiltonian dynamics in atomic quantum simulators of many-body systems, taking advantage of the continuous Zeno effect. After discussing the general theoretical framework, we focus on applications in the context of lattice gauge theories, where imposing exotic, quasi-local constraints is usually challenging. We demonstrate the effectiveness of the scheme for both Abelian and non-Abelian gauge theories, and discuss how engineering dissipative constraints substitutes complicated, non-local interaction patterns by global coupling to laser fields.
3. arXiv:1308.0453 [pdf, ps, other]
Entanglement generation in quantum networks of Bose-Einstein condensates
Alexey N. Pyrkov, Tim Byrnes
Two component (spinor) Bose-Einstein condensates (BECs) are considered as the nodes of an interconnected quantum network. Unlike standard single-system qubits, in a BEC the quantum information is duplicated in a large number of identical bosonic particles, thus can be considered to be a "macroscopic" qubit. One of the difficulties with such a system is how to effectively interact such qubits together in order to transfer quantum information and create entanglement. Here we propose a scheme of cavities containing spinor BECs coupled by optical fiber in order to achieve this task. We discuss entanglement generation and quantum state transfer between nodes using such macroscopic BEC qubits.
Aug 6
1. arXiv:1308.0922 [pdf, ps, other]
Fermionic condensation in ultracold atoms, nuclear matter and neutron stars
Luca Salasnich
We investigate the Bose-Einstein condensation of fermionic pairs in three different superfluid systems: ultracold and dilute atomic gases, bulk neutron matter, and neutron stars. In the case of dilute gases made of fermionic atoms the average distance between atoms is much larger than the effective radius of the inter-atomic potential. Here the condensation of fermionic pairs is analyzed as a function of the s-wave scattering length, which can be tuned in experiments by using the technique of Feshbach resonances from a small and negative value (corresponding to the Bardeen-Cooper-Schrieffer (BCS) regime of Cooper Fermi pairs) to a small and positive value (corresponding to the regime of the Bose-Einstein condensate (BEC) of molecular dimers), crossing the unitarity regime where the scattering length diverges. In the case of bulk neutron matter the s-wave scattering length of neutron-neutron potential is negative but fixed, and the condensate fraction of neutron-neutron pairs is studied as a function of the total neutron density. Our results clearly show a BCS-quasiunitary-BCS crossover by increasing the neutron density. Finally, in the case of neutron stars, where again the neutron-neutron scattering length is negative and fixed, we determine the condensate fraction as a function of the distance from the center of the neutron star, finding that the maximum condensate fraction appears in the crust of the neutron star.
2. arXiv:1308.0756 [pdf, other]
Hermitian and non-Hermitian thermal Hamiltonians
Adrian E. Feiguin, Israel Klich
Thermal density matrices can be described by a pure quantum state within the thermofield formalism. Here we show how to construct a class of Hamiltonians realizing a thermofield state as their ground state. These Hamiltonians are frustration-free, and can be Hermitian or non-Hermitian, allowing one to use ground-state methods to understand the thermodynamic properties of the system. In particular this approach gives an explicit mapping of thermal phase transitions into quantum phase transitions. In the non-Hermitian case, the quantum phase transition is not accompanied by a change in the spectrum of the Hamiltonian, which remains gapped. We illustrate these ideas for the classical 2D Ising model.
3. arXiv:1308.0603 [pdf, ps, other]
Realizing a Kondo-correlated state with ultracold atoms
Johannes Bauer, Christophe Salomon, Eugene Demler
We propose a novel realization of Kondo physics with ultracold atomic gases. It is based on a Fermi sea of two different hyperfine states of one atom species forming bound states with a different species, which is spatially confined in a trapping potential. We show that different situations displaying Kondo physics can be realized when Feshbach resonances between the species are tuned by a magnetic field and the trapping frequency is varied. We illustrate that a mixture of ${}^{40}$K and ${}^{23}$Na atoms can be used to generate a Kondo correlated state and that momentum resolved radio frequency spectroscopy can provide unambiguous signatures of the formation of Kondo resonances at the Fermi energy. We discuss how tools of atomic physics can be used to investigate open questions for Kondo physics, such as the extension of the Kondo screening cloud.
Aug 7
1. arXiv:1308.1091 [pdf, other]
Ferromagnetic response of a "high-temperature" quantum antiferromagnet
Xin Wang, Rajdeep Sensarma, Sankar Das Sarma
We study the finite temperature antiferromagnetic phase of the ionic Hubbard model in the strongly interacting limit using Quantum Monte Carlo based dynamical mean field theory (DMFT). We find that the ionic potential plays a dual role in determining the antiferromagnetic order. A small ionic potential (compared to Hubbard repulsion) increases the super-exchange coupling in the projected sector of the model, leading to an increase in the Neel temperature of the system. A large ionic potential leads to resonance between projected antiferromagnetically ordered configurations and density ordered configurations with double occupancies, thereby killing antiferromagnetism in the system. This novel way of degrading antiferromagnetism leads to spin polarization of the low energy single particle density of states. The dynamic response of the system thus mimics ferromagnetic behaviour, although the system is still an antiferromagnet in terms of the static spin order, thus creating a paradoxical situation of ferromagnetism by antiferromagnetism!
2. arXiv:1308.1226 [pdf, other]
Optomechanical self-structuring in cold atomic gases
Guillaume Labeyrie, Enrico Tesio, Pedro M. Gomes, Gian-Luca Oppo, William J. Firth, Gordon R. M. Robb, Aidan S. Arnold, Robin Kaiser, Thorsten Ackemann
The rapidly developing field of optomechanics aims at the combined control of optical and mechanical (solid-state or atomic) modes. In particular, laser cooled atoms have been used to exploit optomechanical coupling for self-organization in a variety of schemes where the accessible length scales are constrained by a combination of pump modes and those associated to a second imposed axis, typically a cavity axis. Here, we consider a system with many spatial degrees of freedom around a single distinguished axis, in which two symmetries - rotations and translations in the plane orthogonal to the pump axis - are spontaneously broken. We observe the simultaneous spatial structuring of the density of a cold atomic cloud and an optical pump beam. The resulting patterns have hexagonal symmetry. The experiment demonstrates the manipulation of matter by opto-mechanical self-assembly with adjustable length scales and can be potentially extended to quantum degenerate gases.
Aug 8
1. arXiv:1308.1648 [pdf, ps, other]
Transport Regimes in a Double Quantum Dot Device
L. Costa Ribeiro, I. J. Hamad, G. Chiappe, E. V. Anda
We analyze the transport properties of a double quantum dot device with both dots coupled to perfect conducting leads and to a finite chain of N non-interacting sites connecting both of them. The inter-dot chain strongly influences the transport across the system and the Local Density of States of the dots. We study the case of small number of sites, so that Kondo box effects are present, varying the coupling between the dots and the chain. For odd N and small coupling between the inter-dot chain and the dots, a state with two coexisting Kondo regimes develops: the bulk Kondo due to the quantum dots connected to leads and the one produced by the screening of the quantum dots spins by the spin in the finite chain at the Fermi level. As the coupling to the inter-dot chain increases, there is a crossover to a molecular Kondo effect, due to the screening of the molecule (formed by the finite chain and the quantum dots) spin by the leads. For even N the two-Kondo temperatures regime does not develop and the physics is dominated by the usual competition between Kondo and antiferromagnetism between the quantum dots. We finally study how the transport properties are affected as N is increased. For the study we used exact multi-configurational Lanczos calculations and finite U slave-boson mean-field theory at T = 0. The results obtained with both methods describe qualitatively and also quantitatively the same physics.
2. arXiv:1308.1610 [pdf, ps, other]
Kinetic description of thermalization dynamics in weakly interacting quantum systems
Michael Stark, Marcus Kollar
After a sudden disruption, weakly interacting quantum systems first relax to a prethermalized state that can be described by perturbation theory and a generalized Gibbs ensemble. Using these properties of the prethermalized state we perturbatively derive a kinetic equation which becomes a quantum Boltzmann equation in the scaling limit of vanishing interaction. Applying this to interaction quenches in the fermionic Hubbard model we find that the momentum distribution relaxes to the thermal prediction of statistical mechanics. For not too large interaction, this two-stage scenario provides a quantitative understanding of the time evolution leading from the initial pure via a metastable prethermal to the final thermal state.
3. arXiv:1308.1431 [pdf, other]
Realizing the Harper Hamiltonian with Laser-Assisted Tunneling in Optical Lattices
Hirokazu Miyake, Georgios A. Siviloglou, Colin J. Kennedy, William Cody Burton, Wolfgang Ketterle
We experimentally implement the Harper Hamiltonian for neutral particles in optical lattices using laser-assisted tunneling and a potential energy gradient provided by gravity or magnetic field gradients. This Hamiltonian describes the motion of charged particles in strong magnetic fields. Laser-assisted tunneling processes are characterized by studying the expansion of the atoms in the lattice. The band structure of this Hamiltonian should display Hofstadter's butterfly. For fermions, this scheme should realize the quantum Hall effect and chiral edge states.
Aug 9
1. arXiv:1308.1672 [pdf, ps, other]
Ferromagnetism of a Repulsive Atomic Fermi gas in an Optical Lattice: a Quantum Monte Carlo study
S. Pilati, I. Zintchenko, M. Troyer
Using continuous-space quantum Monte Carlo methods we investigate the zero-temperature ferromagnetic behavior of a two-component repulsive Fermi gas under the influence of periodic potentials that describe the effect of a simple-cubic optical lattice. Simulations are performed with balanced and with imbalanced components, including the case of a single impurity immersed in a polarized Fermi sea (repulsive polaron). For an intermediate density below half-filling, we locate the transitions between the paramagnetic, the partially and the fully ferromagnetic phases. As the intensity of the optical lattice increases the ferromagnetic instability takes place at weaker interactions, indicating a possible route to observe ferromagnetism in experiments performed with ultracold atoms. We compare our findings with previous predictions based on the standard computational method used in material science, namely density functional theory, and with results based on tight-binding models.
2. arXiv:1308.1680 [pdf, other]
Experimental entanglement activation from discord in a programmable quantum measurement
Gerardo Adesso, Vincenzo D'Ambrosio, Eleonora Nagali, Marco Piani, Fabio Sciarrino
To acquire knowledge about nature we need to observe its constituents. In quantum mechanics, observing is not a passive act. Consider a system of two quantum particles A and B: if a measurement apparatus M is used to make an observation on particle B, then the overall state of the system AB will typically be altered. When this happens no matter which local measurement is performed, the two objects A and B are revealed to possess peculiar correlations known as quantum discord. Here we demonstrate that the very act of local observation gives rise to an "activation protocol" which converts discord into distillable entanglement, a stronger and more useful form of quantum correlations, between the apparatus M and the composite system AB. In order to experimentally implement such activation protocol, we adopt a flexible two-photon setup to realize a three-qubit system (A,B,M) with programmable degrees of initial correlations, measurement interaction, and characterization processes. Our experiment demonstrates the fundamental mechanism underpinning the ubiquitous act of observing the quantum world, and unlocks the potential of discord for entanglement generation, a primitive for quantum technology.
3. arXiv:1308.1717 [pdf, ps, other]
Equilibration of quantum chaotic systems
Quntao Zhuang, Biao Wu
Quantum ergordic theorem was proved by von Neumann [Z. Phys. {\bf 57}, 30 (1929)] and again by Reimann [Phys. Rev. Lett. {\bf 101}, 190403 (2008)] in a more practical and well-defined form. This theorem is illustrated and numerically verified for quantum chaotic systems. Our numerical results show that a quantum chaotic system with an initial low-entropy state will dynamically relax to a high-entropy state and reach equilibrium. The quantum equilibrium state reached after dynamical relaxation bears a remarkable resemblance to the classical micro-canonical ensemble. In contrast, the fluctuations around equilibrium are distinct: the quantum fluctuations are exponential while the classical fluctuations are Gaussian.
