Jan 2016

Jan 4-Jan 8 Bo Liu, Jan 11-Jan 15 Max, Jan 18-Jan 22 Haiyuan Zou, Jan 25-Jan 29 Ahmet Keles Jan 25-29

arXiv:1601.07764 [pdf, other]
What is a particle-conserving Topological Superfluid?
Gerardo Ortiz, Emilio Cobanera
Subjects: Superconductivity (cond-mat.supr-con); Strongly Correlated Electrons (cond-mat.str-el); Mathematical Physics (math-ph); Quantum Physics (quant-ph)
We establish a criterion for characterizing superfluidity in interacting, particle-number conserving systems of fermions as topologically trivial or non-trivial. Because our criterion is based on the concept of many-body fermionic parity switches, it is directly associated to the observation of the fractional Josephson effect and indicates the emergence of zero-energy modes that anticommute with fermionic parity. We tested these ideas on the Richardson-Gaudin-Kitaev chain, a particle-number conserving system that is solvable by way of the algebraic Bethe ansatz, and reduces to a long-range Kitaev chain in the mean-field approximation. Guided by its closed-form solution, we introduce a procedure for constructing many-body Majorana zero-energy modes of gapped topological superfluids in terms of coherent superpositions of states with different number of fermions. We discuss their significance and the physical conditions required to enable quantum control in the light of superselection rules. 

arXiv:1601.07746 [pdf, other]
Pseudopotentials for an ultracold dipolar gas
T. M. Whitehead, G. J. Conduit
Comments: 14 pages, 12 figures, to be published in Physical Review A
Subjects: Quantum Gases (cond-mat.quant-gas)
A gas of ultracold molecules interacting via the long-range dipolar potential offers a highly controlled environment in which to study strongly correlated phases. However, at particle coalescence the divergent $1/r^3$ dipolar potential and associated pathological wavefunction hinder computational analysis. For a dipolar gas constrained to two dimensions we overcome these numerical difficulties by proposing a pseudopotential that is explicitly smooth at particle coalescence, resulting in a 2000-times speedup in diffusion Monte Carlo calculations. The pseudopotential delivers the scattering phase shifts of the dipolar interaction with an accuracy of $10^{-5}$ and predicts the energy of a dipolar gas to an accuracy of $10^{-4}E_\mathrm{F}$ in a diffusion Monte Carlo calculation. 

arXiv:1601.07675 [pdf, ps, other]
Phase diagram and multicritical behaviors of mixtures of 3D bosonic gases
Giacomo Ceccarelli, Jacopo Nespolo, Andrea Pelissetto, Ettore Vicari
Comments: 16 pages
Subjects: Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech)
We investigate the Bose-Einstein condensation patterns, the critical and multicritical behaviors of three-dimensional mixtures of bosonic gases with short-range density-density interactions. These systems have a global U(1)+U(1) symmetry, as the system Hamiltonian is invariant under independent U(1) transformations acting on each species. In particular, we consider the three-dimensional Bose-Hubbard model for two lattice bosonic gases coupled by an on-site inter-species density-density interaction. We study the phase diagram and the critical behaviors along the transition lines characterized by the Bose-Einstein condensation of one or both species. We present mean-field calculations and numerical finite-size scaling analyses of quantum Monte Carlo data. We also consider multicritical points, close to which it is possible to observe the condensation of both gas components. We determine the possible multicritical behaviors by using field-theoretical perturbative methods. We consider the U(1)+U(1)-symmetric Landau-Ginzburg-Wilson Phi4 theory and determine the corresponding stable fixed points of the renormalization-group flow. The analysis predicts that, in all cases, the multicritical behavior is analogous to the one that would be observed in systems of two identical gases, with an additional Z_2 exchange symmetry. 
arXiv:1601.06413 [pdf, other]
Collectively induced many-vortices topology via rotatory Dicke quantum phase transition
Priyam Das, Mehmet Emre Tasgin, Ozgur E. Mustecaplioglu
Comments: 5 pages, 7 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We examine the superradiance of a Bose-Einstein condensate pumped with a Laguerre-Gaussian laser of high winding number, e.g., $\ell = 7$. The laser beam transfers its orbital angular momentum to the condensate at once due to the collectivity of the superradiance. We let the spatial profile of superradiant scattering to evolve self-consistently within the equations of motion. An $\ell$-fold rotational symmetric structure emerges with the take place of rotatory superradiance. Even though the pump and the condensate profiles initially have cylindrical symmetry, we observe that it is broken to $\ell$-fold rotational symmetry after the superradiance. Self-organization of the condensate is the rotational analog of the ordering in the experiment by Esslinger and colleagues [Nature, {\bf 264}, 1301 (2010)]. We show that the critical point for the onset of the self-organization follows the form of the Dicke quantum phase transition.

arXiv:1601.06197 [pdf, other]
Formation of Bose-Einstein condensates
Matthew J. Davis, Tod M. Wright, Thomas Gasenzer, Simon A. Gardiner, Nick P. Proukakis
Comments: Unedited version of chapter to appear in "Universal Themes of Bose-Einstein Condensation", edited by D. W. Snoke, N. P. Proukakis and P. B. Littlewood (Cambridge University Press). Comments are invited and appreciated
Subjects: Quantum Gases (cond-mat.quant-gas)

The problem of understanding how a coherent, macroscopic Bose-Einstein condensate (BEC) emerges from the cooling of a thermal Bose gas has attracted significant theoretical and experimental interest over several decades. The pioneering achievement of BEC in weakly-interacting dilute atomic gases in 1995 was followed by a number of experimental studies examining the growth of the BEC number, as well as the development of its coherence. More recently there has been interest in connecting such experiments to universal aspects of nonequilibrium phase transitions, in terms of both static and dynamical critical exponents. Here, the spontaneous formation of topological structures such as vortices and solitons in quenched cold-atom experiments has enabled the verification of the Kibble-Zurek mechanism predicting the density of topological defects in continuous phase transitions, first proposed in the context of the evolution of the early universe. This chapter reviews progress in the understanding of BEC formation, and discusses open questions and future research directions in the dynamics of phase transitions in quantum gases.

arXiv:1601.06185 [pdf, other]
Superfluid--Insulator Transition in Strongly Disordered One-dimensional Systems
Zhiyuan Yao, Lode Pollet, Nikolay Prokof'ev, Boris Svistunov
Comments: 12 pages, 4 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Disordered Systems and Neural Networks (cond-mat.dis-nn); Statistical Mechanics (cond-mat.stat-mech)

We present an asymptotically exact renormalization-group theory of the superfluid--insulator transition in one-dimensional disordered systems, with emphasis on an accurate description of the interplay between the Giamarchi--Schulz (instanton--anti-instanton) and weak-link (scratched-XY) criticalities. Combining the theory with extensive quantum Monte Carlo simulations allows us to shed new light on the ground-state phase diagram of the one-dimensional disordered Bose-Hubbard model at unit filling.

arXiv:1601.06172 [pdf, ps, other]
Competing ground states of strongly correlated bosons in the Harper-Hofstadter-Mott model
Stefan S. Natu, Erich J. Mueller, S. Das Sarma
Comments: 10 pages, 6 figures. Comments are welcome
Subjects: Quantum Gases (cond-mat.quant-gas)

Using an efficient cluster approach, we study the physics of two-dimensional lattice bosons in a strong magnetic field in the regime where the tunneling is much weaker than the on-site interaction strength. We study both dilute, hard core bosons at filling factors much smaller than unity occupation per site, and the physics in the vicinity of the superfluid-Mott lobes as the density is tuned away from unity. For hardcore bosons, we carry out extensive numerics for a fixed flux per plaquette $\phi=1/5$ and $\phi = 1/3$. At large flux, the lowest energy state is a strongly correlated superfluid, analogous to He-$4$, in which the order parameter is dramatically suppressed, but non-zero. At filling factors $\nu=1/2,1$, we find competing incompressible states which are metastable. These appear to be commensurate density wave states. For small flux, the situation is reversed, and the ground state at $\nu = 1/2$ is an incompressible density-wave solid. Here, we find a metastable lattice supersolid phase, where superfluidity and density-wave order coexist. We then perform careful numerical studies of the physics near the vicinity of the Mott lobes for $\phi = 1/2$ and $\phi = 1/4$. At $\phi = 1/2$, the superfluid ground state has commensurate density-wave order. At $\phi = 1/4$, incompressible phases appear outside the Mott lobes at densities $n = 1.125$ and $n = 1.25$, corresponding to filling fractions $\nu = 1/2$ and $1$ respectively. These phases, which are absent in single-site mean-field theory are metastable, and have slightly higher energy than the superfluid, but the energy difference between them shrinks rapidly with increasing cluster size, suggestive of an incompressible ground state. We thus explore the interplay between Mott physics, magnetic Landau levels, and superfluidity, finding a rich phase diagram of competing compressible and incompressible states.


arXiv:1601.06038 [pdf, other]
Optimizing Quantum Gas Production by an Evolutionary Algorithm
Tobias Lausch, Michael Hohmann, Farina Kindermann, Daniel Mayer, Felix Schmidt, Artur Widera
Comments: 7 pages, 6 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph)

We report on the application of an evolutionary algorithm (EA) to enhance performance of an ultra-cold quantum gas experiment. The production of a $^{87}$Rubidium Bose-Einstein condensate (BEC) can be divided into fundamental cooling steps, specifically magneto optical trapping of cold atoms, loading of atoms to a far detuned crossed dipole trap and finally the process of evaporative cooling. The EA is applied separately for each of these steps with a particular definition for the feedback the so-called fitness. We discuss the principles of an EA and implement an enhancement called differential evolution. Analyzing the reasons for the EA to improve \eg, the atomic loading rates and increase the BEC phase-space density, yields an optimal parameter set for the BEC production and enables us to reduce the BEC production time significantly. Furthermore, we focus on how additional information about the experiment and optimization possibilities can be extracted and how the correlations revealed allow for further improvement. Our results illustrate that EAs are powerful optimization tools for complex experiments and exemplify that the application yields useful information on the dependence of these experiments on the optimized parameters.

arXiv:1601.05891 [pdf]
Emergent Behavior in Strongly Correlated Electron Systems
David Pines
Comments: 25 pages, submitted to Rep. Prog. Phys
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)

I describe early work on strongly correlated electron systems [SCES] from the perspective of a theoretical physicist who, while a participant in their reductionist top-down beginnings, is now part of the paradigm change to a bottom-up "emergent" approach with its focus on using phenomenology to find the organizing principles responsible for their emergent behavior disclosed by experiment---and only then constructing microscopic models that incorporate these. After considering the organizing principles responsible for plasmons, quasiparticles, and conventional superconductivity in SCES, I consider their extension to their sister systems, the helium liquids, nuclei, and the nuclear matter found in neutron stars. I note some recent applications of the random phase approximation and examine briefly the role that paradigm change is playing in two central problems in our field: understanding the emergence and subsequent behavior of heavy electrons in Kondo lattice materials; and the mechanism for the unconventional superconductivity found in heavy electron, organic, and cuprate materials.

arXiv:1601.05869 [pdf, other]
Properties of a dipolar condensate with three-body interactions
P. B. Blakie
Comments: 7 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We obtain the phase diagram for a harmonically trapped dilute dipolar condensate with a short ranged conservative three-body interaction. We show that this system supports two distinct fluid states: a usual condensate state and a self-cohering droplet state. We develop a simple model to quantify the energetics of these states, which we verify with full numerical calculations. Based on our simple model we develop a phase diagram showing that there is a first order phase transition between the states. Using dynamical simulations we explore the phase transition dynamics, revealing that the droplet crystal observed in previous work is an excited state that arises from heating as the system crosses the phase transition. Utilising our phase diagram we show it is feasible to produce a single droplet by dynamically adjusting the confining potential.

Jan 22
arXiv:1601.05780 [pdf, other]
Majorana-time-reversal symmetries: a fundamental principle for sign-problem-free quantum Monte Carlo simulations
Zi-Xiang Li, Yi-Fan Jiang, Hong Yao
Comments: 5 pages + Supplementary Materials, 3 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Superconductivity (cond-mat.supr-con); High Energy Physics - Lattice (hep-lat)

A fundamental open issue in physics is whether and how the fermion-sign-problem in quantum Monte Carlo (QMC) can be solved generically. Here, we show that Majorana-time-reversal (MTR) symmetries can provide a unifying principle to solve the fermion-sign-problem in interacting fermionic models. By systematically classifying Majorana-bilinear operators according to the anti-commuting MTR symmetries they respect, we rigorously proved that there are two and only two fundamental symmetry classes which are sign-problem-free and which we call "Majorana-class" and "Kramers-class". All other sign-problem-free symmetry classes have higher symmetries than these two fundamental classes. Sign-problem-free models in the Majorana-class include interacting topological superconductors, for which we performed sign-problem-free Majorana QMC simulations and found that with increasing interactions the topological superconductor's helical edge states first undergo spontaneous symmetry-breaking while the bulk is still topologically-nontrivial. Remarkably, we discovered emergent spacetime supersymmetry (SUSY) at the edge quantum critical point.
Jan 21
arXiv:1601.05262 [pdf, other]
Quantum-Classical Correspondence Principle for Work Distributions in a Chaotic System
Long Zhu, Zongping Gong, Biao Wu, H.T.Quan
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)
Through proving the correspondence principle of work distributions, recent work [C. Jarzynski, H. T. Quan, and S. Rahav, Phys. Rev. X 5, 031038 (2015)] gives some justification to the definition of quantum work via two point energy measurements in one-dimensional (1D) integrable systems. In a chaotic system, however, the correspondence principle of work distributions has not been explored so far. In this paper we numerically study the work distribution in a chaotic system, and examine the relationship between the quantum and the classical work. Our numerical results suggest that there exists the correspondence principle of work distributions in a chaotic system as well. Our investigation further justifies the definition of quantum work via two point energy measurements in a chaotic system.


Jan 20
arXiv:1601.04929 [pdf, other]
Simulation of the many-body dynamical quantum Hall effect in an optical lattice
Dan-Wei Zhang, Xu-Chen Yang
Comments: 8 pages, 3 figures; accepted in Quantum Information Processing
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

We propose an experimental scheme to simulate the many-body dynamical quantum Hall effect with ultra-cold bosonic atoms in a one-dimensional optical lattice. We first show that the required model Hamiltonian of a spin-1/2 Heisenberg chain with an effective magnetic field and tunable parameters can be realized in this system. For dynamical response to ramping the external fields, the quantized plateaus emerge in the Berry curvature of the interacting atomic spin chain as a function of the effective spin-exchange interaction. The quantization of this response in the parameter space with the interaction-induced topological transition characterizes the many-body dynamical quantum Hall effect. Furthermore, we demonstrate that this phenomenon can be observed in practical cold-atom experiments with numerical simulations.

Jan 19
arXiv:1601.04616 [pdf, ps, other]
Quantum optics and frontiers of physics: The third quantum revolution
Alessio Celi, Anna Sanpera, Veronica Ahufinger, Maciej Lewenstein
Comments: 15 pag. arXiv admin note: text overlap with arXiv:cond-mat/0606771
Subjects: Quantum Gases (cond-mat.quant-gas)

The year 2015 was the International Year of Light. It marked, however, also the 20th anniversary of the first observation of Bose-Einstein condensation in atomic vapors by Eric Cornell, Carl Wieman and Wolfgang Ketterle. This discovery can be considered as one of the greatest achievements of quantum optics that has triggered an avalanche of further seminal discoveries and achievements. For this reason we devote this essay for focus issue on "Quantum Optics in the International Year of Light" to the recent revolutionary developments in quantum optics at the frontiers of all physics: atomic physics, molecular physics, condensed matter physics, high energy physics and quantum information science. We follow here the lines of the introduction to our book "Ultracold atoms in optical lattices: Simulating quantum many-body systems" [1]. The book, however, was published in 2012, and many things has happened since then -- the present essay is therefore upgraded to include the latest developments.

Jan 18
arXiv:1601.03742 [pdf, other]
Quantum Spin Liquids
Lucile Savary, Leon Balents
Comments: 60 pages, 8 figures, 1 table
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

Quantum spin liquids may be considered "quantum disordered" ground states of spin systems, in which zero point fluctuations are so strong that they prevent conventional magnetic long range order. More interestingly, quantum spin liquids are prototypical examples of ground states with massive many-body entanglement, of a degree sufficient to render these states distinct phases of matter. Their highly entangled nature imbues quantum spin liquids with unique physical aspects, such as non-local excitations, topological properties, and more. In this review, we discuss the nature of such phases and their properties based on paradigmatic models and general arguments, and introduce theoretical technology such as gauge theory and partons that are conveniently used in the study of quantum spin liquids. An overview is given of the different types of quantum spin liquids and the models and theories used to describe them. We also provide a guide to the current status of experiments to study quantum spin liquids, and to the diverse probes used therein.

Jan 12
1. arXiv:1601.02106 [pdf, other]

Entanglement growth and correlation spreading with variable-range interactions in spin and fermionic tunnelling models

Anton S. Buyskikh, Maurizio Fagotti, Johannes Schachenmayer, Fabian Essler, Andrew J. Daley

We investigate the dynamics following a global parameter quench for two 1D models with variable-range power-law interactions: a long-range transverse Ising model, which has recently been realised in chains of trapped ions, and a long-range lattice model for spinless fermions with long-range tunnelling. For the transverse Ising model, the spreading of correlations and growth of entanglement are computed using numerical matrix product state techniques, and are compared with exact solutions for the fermionic tunnelling model. We identify transitions between regimes with and without an apparent linear light cone for correlations, which correspond closely between the two models. For long-range interactions (in terms of separation distance r, decaying slower than 1/r), we find that despite the lack of a light-cone, correlations grow slowly as a power law at short times, and that -- depending on the structure of the initial state -- the growth of entanglement can also be sublinear. These results are understood through analytical calculations, and should be measurable in experiments with trapped ions.


Jan 8
1. arXiv:1601.01541 [pdf, other]
Fractional-charge vortex in a spinor Bose-Einstein condensate
Sandeep Gautam, S. K. Adhikari
We classify all possible fractional charge vortices of charge less than unity in spin-1 and spin-2 polar and cyclic Bose-Einstein condensates (BECs) with zero magnetization. Statics and dynamics of these vortices in quasi-two-dimensional spinor BECs are studied employing accurate numerical solution and a Lagrange variational approximation. The results for density and collective-mode oscillation are illustrated using fractional-charge BEC vortex of 23Na and 87Rb atoms with realistic interaction and trapping potential parameters.


2. arXiv:1601.01519 [pdf, other]
Stable and mobile two-dimensional dipolar ring-dark-in-bright Bose-Einstein condensate soliton
S. K. Adhikari
We demonstrate robust, stable, mobile two-dimensional (2D) dipolar ring-dark-in-bright (RDB) Bose-Einstein condensate (BEC) solitons for repulsive contact interaction, subject to a harmonic trap along the y direction perpendicular to the polarization direction z. Such a RDB soliton has a ring-shaped notch (zero in density) imprinted on a 2D bright soliton free to move in the x−z plane. At medium velocity the head-on collision of two such solitons is found to be quasi elastic with practically no deformation. The possibility of creating the RDB soliton by phase imprinting is demonstrated. The findings are illustrated using numerical simulation employing realistic interaction parameters in a dipolar 164Dy BEC.


Jan 7
1. arXiv:1601.01185 [pdf, other]
Topological phase transitions on a triangular optical lattice with non-Abelian gauge fields
M. Iskin
We study the mean-field BCS-BEC evolution of a uniform Fermi gas on a single-band triangular lattice, and construct its ground-state phase diagrams, showing a wealth of topological quantum phase transitions between gapped and gapless superfluids that are induced by the interplay of an out-of-plane Zeeman field and a generic non-Abelian gauge field.


2. arXiv:1601.01004 [pdf, other]
Dynamics of interacting fermions in spin-dependent potentials
Andrew P. Koller, Michael L. Wall, Josh Mundinger, Ana Maria Rey
Recent experiments with dilute trapped Fermi gases observed that weak interactions can drastically modify spin transport dynamics and give rise to robust collective effects whose fundamental quantum origins are not well understood, including global demagnetization, macroscopic spin waves, spin segregation, and spin self-rephasing. In this work, we develop a framework for analyzing the dynamics of weakly interacting fermionic gases following a spin-dependent change of the trapping potential in regimes where standard mean-field and kinetic theory treatments are invalid. The key idea is the projection of the dynamics onto a set of lattice spin models defined on the single-particle mode space. Collective phenomena, including the global spreading of quantum correlations in real space, arise as a consequence of the long-ranged character of the spin model couplings. The spin model formulation provides a simple analytic picture of the experimental observations and illuminates the interplay between spin, motion, Fermi statistics, and interactions. Our results suggest a number of directions for future experiments in the weakly interacting regime.


Jan 6
1. arXiv:1601.00958 [pdf, other]
Chiral spin liquids in triangular lattice SU(N) fermionic Mott insulators with artificial gauge fields
Pierre Nataf, Miklós Lajkó, Alexander Wietek, Karlo Penc, Frédéric Mila, Andreas M. Läuchli
We show that, in the presence of a π/2 artificial gauge field per plaquette, Mott insulating phases of ultra-cold fermions with SU(N) symmetry and one particle per site generically possess an extended chiral phase with intrinsic topological order characterized by a multiplet of N low-lying singlet excitations for periodic boundary conditions, and by chiral edge states described by the SU(N)1 Wess-Zumino-Novikov-Witten conformal field theory for open boundary conditions. This has been achieved by extensive exact diagonalizations for N between 3 and 9, and by a parton construction based on a set of N Gutzwiller projected fermionic wave-functions with flux π/N per triangular plaquette. Experimental implications are briefly discussed.


 Jan 5
1. arXiv:1601.00628 [pdf, other]
Quantum impurities: from mobile Josephson junctions to depletons
Michael Schecter, Dimitri M. Gangardt, Alex Kamenev
We overview the main features of mobile impurities moving in one-dimensional superfluid backgrounds by modeling it as a mobile Josephson junction, which leads naturally to the periodic dispersion of the impurity. The dissipation processes, such as radiative friction and quantum viscosity, are shown to result from the interaction of the collective phase difference with the background phonons. We develop a more realistic depleton model of an impurity-hole bound state that provides a number of exact results interpolating between the semiclassical weakly-interacting picture and the strongly interacting Tonks-Girardeau regime. We also discuss the physics of a trapped impurity, relevant to current experiments with ultra cold atoms.