Mar 2015

Mar 2-Mar 6 Zhifang Xu & Xuguang Yue, Mar 9-Mar 13 Bo Liu & Max, Mar 16-mar 20, Zhenyu Zhou & Jinlong Yu, Mar 23-Mar 27, Jiyao chen & Jianhui zhou, Mar 30-Apr 3, Haiyuan Zou & Ahmet Keles,

Apr 3
arXiv:1504.00369 [pdf, other]
Observation of the Chern-Simons gauge anomaly
Sunil Mittal, Sriram Ganeshan, Jingyun Fan, Abolhassan Vaezi, Mohammad Hafezi
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
Topological Quantum Field Theories (TQFTs) are powerful tools to describe universal features of topological orders. A hallmark example of a TQFT is the 2+1 D Chern-Simons (CS) theory which describes topological properties of both integer and fractional quantum Hall effects. The gauge invariant form of the CS theory with boundaries, encompassing both edge and bulk terms, provides an unambiguous way to relate bulk topological invariants to the edge dynamics. This bulk-edge correspondence is manifested as a gauge anomaly of the chiral dynamics at the edge, and provides a direct insight into the bulk topological order. Such an anomaly has never been directly observed in an experiment. In this work, we experimentally implement the integer quantum Hall model in a photonic system, described by the corresponding CS theory. By selectively manipulating and probing the edge, we exploit the gauge anomaly of the CS theory, for the first time. The associated spectral edge flow allows us to unambiguously measure topological invariants. This experiment provides a new approach for direct measurement of topological invariants, independent of the microscopic details, and thus could be extended to probe strongly correlated topological orders.

Apr 2
arXiv:1504.00016 [pdf, other]
Energy dependent Ergodicity and Many Body Localization: Effects of a Single Particle Mobility Edge
Xiaopeng Li, Sriram Ganeshan, J. H. Pixley, S. Das Sarma
Comments: 5+3 pages, 5 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

We investigate many body localization in the presence of a single particle mobility edge. The mobility edge in the model we consider leads to an energy region where the non-interacting extended many body states are mixed with partially-localized states. We find through exact diagonalization, that such a mixed energy regime survives in the presence of interactions. Although not fully localized, states in this mixed regime are non-ergodic in nature, giving rise to an energy dependent violation of the eigenstate thermalization hypothesis. Surprisingly, we find that the many body system could be completely localized even if there are extended single-particle eigenstates.

Apr 1
arXiv:1503.08887 [pdf, other]
Theory of excitations and dielectric response at a spin-orbital quantum critical point
Daniel Ish, Leon Balents
Comments: 8 pages, 3 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
Despite possessing a local spin 2 moment on the iron site and a Curie-Weiss temperature of 45K, the A site spinel FeSc2S4 does not magnetically order down to 50mK. Previous theoretical work by Chen and Balents advanced an explanation for this observation in the form of the "J2-λ" model which places FeSc2S4 close to a quantum critical point on the disordered side of a quantum phase transition between a N\'{e}el ordered phase and a "Spin-Orbital Liquid" in which spins and orbitals are entangled, quenching the magnetization. We present new theoretical studies of the optical properties of the J2-λ model, including a computation of the dispersion relation for the quasiparticle excitations and the form of the collective response to electric field. We argue that the latter directly probes a low energy excitation continuum characteristic of quantum criticality, and that our results reinforce the consistency of this model with experiment.


Mar 31
arXiv:1503.08592 [pdf, other]
Exact holographic mapping in free fermion systems
Ching Hua Lee, Xiao-Liang Qi
Comments: 32 pages, 7 figures
Subjects: High Energy Physics - Theory (hep-th); Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)
In this paper, we shall perform a detailed analysis of the Exact Holographic Mapping first introduced in arXiv:1309.6282, which was proposed as an explicit example of holographic duality between quantum many-body systems and gravitational theories. We obtain analytic results for free-fermion systems that not only confirm previous numerical results, but also elucidate the exact relationships between the various physical properties of the bulk and boundary systems. Our analytic results allow us to study the asymptotic properties that are difficult to probe numerically, such as the near horizon regime of the black hole geometry. We shall also explore a few interesting but hitherto unexplored bulk geometries, such as that corresponding to a boundary critical fermion with nontrivial dynamic critical exponent. Our analytic framework also allows us to study the holographic mapping of some of these boundary theories in dimensions 2+1 or higher.

arXiv:1503.08243 [pdf, other]
Observation of Bose-Einstein Condensation in a Strong Synthetic Magnetic Field
Colin J. Kennedy, William Cody Burton, Woo Chang Chung, Wolfgang Ketterle
Comments: 6 pages, 5 figures. Supplement: 6 pages, 4 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Atomic Physics (physics.atom-ph)Extensions of Berry's phase and the quantum Hall effect have led to the discovery of new states of matter with topological properties. Traditionally, this has been achieved using gauge fields created by magnetic fields or spin orbit interactions which couple only to charged particles. For neutral ultracold atoms, synthetic magnetic fields have been created which are strong enough to realize the Harper-Hofstadter model. Despite many proposals and major experimental efforts, so far it has not been possible to prepare the ground state of this system. Here we report the observation of Bose-Einstein condensation for the Harper-Hofstadter Hamiltonian with one-half flux quantum per lattice unit cell. The diffraction pattern of the superfluid state directly shows the momentum distribution on the wavefuction, which is gauge-dependent. It reveals both the reduced symmetry of the vector potential and the twofold degeneracy of the ground state. We explore an adiabatic many-body state preparation protocol via the Mott insulating phase and observe the superfluid ground state in a three-dimensional lattice with strong interactions.





Mar 30
arXiv:1503.07976 [pdf, ps, other]
Multiple Period States of the Superfluid Fermi Gas in an Optical Lattice
Sukjin Yoon, Franco Dalfovo, Takashi Nakatsukasa, Gentaro Watanabe
Comments: 5 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Solar and Stellar Astrophysics (astro-ph.SR); Superconductivity (cond-mat.supr-con); Nuclear Theory (nucl-th)

We study multiple period states of a superfluid Fermi gas in an optical lattice along the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensate (BEC) crossover. The existence of states whose period is a multiple of the lattice spacing is a consequence of the non-linear behavior of the gas, which is due to the presence of the order parameter associated with superfluidity. By solving Bogoliubov-de Gennes equations we find that, in the BCS side of the crossover, the multiple period states can be energetically favorable compared to the normal Bloch states and their survival time against dynamical instability drastically increases, suggesting that these states can be accessible in current experiments with ultracold gases. This is in sharp contrast to the situation in BECs.

Mar 27
1. arXiv:1503.07784 [pdf, other]
Odd-Frequency Triplet Superconductivity at the Helical Edge of a Toplogical Insulator
François Crépin, Pablo Burset, Björn Trauzettel

2. arXiv:1503.07620 [pdf, ps, other]
Many body localization in the presence of a single particle mobility edge
Ranjan Modak, Subroto Mukerjee


Mar 26
1. arXiv:1503.07456 [pdf, other]
Topological classification of k.p Hamiltonians for Chern insulators
Frank Kirtschig, Jeroen van den Brink, Carmine Ortix
2. arXiv:1503.07292 [pdf, ps, other]
Symmetry protection of critical phases and global anomaly in $1+1$ dimensions
Shunsuke C. Furuya, Masaki Oshikawa
3. arXiv:1503.07271 [pdf, ps, other]
Fermionic spinon and holon statistics in the pyrochlore quantum spin liquid
B. Normand, Z. Nussinov
Comments: 5 pages, 4 figures

4. arXiv:1503.07167 [pdf, other]
Dynamical detection of a topological phase transition in one-dimensional spin-orbit-coupled Fermi gases
F. Setiawan, K. Sengupta, I. B. Spielman, Jay D. Sau





Mar 25
1. arXiv:1503.06979 [pdf, other]
Quasiparticle properties of a mobile impurity in a Bose-Einstein condensate
Rasmus Søgaard Christensen, Jesper Levinsen, Georg M. Bruun
2. arXiv:1503.06808 [pdf, other]
Observation of phononic helical edge states in a mechanical 'topological insulator'
Roman Süsstrunk, Sebastian D. Huber
3. arXiv:1503.06812 [pdf, other]
Theory of Twist Liquids: Gauging an Anyonic Symmetry
Jeffrey C. Y. Teo, Taylor L. Hughes, Eduardo Fradkin




Mar 24
1. arXiv:1503.06508 [pdf, ps, other]
Many-body localization in a random $x-y$ model with the long-range interaction
Alexander L. Burin
2. arXiv:1503.06295 [pdf]
Synthetic gauge fields and Weyl point in Time-Reversal Invariant Acoustic Systems
Meng Xiao, Wen-Jie Chen, Wen-Yu He, Z. Q. Zhang, C. T. Chan





Fri, Mar 20
1. arXiv:1503.05565 [pdf, other]
Measuring the dynamic structure factor of a quantum gas undergoing a structural phase transition
Renate Landig, Ferdinand Brennecke, Rafael Mottl, Tobias Donner, Tilman Esslinger
Comments: 17 pages including supplementary information
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

The dynamic structure factor is a central quantity describing the physics of quantum many-body systems, capturing structure and collective excitations of a material. In condensed matter, it can be measured via inelastic neutron scattering, which is an energy-resolving probe for the density fluctuations. In ultracold atoms, a similar approach could so far not be applied due to the diluteness of the system. Here, we report on a direct, real-time and non-destructive measurement of the dynamic structure factor of a quantum gas exhibiting cavity-mediated long-range interactions. The technique relies on inelastic scattering of photons, stimulated by the enhanced vacuum field inside a high finesse optical cavity. We extract the density fluctuations, their energy and lifetime while the system undergoes a structural phase transition. We observe an occupation of the relevant quasi-particle mode on the level of a few excitations, and provide a theoretical description of this dissipative quantum many-body system.

2. arXiv:1503.05648 [pdf, other]
Observation of Half-Quantum Vortices in an Antiferromagnetic Spinor Bose-Einstein Condensate
Sang Won Seo, Seji Kang, Woo Jin Kwon, Yong-il Shin
Comments: 5 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We report the observation of half-quantum vortices (HQVs) in the easy-plane polar phase of an antiferromagnetic spinor Bose-Einstein condensate. Using in-situ magnetization-sensitive imaging, we observe that a singly charged quantum vortex dissociates into a pair of HQVs with opposite core magnetization. The dissociation dynamics is investigated by measuring the temporal evolution of the separation distance of the HQV pair. The separating behavior of the HQVs shows the short-range nature of their interactions. We observe that the dissociation process is activated faster with larger thermal spin fluctuations in the condensate, confirming the energetic instability of a singly charged vortex state.


Thus, Mar 19
1. arXiv:1503.05549 [pdf, other]
Exploring competing density order in the ionic Hubbard model with ultracold fermions
Michael Messer, Rémi Desbuquois, Thomas Uehlinger, Gregor Jotzu, Sebastian Huber, Daniel Greif, Tilman Esslinger
Comments: 5+3 pages
Subjects: Quantum Gases (cond-mat.quant-gas); Materials Science (cond-mat.mtrl-sci); Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)
We realize and study the ionic Hubbard model using an interacting two-component gas of fermionic atoms loaded into an optical lattice. The bipartite lattice has honeycomb geometry with a staggered energy-offset that explicitly breaks the inversion symmetry. Distinct density-ordered phases are identified using noise correlation measurements of the atomic momentum distribution. For weak interactions the geometry induces a charge density wave. For strong repulsive interactions a Mott insulator forms and we observe a restoration of the inversion symmetry. The local density distributions in different configurations are characterized by measuring the number of doubly occupied lattice sites as a function of interaction and energy-offset. We further probe the excitations of the system using direction dependent modulation spectroscopy and discover a complex spectrum, which we compare with a theoretical model.


2. arXiv:1503.05498 [pdf, other]
Adiabatic Control of Atomic Dressed States for Transport and Sensing
N. R. Cooper, A. M. Rey
Subjects: Quantum Gases (cond-mat.quant-gas)
We describe forms of adiabatic transport that arise for dressed-state atoms in optical lattices. Focussing on the limit of weak tunnel-coupling between nearest-neighbour lattice sites, we explain how adiabatic variation of optical dressing allows control of atomic motion between lattice sites: allowing adiabatic particle transport in a direction that depends on the internal state, and force measurements via spectroscopic preparation and readout. For uniformly filled bands these systems display topologically quantised particle transport.

3. arXiv:1503.05472 [pdf, other]
Twisted complex superfluids in optical lattices
Ole Jürgensen, Klaus Sengstock, Dirk-Sören Lühmann
Subjects: Quantum Gases (cond-mat.quant-gas)We show that correlated pair tunneling drives a phase transition to a twisted superfluid with a complex order parameter. This unconventional superfluid phase spontaneously breaks the time-reversal symmetry and is characterized by a twisting of the complex phase angle between adjacent lattice sites. We discuss the entire phase diagram of the extended Bose--Hubbard model for a honeycomb optical lattice showing a multitude of quantum phases including twisted superfluids, pair superfluids, supersolids and twisted supersolids. Furthermore, we show that the nearest-neighbor interactions breaks the inversion symmetry of the lattice and gives rise to dimerized density-wave insulators, where particles are delocalized on dimers. For two components, we find twisted superfluid phases with strong correlations between the species already for surprisingly small pair-tunneling amplitudes. Interestingly, this ground state shows an infinite degeneracy ranging continuously from a supersolid to a twisted superfluid.

4. arXiv:1503.05234 [pdf, ps, other]
Proposal to directly observe the Kondo effect through enhanced photo-induced scattering of cold fermionic and bosonic atoms
Bhuvanesh Sundar, Erich J. Mueller
Comments: 10 pages, 8 figuresSubjects: Quantum Gases (cond-mat.quant-gas)We propose an experimental protocol to directly observe the Kondo effect by scattering ultracold atoms with spin-dependent interactions. We propose using an optical Feshbach resonance to engineer Kondo-type spin-dependent interactions in a system with ultracold 6Li and 87Rb gases. We calculate the momentum transferred from the 87Rb to the 6Li gas in a scattering experiment and show that it has a logarithmically enhanced temperature dependence, characteristic of the Kondo effect and analogous to the resistivity of alloys with magnetic impurities. Experimentally detecting this enhancement will give a different perspective on the Kondo effect, and allow us to explore a rich variety of problems such as the Kondo lattice problem and heavy-fermion systems.


Wed, Mar 18
1. arXiv:1503.04993 [pdf, other]
Numerical calculation of spectral functions of the Bose-Hubbard model using B-DMFT
Jaromir Panas, Anna Kauch, Jan Kuneš, Dieter Vollhardt, Krzysztof Byczuk
Subjects: Quantum Gases (cond-mat.quant-gas)
We calculate the momentum dependent spectral function of the Bose-Hubbard model on a simple cubic lattice in three dimensions within the bosonic dynamical mean-field theory (B-DMFT). The continuous-time quantum Monte Carlo method is used to solve the self-consistent B-DMFT equations together with the maximum entropy method for the analytic continuation to real frequencies. Results for weak, intermediate, and strong interactions are presented. In the limit of weak and strong interactions very good agreement with results obtained by perturbation theory is found. By contrast, at intermediate interactions the results differ significantly, indicating that in this regime perturbative methods fail do describe the dynamics of interacting bosons.

2. arXiv:1503.04966 [pdf, other]
Polarons in Ultracold Fermi Superfluids
Wei Yi, Xiaoling Cui
Comments: 8 pages, 6 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Superconductivity (cond-mat.supr-con)


We study a new type of Fermi polaron induced by an impurity interacting with an ultracold Fermi superfluid. Due to the three-component nature of the system, the polaron can become trimer-like with a non-universal energy spectrum. We identify multiple avoided crossings between impurity- and trimer-like solutions in both the attractive and the repulsive polaron spectra. In particular, the widths of avoided crossings gradually increase as the Fermi superfluid undergoes a crossover from the BCS side towards the BEC side, which suggests instabilities towards three-body losses. Such losses can be reduced for interaction potentials with small effective ranges. We also demonstrate, using the second-order perturbation theory, that the mean-field evaluation of the fermion-impurity interaction energy is inadequate even for small fermion-impurity scattering lengths, due to the essential effects of Fermi superfluid and short-range physics in such a system. Our results are practically useful for cold atom experiments on mixtures.

3. arXiv:1503.05026 (cross-list from cond-mat.str-el) [pdf, ps, other]
Pfaffian-like ground states for bosonic atoms and molecules in 1D optical lattices
Tanja Duric, Nicholas Chancellor, Philip J. D. Crowley, Pierfrancesco Di Cintio, Andrew G. Green
Comments: 10 pages, 10 figures. arXiv admin note: text overlap with arXiv:cond-mat/0608012 by other authorsSubjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)


We study ground states of a system of bosonic atoms and diatomic Feshbach molecules trapped in a one-dimensional optical lattice using exact diagonalization and Variational Monte Carlo methods. We primarily study the case of an average filling of one boson per site. In agreement with bosonization theory, we show that the ground state of the system in the thermodynamic limit corresponds to the Pfaffian-like state when the system is tuned towards the superfluid to Mott insulator quantum phase transition. Our study clarifies possibility of the creation of exotic Pfaffian-like states in realistic one-dimensional systems. Such states could be used as the basis to create non-Abelian anyons in one-dimensional systems, with potential application for fault tolerant topological quantum computation.



Tue, Mar 17
1. arXiv:1503.04664 [pdf]
Intertwined orders of ultracold fermions loaded in two-dimensional optical lattices
A. Leprévost, O. Juillet, R. Frésard
Comments: 18 pages, 9 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
The generic quantum phase diagram of repulsively interacting spin-1/2 ultracold atoms, moving in a two-dimensional optical lattice, is investigated by means of unbiased energy minimizations with symmetry-adapted unrestricted wavefunctions. In the strongly correlated regime, we highlight the intertwining of spin-, charge-, and pair-density waves embedded in a uniform d-wave superfluid background. As the lattice filling increases, this phase emerges from homogenous states exhibiting spiral magnetism and evolves towards a doped antiferromagnet. A concomitant enhancement of long-ranged d-wave pairing correlations is also found. Nevertheless, superfluidity is suppressed when the charge period and the inverse of the hole doping from half-filling are commensurate.


Mon, Mar 16
1. arXiv:1503.03999 [pdf, other]
Entanglement like properties in Spin-Orbit Coupled Ultra Cold Atom and violation of Bell like Inequality
Rahul Kumar, Sankalpa Ghosh
Comments: Latex file with 4 pdf figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We show that the general quantum state of synthetically spin-orbit coupled ultra cold bosonic atom whose condensate was experimentally created recently ( Y. J. Lin {\it et al.}, Nature, {\bf 471}, 83, (2011)), shows entanglement between motional degrees of freedom ( momentum) and internal degrees of freedom (hyperfine spin). We demonstrate the violation of Bell-like inequality (CHSH) for such states that provides a unique opportunity to verify fundamental principle like quantum non-contextuality for commutating observables which are not spatially separated. We analyze in detail the Rabi oscillation executed by such atom-laser system and how that influneces quantities like entanglement entropy, violation of Bell like Inequality etc. We also discuss the implication of our result in testing the quantum non-contextuality and Bell's Inequality vioaltion by macroscopic quantum object like Bose-Einstein Condensate of ultra cold atoms.

Fri, 13
1. arXiv:1503.03725 [pdf, ps, other]
Quantum Impurity in a One-dimensional Trapped Bose Gas
A. S. Dehkharghani, A. G. Volosniev, N. T. Zinner
We present a new theoretical framework for describing an impurity in a trapped Bose system in one spatial dimension. The theory handles any external confinement, arbitrary mass ratios, and a weak interaction may be included between the Bose particles. To demonstrate our technique, we calculate the ground state energy and properties of a sample system with eight bosons and find an excellent agreement with numerically exact results. Our theory can thus provide definite predictions for experiments in cold atomic gases.




Thu, 12

1. arXiv:1503.03075 (cross-list from cond-mat.supr-con) [pdf, other]
Local origin of the pseudogap in the attractive Hubbard model
Robert Peters, Johannes Bauer
We provide a new perspective on the pseudogap physics for attractive fermions as described by the three-dimensional Hubbard model. The pseudogap in the single-particle spectral function, which occurs for temperatures above the critical temperature Tc of the superfluid transition, is often interpreted in terms of preformed, uncondensed pairs. Here we show that the occurrence of pseudogap physics can be consistently understood in terms of local excitations which lead to a splitting of the quasiparticle peak for sufficiently large interaction. This effect becomes prominent at intermediate and high temperatures when the quantum mechanical hopping is incoherent. We clarify the existence of a conjectured temperature below which pseudogap physics is expected to occur. Our results are based on approximating the physics of the three-dimensional Hubbard model by dynamical mean field theory calculations and a momentum independent self-energy. Our predictions can be tested with ultracold atoms in optical lattices with currently available temperatures and spectroscopic techniques.




Wed, 11
1. arXiv:1503.02816 [pdf, other]
Laser operation and Bose-Einstein condensation: analogies and differences
Alessio Chiocchetta, Andrea Gambassi, Iacopo Carusotto
After reviewing the interpretation of laser operation as a non-equilibrium Bose-Einstein condensation phase transition, we illustrate the novel features arising from the non-equilibrium nature of photon and polariton Bose-Einstein condensates recently observed in experiments. We then proposea quantitative criterion to experimentally assess the equilibrium vs. non-equilibrium nature of a specific condensation process, based on fluctuation-dissipation relations. The power of this criterion is illustrated on two models which shows very different behaviours.




Tue, 10

1. arXiv:1503.02452 (cross-list from physics.atom-ph) [pdf, other]
Microscopy of a scalable superatom
Johannes Zeiher, Peter Schauß, Sebastian Hild, Tommaso Macrì, Immanuel Bloch, Christian Gross
Strong interactions can amplify quantum effects such that they become important on macroscopic scales. Controlling these coherently on a single particle level is essential for the tailored preparation of strongly correlated quantum systems and opens up new prospects for quantum technologies. Rydberg atoms offer such strong interactions which lead to extreme nonlinearities in laser coupled atomic ensembles. As a result, multiple excitation of a Micrometer sized cloud can be blocked while the light-matter coupling becomes collectively enhanced. The resulting two-level system, often called "superatom", is a valuable resource for quantum information, providing a collective Qubit. Here we report on the preparation of two orders of magnitude scalable superatoms utilizing the large interaction strength provided by Rydberg atoms combined with precise control of an ensemble of ultracold atoms in an optical lattice. The latter is achieved with sub shot noise precision by local manipulation of a two-dimensional Mott insulator. We microscopically confirm the superatom picture by in-situ detection of the Rydberg excitations and observe the characteristic square root scaling of the optical coupling with the number of atoms. Furthermore, we verify the presence of entanglement in the prepared states and demonstrate the coherent manipulation of the superatom. Finally, we investigate the breakdown of the superatom picture when two Rydberg excitations are present in the system, which leads to dephasing and a loss of coherence.


2. arXiv:1503.02312 (cross-list from quant-ph) [pdf, other]
Quantum Simulations of Lattice Gauge Theories using Ultracold Atoms in Optical Lattices
Erez Zohar, J. Ignacio Cirac, Benni Reznik
Can high energy physics can be simulated by low-energy, nonrelativistic, many-body systems, such as ultracold atoms? Such ultracold atomic systems lack the type of symmetries and dynamical properties of high energy physics models: in particular, they manifest neither local gauge invariance nor Lorentz invariance, which are crucial properties of the quantum field theories which are the building blocks of the standard model of elementary particles.
However, it turns out, surprisingly, that there are ways to configure atomic system to manifest both local gauge invariance and Lorentz invariance. In particular, local gauge invariance can arise either as an effective, low energy, symmetry, or as an "exact" symmetry, following from the conservation laws in atomic interactions. Hence, one could hope that such quantum simulators may lead to new type of (table-top) experiments, that shall be used to study various QCD phenomena, as the con?nement of dynamical quarks, phase transitions, and other effects, which are inaccessible using the currently known computational methods.
In this report, we review the Hamiltonian formulation of lattice gauge theories, and then describe our recent progress in constructing quantum simulation of Abelian and non-Abelian lattice gauge theories in 1 + 1 and 2 + 1 dimensions using ultracold atoms in optical lattices.

Mon, 9

1. arXiv:1503.02005 [pdf, other]
Single-atom imaging of fermions in a quantum-gas microscope
Elmar Haller, James Hudson, Andrew Kelly, Dylan A. Cotta, Bruno Peaudecerf, Graham D. Bruce, Stefan Kuhr
Single-atom-resolved detection in optical lattices using quantum-gas microscopes has enabled a new generation of experiments in the field of quantum simulation. Fluorescence imaging of individual atoms has so far been achieved for bosonic species with optical molasses cooling, whereas detection of fermionic alkaline atoms in optical lattices by this method has proven more challenging. Here we demonstrate single-site- and single-atom-resolved fluorescence imaging of fermionic potassium-40 atoms in a quantum-gas microscope setup using electromagnetically-induced-transparency cooling. We detected on average 1000 fluorescence photons from a single atom within 1.5s, while keeping it close to the vibrational ground state of the optical lattice. Our results will enable the study of strongly correlated fermionic quantum systems in optical lattices with resolution at the single-atom level, and give access to observables such as the local entropy distribution and individual defects in fermionic Mott insulators or anti-ferromagnetically ordered phases.


2. arXiv:1503.01797 (cross-list from cond-mat.mes-hall) [pdf, other]
Hierarchy of Floquet gaps and edge states for driven honeycomb lattices
Pablo M. Perez-Piskunow, Luis E. F. Foa Torres, Gonzalo Usaj
Electromagnetic driving in a honeycomb lattice can induce gaps and topological edge states with a structure of increasing complexity as the frequency of the driving lowers. While the high frequency case is the most simple to analyze we focus on the multiple photon processes allowed in the low frequency regime to unveil the hierarchy of Floquet edge-states. In the case of low intensities an analytical approach allows us to derive effective Hamiltonians and address the topological character of each gap in a constructive manner. At high intensities we obtain the net number of edge states, given by the winding number, with a numerical calculation of the Chern numbers of each Floquet band. Using these methods, we find a hierarchy that resembles that of a Russian nesting doll. This hierarchy classifies the gaps and the associated edge states in different orders according to the electron-photon coupling strength. For large driving intensities, we rely on the numerical calculation of the winding number, illustrated in a map of topological phase transitions. The hierarchy unveiled with the low energy effective Hamiltonians, alongside with the map of topological phase transitions discloses the complexity of the Floquet band structure in the low frequency regime. The proposed method for obtaining the effective Hamiltonian can be easily adapted to other Dirac Hamiltonians of two dimensional materials and even the surface of a 3D topological insulator.




Mar 6
1.arXiv:1503.01516 [pdf, ps, other]
Fano resonance through Higgs bound states in tunneling of Nambu-Goldstone modes
Takeru Nakayama, Ippei Danshita, Tetsuro Nikuni, Shunji Tsuchiya
Comments: 14 pages, 9 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)

We study collective modes of superfluid Bose gases in optical lattices combined with potential barriers. We assume that the system is in the vicinity of the quantum phase transition to a Mott insulator at a commensurate filling, where emergent particle-hole symmetry gives rise to two types of collective mode, namely, a gapless Nambu-Goldstone (NG) phase mode and a gapful Higgs amplitude mode. We consider two kinds of potential barrier: one does not break the particle-hole symmetry while the other does. In the presence of the former barrier, we find Higgs bound states that have binding energies lower than the bulk Higgs gap and are localized around the barrier. We analyze tunneling properties of the NG mode incident to both barriers to show that the latter barrier couples the Higgs bound states with the NG mode, leading to Fano resonance mediated by the bound states. Thanks to the universality of the underlying field theory, it is expected that Higgs bound states may be present also in other condensed matter systems with a particle-hole symmetry and spontaneous breaking of a continuous symmetry, such as quantum dimer antiferromagnets, superconductors, and charge-density-wave materials.

Mar 5
1.arXiv:1503.01164 [pdf, other]
Tuning the Chern number and Berry curvature with spin-orbit coupling and magnetic textures
Timothy M. McCormick, Nandini Trivedi
Comments: 8 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We obtain the band structure of a particle moving in a magnetic spin texture, classified by its chirality and structure factor, in the presence of spin-orbit coupling. This rich interplay leads to a variety of novel topological phases characterized by the Berry curvature and their associated Chern numbers. We suggest methods of experimentally exploring these topological phases by Hall drift measurements of the Chern number and Berry phase interferometry to map the Berry curvature.

Mar 3
1.arXiv:1502.07350 (cross-list from quant-ph) [pdf, other]
Fundamental limitations and design of effective Hamiltonians in driven lattices
Albert Verdeny, Florian Mintert
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas)

Driven lattices permit the engineering of effective Hamiltonians with well-controllable tunneling properties. We discuss the design of such effective Hamiltonians and identify fundamental constraints imposed by the underlying lattice geometry. With the specific example of a hexagonal lattice we show how suitably chosen driving forces allow the realization of non-trivial topological phases and overcome limitations of monochromatic driving.

2.arXiv:1503.00300 [pdf, other]
Spinor Condensates on a Cylindrical Surface in Synthetic Gauge Fields
Tin-Lun Ho, Biao Huang
Comments: 4+\epsilon pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We point out that by modifying the setup of a recent experiment that generates a Dirac string, one can create a quasi 2D spinor Bose condensate on a cylindrical surface with a synthetic magnetic field pointing radially outward from the cylindrical surface. The synthetic magnetic field takes the form of the Landau gauge. It is generated by the Berry's phase of a spin texture, frozen by an external quadrupolar magnetic field. Unlike in the planar case, there are two types of vortices (called A and B) with the same vorticity. The ground state for 5\le S\le 9 consists of a row of alternating AB vortices lying at the equatorial circle of the cylinder. For higher values of S, the A and B vortices split into two rows and are displaced from each other along the cylindrical axis z. The fact that many properties of a BEC are altered in a cylindrical surface implies many rich phenomena will emerge for ground states in curved surfaces.

Mar 2
1.arXiv:1502.07944 [pdf, ps, other]
Density-dependent synthetic magnetism for ultra-cold atoms in optical lattices
Sebastian Greschner, Daniel Huerga, Gaoyong Sun, Dario Poletti, Luis Santos
Comments: 5 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

Raman-assisted hopping can allow for the creation of density-dependent synthetic magnetism for cold neutral gases in optical lattices. We show that the density-dependent fields lead to a non-trivial interplay between density modulations and chirality. This interplay results in a rich physics for atoms in two-leg ladders, characterized by a density-driven Meissner- to vortex-superfluid transition, and a non-trivial dependence of the density imbalance between the legs. Moreover, the strong rung-coupling regime presents direct transitions between different rung-singlet phases, as well as rung-singlet phases with finite rung string order. Density-dependent fields also lead to an intriguing physics in square lattices, characte by an occupation-driven transition between a non-chiral and a chiral superfluid, and a non-trivial density-wave amplitude. We finally show how the density-dependent fields may be easily probed in experiments by monitoring the expansion of doublons and holes in a Mott insulator, which presents a remarkable dependence on quantum fluctuations.

2.arXiv:1502.08052 [pdf, other]
Quantum phases of Bose-Einstein condensates with synthetic spin - orbital angular momentum coupling
Chunlei Qu, Kuei Sun, Chuanwei Zhang
Comments: 8 pages, 6 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

The experimental realization of emergent spin-orbit coupling through laser-induced Raman transitions in ultracold atoms paves a way for exploring novel superfluid physics and simulating exotic many-body phenomena. A recent proposal with the use of Laguerre-Gaussian lasers enables another fundamental type of coupling between spin and orbital angular momentum (SOAM) in ultracold atoms. We hereby study quantum phases of a realistic Bose-Einstein condensate (BEC) with this synthetic SOAM coupling in a disk-shaped geometry, respecting radial inhomogeneity of the Raman coupling. We find that the experimental system naturally resides in a strongly interacting regime in which the phase diagram significantly deviates from the single-particle picture. The interplay between SOAM coupling and interaction leads to rich structures in spin-resolved position and momentum distributions, including a stripe phase and various types of immiscible states. Our results would provide a guide for experimental investigation on SOAM coupled BECs.