Arxiv Selection Sep 2018

Aug 27-Sep 2 Jiansong Pan, Sep 3-Sep 9 Ahmet Keles, Sep 10-Sep 16 Max Arzamasovs, Sep 17-Sep 23 Haiping Hu, Sep 24-Sep 30 Biao Huang

Sep. 28

arXiv:1809.09282
A Quantum Walk of Ultra-Cold Atoms in Momentum Space
Siamak Dadras, Alexander Gresch, Sandro Wimberger, Gil S. Summy
We report on a discrete-time quantum walk that uses the momentum of ultra-cold rubidium-87 atoms as the walk space and two internal atomic states as the coin degree of freedom. Each step of the walk consists of a coin toss (a microwave pulse) followed by a unitary shift operator (a resonant quantum ratchet). We review the signatures of our quantum walk and carry out a comprehensive study on the effects of various parameters, including the ratchet strength, coin parameters, noise, and the initial system characteristics on the behavior of the walk. We show how the walk dynamics can be steered or reversed with potential applications in atom interferometry and investigation of possible biases.

arXiv:1809.10275
Fractonic Matter in Symmetry-Enriched U(1) Gauge Theory
Dominic J. Williamson, Zhen Bi, Meng Cheng
(Submitted on 27 Sep 2018) In this work we explore the interplay between global symmetry and the mobility of quasiparticle excitations. We show that fractonic matter naturally appears in a three dimensional U(1) gauge theory, enriched by global U(1) and translational symmetries, via the mechanism of anyonic spin-orbital coupling. We develop a systematic understanding of such symmetry-enforced mobility restrictions in terms of the classification of U(1) gauge theories enriched by U(1) and translational symmetries. We provide a unified construction of these phases by gauging layered symmetry-protected topological phases.

Sep. 23

arXiv:1805.07555
Second-order photonic topological insulator with corner states
Bi Ye Xie, Hong Fei Wang, Hai-Xiao Wang, Xue Yi Zhu, Jian-Hua Jiang, Ming Hui Lu, Yan Feng Chen
Higher-order topological insulators (HOTIs) which go beyond the description of conventional bulk-boundary correspondence, broaden the understanding of topological insulating phases. Being mainly focused on electronic materials, HOTIs have not been found in photonic systems yet. In this article, we propose a type of two-dimensional second-order photonic crystals with zero-dimensional corner states and one-dimensional boundary states for optical frequencies. All of these states are topologically non-trivial and can be understood based on the theory of topological polarization. Moreover, by tuning the easily-fabricated structure of the photonic crystals, we can realize different topological phases with unique topological boundary states straightforwardly. Our result can be generalized to higher dimensions and provides unprecedented venues for higher-order photonic topological insulators and semimetals.

Sep. 20

arXiv:1809.07325
Classification and construction of higher-order symmetry protected topological phases of interacting bosons
Alex Rasmussen, Yuan-Ming Lu
Motivated by the recent discovery of higher-order topological insulators, we study their counterparts in strongly interacting bosons: `higher-order symmetry protected topological (HOSPT) phases'. While the usual (1st-order) SPT phases in d spatial dimensions support anomalous (d-1)-dimensional surface states, HOSPT phases in d dimensions are characterized by topological boundary states of dimension (d-2) or smaller, protected by certain global symmetries and robust against disorders. Based on a dimensional reduction analysis, we show that HOSPT phases can be built from lower-dimensional SPT phases in a way that preserves the associated crystalline symmetries. When the total symmetry is a direct product of global and crystalline symmetry groups, we are able to classify the HOSPT phases using the K\"unneth formula of group cohomology. Based on a decorated domain wall picture of the K\"unneth formula, we show how to systematically construct the HOSPT phases, and demonstrate our construction with many examples in two and three dimensions.

Sep. 19

arXiv:1809.06891
Interaction Spectroscopy of a Two-component Mott Insulator
Jesse Amato-Grill, Niklas Jepsen, Ivana Dimitrova, William Lunden, Wolfgang Ketterle
We prepare and study a two-component Mott insulator of bosonic atoms with two particles per site. The mapping of this system to a magnetic spin model, and the subsequent study of its quantum phases, require a detailed knowledge of the interaction strengths of the two components. In this work, we use radio frequency (RF) transitions and an on-site interaction blockade for precise, empirical determination of the interaction strengths of different combinations of hyperfine states on a single lattice site. We create a map of the interactions of the lowest two hyperfine states of 7Li as a function of magnetic field, including measurements of several Feshbach resonances with unprecedented sensitivity, and we identify promising regions for the realization of magnetic spin models.

arXiv:1809.02122
A Bose-Einstein Condensate on a Synthetic Hall Cylinder
Chuan-Hsun Li, Yangqian Yan, Sayan Choudhury, David B. Blasing, Qi Zhou, Yong P. Chen
Interplay between matter and fields in physical spaces with nontrivial geometries gives rise to many exotic quantum phenomena. However, their realizations are often impeded by experimental constraints. Here, we realize a Bose-Einstein condensate (BEC) on a synthetic cylindrical surface subject to a net radial synthetic magnetic flux, topologically equivalent to a two-dimensional (2D) Hall ribbon with two edges connected. This cylindrical surface comprises a real spatial dimension and a curved synthetic dimension formed by cyclically-coupled spin states. The BEC on such a Hall cylinder has counterintuitive properties unattainable by its counterparts in 2D planes. We observe Bloch oscillations of the BEC with doubled periodicity of the band structure, analogous to traveling on a Mobi\"us strip, reflecting the BEC's emergent crystalline order with nonsymmorphic symmetry-protected band crossings. We further demonstrate such topological operations as gapping the band crossings and unzipping the cylinder. Our work opens the door to engineering synthetic curved spaces and observing intriguing quantum phenomena inherent to the topology of spaces.

Sep. 18

arXiv:1809.02125
Anatomy of skin modes and topology in non-Hermitian systems
Ching Hua Lee, Ronny Thomale
A non-Hermitian system can exhibit extensive sensitivity of its complex energy spectrum to the imposed boundary conditions, which is beyond any known phenomenon from Hermitian systems. In addition to topologically protected boundary modes, macroscopically many `skin' boundary modes may appear under open boundary conditions. We rigorously derive universal results for characterizing all avenues of boundary modes in non-Hermitian systems. We show, for the first time, how the exact energies and decay lengths of skin modes can be obtained by threading an imaginary flux. Secondly, we also derive a novel and very straightforward criterion for the existence of generic 1D topological boundary modes which does not require a contour specifically tailored to the system at hand. It also reveals that the topologically nontrivial phase is partitioned into regions where the boundary mode decay length depend differently on complex momenta roots. These results generalize known winding number approaches that are not necessarily valid beyond the simplest models, and are intimately based on the complex analytical properties of in-gap exceptional points.

Sep. 17

arXiv:1809.05721
Phononic topological insulators with tunable pseudospin physics
Yizhou Liu, Yong Xu, Wenhui Duan
Efficient control of phonons is crucial to energy-information technology, but limited by the lacking of tunable degrees of freedom like charge or spin. Here we suggest to utilize crystalline symmetry-protected pseudospins as new quantum degrees of freedom to manipulate phonons. Remarkably, we reveal a duality between phonon pseudospins and electron spins by presenting Kramers-like degeneracy and pseudospin counterparts of spin-orbit coupling, which lays the foundation for "pseudospin phononics". Furthermore, we report two types of three-dimensional phononic topological insulators, which give topologically protected, gapless surface states with linear and quadratic band degeneracies, respectively. These topological surface states display unconventional phonon transport behaviors attributed to the unique pseudospin-momentum locking, which are useful for phononic circuits, transistors, antennas, etc. The emerging pseudospin physics offers new opportunities to develop future phononics.

arXiv:1809.05554
Controlling and characterizing Floquet prethermalization in a driven quantum system
K. Singh, K. M. Fujiwara, Z. A. Geiger, E. Q. Simmons, M. Lipatov, A. Cao, P. Dotti, S. V. Rajagopal, R. Senaratne, T. Shimasaki, M. Heyl, A. Eckardt, D. M. Weld
Strong time periodic driving represents a powerful tool for engineering tunable many-body quantum states. Such Floquet engineering must avoid thermalization toward a delocalized infinite-temperature phase and thus relies on the existence of a localized prethermal regime. However, the understanding of Floquet prethermalization remains incomplete. We report experiments on a many-body Floquet system consisting of ultracold atoms in an amplitude-modulated optical lattice. A double quench protocol enables measurement of an inverse participation ratio (IPR) quantifying the degree of localization in the prethermal regime. We demonstrate control of prethermalization by tuning of integrability-breaking interactions. Following the time evolution of the driven system, we observe the sequential formation of two localized prethermal plateaux, characterized by different degrees of localization, as well as interaction-driven delocalization. The experimental control and quantitative characterization of Floquet prethermalization opens new possibilities for dynamical quantum engineering.

Sep. 14

arXiv:1809.04812
Induced p-wave pairing in Bose-Fermi mixtures
Jami J. Kinnunen, Zhigang Wu, Georg M. Bruun
Cooper pairing caused by an induced interaction represents a paradigm in our description of fermionic superfluidity. Here, we present a strong coupling theory for the critical temperature of p-wave pairing between spin polarised fermions immersed in a Bose-Einstein condensate. The fermions interact via the exchange of phonons in the condensate, and our self-consistent theory takes into account the full frequency/momentum dependence of the resulting induced interaction. We demonstrate that both retardation and self-energy effects are important for obtaining a reliable value of the critical temperature. Focusing on experimentally relevant systems, we perform a systematic analysis varying the boson-boson and boson-fermion interaction strength as well as their masses, and identify the most suitable system for realising a p-wave superfluid. Our results show that such a superfluid indeed is experimentally within reach using light bosons mixed with heavy fermions.

Sep. 13

arXiv:1809.03023
Three-Body Problem of Bosons nearby a d-wave Resonance
Juan Yao, Pengfei Zhang, Ran Qi, Hui Zhai
Motivated by recent experimental progresses, we investigate few-body properties of interacting spinless bosons nearby a d-wave resonance. Using the Skorniakov-Ter-Martirosion (STM) equations, we calculate the scattering length between an atom and a d-wave dimer, and we find that the atom-dimer scattering length is positive and is much smaller the result from the mean-field approximation. We also reveal unique properties of the three-body recombination rate for a degenerate Bose condensate nearby the d-wave resonance. We find that the total recombination rate is nearly a constant at the quasi-bound side, in contrast to the behavior of a thermal gas nearby high-partial wave resonance. We also find that the recombination rate monotonically increases across the unitary point toward the bound side, which is due to the largely enhanced coupling between the atom and the d-wave dimer with deeper binding energy. This monotonic behavior is also qualitatively different from that of a degenerate gas nearby an s-wave resonance counterpart.

Sep. 12

arXiv:1809.04055
Observation of many-body localization in a one-dimensional system with single-particle mobility edge
Thomas Kohlert, Sebastian Scherg, Xiao Li, Henrik P. Lüschen, Sankar Das Sarma, Immanuel Bloch, and Monika Aidelsburger
In this work we experimentally study many-body localization (MBL) in a one-dimensional bichromatic quasiperiodic potential with a single-particle mobility edge (SPME) using ultracold atoms. We measure the time evolution of the density imbalance between even and odd lattice sites from an initial charge density wave, and analyze the corresponding relaxation exponents. We find clear signatures of MBL in this system when the corresponding noninteracting model is deep in the localized phase. We also critically compare and contrast our results with those from a tight-binding Aubry-Andre model, which does not exhibit an SPME.

Sep. 11

arXiv:1809.03170
Chiral Majorana edge states in the vortex core of a p+ip Fermi superfluid
Jing-Bo Wang, Wei Yi, and Jian-Song Pan
We study a single vortex in a two-dimensional p+ip Fermi superfluid interacting with a Bose-Einstein condensate. The Fermi superfluid is topologically non-trivial and hosts a zero-energy Majorana bound state at the vortex core. Assuming a repulsive s-wave contact interaction between fermions and bosons, we find that fermions are depleted from the vortex core when the bosonic density becomes sufficiently large. In this case, a dynamically-driven local interface emerges between fermions and bosons, along which chiral Majorana edge states should appear.We examine in detail the variation of vortex-core structures as well as the formation of chiral Majorana edge states with increasing bosonic density. In particular, when the angular momentum of the vortex matches the chirality of the Fermi superfluid, the Majorana zero mode and normal bound states within the core continuously evolve into chiral Majorana edge states. Otherwise, a first-order transition occurs in the lowest excited state within the core, due to the competition between counter-rotating normal bound states in forming chiral Majorana edge states. Such a transition is manifested as a sharp peak in the excitation gap above the Majorana zero mode, at which point the Majorana zero mode is protected by a large excitation gap.Our study presents an illuminating example on how topological defects can be dynamically controlled in the context of cold atomic gases.

Sep. 10

arXiv:1808.10251
Complex wave fields in the interacting one-dimensional Bose gas
J. Pietraszewicz and P. Deuar
We study the temperature regimes of the 1d interacting gas to determine when the matter wave (c-field) theory is, in fact, correct and usable. The judgment is made by investigating the level of discrepancy in many observables at once in comparison to the exact Yang-Yang theory. We also determine what cutoff maximizes the accuracy of such an approach. Results are given in terms of a bound on accuracy, as well as an optimal cutoff prescription. For a wide range of temperatures the optimal cutoff is independent of density or interaction strength and so its temperature dependent form is suitable for many cloud shapes and, possibly, basis choices. However, this best global choice is higher in energy than most prior determinations. The high value is needed to obtain the correct kinetic energy, but does not detrimentally affect other observables.

Sep. 3- Sep. 9

arXiv:1808.10816
Quantum Optimization for Maximum Independent Set Using Rydberg Atom Arrays
Hannes Pichler, Sheng-Tao Wang, Leo Zhou, Soonwon Choi, Mikhail D. Lukin
We describe and analyze an architecture for quantum optimization to solve maximum independent set (MIS) problems using neutral atom arrays trapped in optical tweezers. Optimizing independent sets is one of the paradigmatic, NP-hard problems in computer science. Our approach is based on coherent manipulation of atom arrays via the excitation into Rydberg atomic states. Specifically, we show that solutions of MIS problems can be efficiently encoded in the ground state of interacting atoms in 2D arrays by utilizing the Rydberg blockade mechanism. By studying the performance of leading classical algorithms, we identify parameter regimes, where computationally hard instances can be tested using near-term experimental systems. Practical implementations of both quantum annealing and variational quantum optimization algorithms beyond the adiabatic principle are discussed.

arXiv:1809.00017
Hydrodynamic model of BEC with anisotropic short range interaction and the bright solitons in the repulsive BEC
Pavel A. Andreev
The quantum hydrodynamic model is developed for the axial symmetric anisotropic short-range interaction. The quantum stress tensor presents the interaction. It is derived up to the third order by the interaction radius. The first order by the interaction radius contains the isotropic part only. It leads to the interaction in the Gross-Pitaevskii approximation. Terms existing in the third order by the interaction radius are caused by the isotropic and nonisotropic parts of the interaction. Each of them introduces the interaction constant. Therefore, three interaction constants are involved in the model. Atoms, except the alkali and alkali-earth atoms, can have anisotropic potential of interaction, particularly it is demonstrated for the lanthanides. The short-wavelength instability caused by the nonlocal terms appears in the Bogoliubov spectrum. Conditions for the stable and unstable behaviour are described. Bright solitons in the repulsive BEC are studied under influence of the anisotropic short-range interaction in the BEC of one species. Area of existence of this bright solitons corresponds to the area of the instability of the Bogoliubov spectrum. Approximate reduction of the nonlocal nonlinearity to the quintic nonlinearity at the description of the bright solitons is demonstrated either.

arXiv:1809.00171
On the response of a Bose-Einstein condensate exposed to two counterpropagating ultra-fast laser beams
Priyam Das, Ayan Khan, Anirban Pathak
The effect of light-matter interaction is investigated for a situation where counter propagating laser pulses of localized nature are incident on the atomic condensate. In contrast to the earlier investigations on the similar systems, it's assumed that the laser beams are ultra-fast and they have a sech2 profile. Specifically, we consider a quasi-homogeneous, later extended to inhomogeneous, Bose-Einstein condensate (BEC), which is exposed to two counter propagating orthogonally polarized ultra-fast laser beams of equal intensity. The electromagnetic field creates an optical potential for the Bose-Einstein condensate, which in turn modifies the optical field. Hence, light and matter are found to contentiously exchange energy and thus to modify themselves dynamically. In the inhomogenous case, a self-similar method is used here to treat a cigar-shaped BEC exposed to light. Our theoretical analysis in a hither to unexplored regime of BEC-light interaction hints at the solitonic bound state formation in the regime where the atom-atom interaction is repulsive and the light-matter interaction is attractive. The energy diagram also indicates a transfer of energy from photon to atom as the light-matter interaction turns repulsive from attractive.

arXiv:1809.00471
Confinement-induced Resonance of Alkaline-earth-metal-like Atoms in Anisotropic Quasi-one-dimensional Traps
Qing Ji, Ren Zhang, Xiang Zhang, Wei Zhang
We study the confinement-induced resonance (CIR) of 173Yb atoms near an orbital Feshbach resonance in a quasi-one-dimensional tube with transversal anisotropy. By solving the two-body scattering problem, we obtain the location of CIR for various anisotropy ratio and magnetic field. Our results show that the anisotropy of the trapping potential can serve as an additional knob to tune the location of CIR. In particular, one can shift the location of CIR to the region attainable in current experiment. We also study the energy spectrum of the system and analyze the properties of CIR from the perspective of bound states. We find that as the orbital Feshbach resonance acquires two nearly degenerate scattering channels, which in general have different threshold energies, CIR takes place when the closed channel bound state energy becomes degenerate with one of the thresholds.

arXiv:1809.00571
Quantum simulation of the Hubbard model with ultracold fermions in optical lattices
Leticia Tarruell (ICFO), Laurent Sanchez-Palencia (CPHT)
Ultracold atomic gases provide a fantastic platform to implement quantum simulators and investigate a variety of models initially introduced in condensed matter physics or other areas. One of the most promising applications of quantum simulation is the study of strongly-correlated Fermi gases, for which exact theoretical results are not always possible with state-of-the-art approaches. Here, we review recent progress of the quantum simulation of the emblematic Fermi-Hubbard model with ultracold atoms. After introducing the Fermi-Hubbard model in the context of condensed matter, its implementation in ultracold atom systems, and its phase diagram, we review landmark experimental achievements, from the early observation of the onset of quantum degeneracy and superfluidity to demonstration of the Mott insulator regime and the emergence of long-range anti-ferromagnetic order. We conclude by discussing future challenges, including the possible observation of high-Tc superconductivity, transport properties, and the interplay of strong correlations and disorder or topology.