Arxiv Selection Feb 2019

Feb 1-Feb 7 Ahmet Keles, Feb 8- Feb 14 Haiping Hu, Feb 15-Feb 21 Biao Huang, Feb 22-Feb 28 Xuguang Yue

Feb 1-Feb 7

Equilibrium trapping of cold atoms using dipole and radiative forces in an optical trap
Taro Mashimo, Masashi Abe, Satoshi Tojo
We report on highly effective trapping of cold atoms by a new method for a stable single optical trap in the near-optical resonant regime. An optical trap with the near-optical resonance condition consists of not only the dipole but also the radiative forces, while a trap using a far-off resonance dominates only the dipole force. We estimate a near-optical resonant trap for ultracold rubidium atoms in the range between -0.373 and -2.23 THz from the resonance. The time dependence of the trapped atoms indicates some difference of the stable center-of-mass positions in the near-optical resonant trap, and also indicates that the differences are caused by the change of the equilibrium condition of the optical dipole and radiative forces. A stable position depends only on laser detuning due to the change in the radiative force; however, the position is ineffective against the change in the laser intensity, which results in a change in the radiative force.

An Optical Tweezer Array of Ultracold Molecules
Loïc Anderegg, Lawrence W. Cheuk, Yicheng Bao, Sean Burchesky, Wolfgang Ketterle, Kang-Kuen Ni, John M. Doyle
Arrays of single ultracold molecules promise to be a powerful platform for many applications ranging from quantum simulation to precision measurement. Here we report on the creation of an optical tweezer array of single ultracold CaF molecules. By utilizing light-induced collisions during the laser cooling process, we trap single molecules. The high densities attained inside the tweezer traps have also enabled us to observe in the absence of light molecule-molecule collisions of laser cooled molecules for the first time.

Critical Opalescence across the Doping Driven Mott Transition in Optical Lattices of Ultracold Atoms
C. Walsh, P. Sémon, G. Sordi, A.-M. S. Tremblay
Phase transitions and their associated crossovers are imprinted in the behavior of fluctuations. Motivated by recent experiments on ultracold atoms in optical lattices, we compute the thermodynamic density fluctuations δN2 of the two-dimensional fermionic Hubbard model with plaquette cellular dynamical mean-field theory. To understand the length scale of these fluctuations, we separate the local from the nonlocal contributions to δN2. We determine the effects of particle statistics, interaction strength U, temperature T and density n. At high temperature, our theoretical framework reproduces the experimental observations in the doping-driven crossover regime between metal and Mott insulator. At low temperature, there is an increase of thermodynamic density fluctuations, analog to critical opalescence, accompanied by a surprising reduction of the absolute value of their nonlocal contributions. This is a precursory sign of an underlying phase transition between a pseudogap phase and a metallic state in doped Mott insulators, which should play an important role in the cuprate high-temperature superconductors. Predictions for ultracold atom experiments are made.

Quantum spiral spin-tensor magnetism
Xiaofan Zhou, Xi-Wang Luo, Gang Chen, Suotang Jia, Chuanwei Zhang
The characterization of quantum magnetism in a large spin (≥1) system naturally involves both spin-vectors and -tensors. While certain types of spin-vector (e.g., ferromagnetic, spiral) and spin-tensor (e.g., nematic in frustrated lattices) orders have been investigated separately, the coexistence and correlation between them have not been well explored. Here we propose and characterize a novel quantum spiral spin-tensor order on a spin-1 Heisenberg chain subject to a spiral spin-tensor Zeeman field, which can be experimentally realized using a Raman-dressed cold atom optical lattice. Through numerical density-matrix renormalization group (DMRG) simulation, we obtain the phase diagram and characterize the coexistence of spin-vector and spin-tensor orders as well as their correlations. Our results may open an avenue for exploring novel magnetic orders and spin-tensor electronics/atomtronics in large-spin systems.

Bloch oscillations of spin-orbit-coupled cold atoms in an optical lattice and spin current generation
Wei Ji, Keye Zhang, Weiping Zhang, Lu Zhou
We study the Bloch oscillation dynamics of a spin-orbit-coupled cold atomic gas trapped inside a one-dimensioanl optical lattice. The eigenspectra of the system is identified as two interpenetrating Wannier-Stark ladder. Based on that, we carefully analyzed the Bloch oscillation dynamics and found out that intraladder coupling between neighboring rungs of Wannier-Stark ladder give rise to ordinary Bloch oscillation while interladder coupling lead to small amplitude high frequency oscillation superimposed on it. Specifically spin-orbit interaction breaks Galilean invariance, which can be reflected by out-of-phase oscillation of the two spin components in the accelerated frame. The possibility of generating spin current in this system are also explored.

Bose-Einstein condensate in Bloch bands with off-diagonal periodic potential
Yue-Xin Huang, Wei Feng Zhuang, Xiang-Fa Zhou, Han Pu, Guang-Can Guo, Ming Gong
We report the Bose-Einstein condensate (BEC) in the Bloch bands with off-diagonal periodic potential (ODPP), which simultaneously plays the role of spin-orbit coupling (SOC) and Zeeman field. This model can be realized using two independent Raman couplings in the same three level system, in which the time-reversal symmetry ensures the energy degeneracy between the two states with opposite momenta. We find that these two Raman couplings can be used to tune the spin polarization in momentum space, thus greatly modifies the effective scatterings over the Bloch bands. We observe a transition from the Bloch plane wave phase with condensate at one wave vector to the Bloch stripe phase with condensates at the two Bloch states with opposite wave vectors. These two phases will exhibit totally different spin textures and density modulations in real space, which are totally different from that in free space. In momentum space multiple peaks differ by some reciprocal lattice vectors can be observed, reflecting the periodic structure of the ODPP. A three-band effective model is proposed to understand these observations. This system can provide a new platform in investigating of various physics, such as collective excitations, polaron and topological superlfuids, over the Bloch bands.

Entanglement structure of a quantum simulator: the two-component Bose-Hubbard model
Ivan Morera, Artur Polls, Bruno Juliá-Díaz
We consider a quantum simulator of the Heisenberg chain with ferromagnetic interactions based on the two-component 1D Bose-Hubbard model at filling equal to two in the strong coupling regime. The entanglement properties of the ground state are compared between the original spin model and the quantum simulator as the interspecies interaction approaches the intraspecies one. A numerical study of the entanglement properties of the quantum simulator state is supplemented with analytical expressions derived from the simulated Hamiltonian. At the isotropic point, the entanglement properties of the simulated system are not properly predicted by the quantum simulator.

Particle-number scaling of the quantum work statistics and Loschmidt echo in Fermi gases with time-dependent traps
Ettore Vicari
We investigate the particle-number dependence of some features of the out-of-equilibrium dynamics of d-dimensional Fermi gases in the dilute regime. We consider protocols entailing the variation of the external potential which confines the particles within a limited spatial region, in particular sudden changes of the trap size. In order to characterize the dynamic behavior of the Fermi gas, we consider various global quantities such as the ground-state fidelity for different trap sizes, the quantum work statistics associated with the protocol considered, and the Loschmidt echo measuring the overlap of the out-of-equilibrium quantum states with the initial ground state. Their asymptotic particle-number dependences show power laws for noninteracting Fermi gases. We also discuss the effects of short-ranged interactions to the power laws of the average work and its square fluctuations, within the Hubbard model and its continuum limit, arguing that they do not generally change the particle-number power laws of the free Fermi gases, in any spatial dimensions.

Feb 11

arXiv:1902.02912 [pdf]

Nanomechanical characterization of quantum interference in a topological insulator nanowire

Minjin Kim, Jihwan Kim, Yasen Hou, Dong Yu, Yong-Joo Doh, Bongsoo Kim, Kun Woo Kim, Junho Suh Comments: 15+16 pages, 4+11 figures

Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

The discovery of two-dimensional gapless Dirac fermions in graphene and topological insulators (TI) has sparked extensive ongoing research toward applications of their unique electronic properties. The gapless surface states in three-dimensional insulators indicate a distinct topological phase of matter with a non-trivial Z2 invariant that can be verified by angle-resolved photoemission spectroscopy or magnetoresistance quantum oscillation. In TI nanowires, the gapless surface states exhibit Aharonov-Bohm (AB) oscillations in conductance, with this quantum interference effect accompanying a change in the number of transverse one-dimensional modes in transport. Thus, while the density of states (DOS) of such nanowires is expected to show such AB oscillation, this effect has yet to be observed. Here, we adopt nanomechanical measurements that reveal AB oscillations in the DOS of a topological insulator. The TI nanowire under study is an electromechanical resonator embedded in an electrical circuit, and quantum capacitance effects from DOS oscillation modulate the circuit capacitance thereby altering the spring constant to generate mechanical resonant frequency shifts. Detection of the quantum capacitance effects from surface-state DOS is facilitated by the small effective capacitances and high quality factors of nanomechanical resonators, and as such the present technique could be extended to study diverse quantum materials at nanoscale.

Feb 12

arXiv:1902.03445 [pdf, other]

A nonlinear, geometric Hall effect without magnetic field

Nicholas B. Schade, David I. Schuster, Sidney R. Nagel Comments: 22 pages, 3 figures

Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall) The classical Hall effect, the traditional means of determining charge-carrier sign and density in a conductor, requires a magnetic field to produce transverse voltages across a current-carrying wire. We show that along curved paths -- without any magnetic field -- geometry alone can produce nonlinear transverse potentials that reflect the charge-carrier sign and density. We demonstrate this effect in curved graphene wires where the transverse potentials are consistent with the doping and change polarity as we switch the carrier sign. In straight wires, we measure transverse potential fluctuations with random polarity demonstrating that the current follows a complex, tortuous path. This geometrically-induced potential offers a sensitive characterization of inhomogeneous current flow in thin films.

Feb 13

arXiv:1902.04076 (cross-list from quant-ph) [pdf, other]

Scrambling and Complexity in Phase Space

Quntao Zhuang, Thomas Schuster, Beni Yoshida, Norman Y. Yao Comments: 28 + 6 pages, 19 figures

Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech); High Energy Physics - Theory (hep-th); Atomic Physics (physics.atom-ph) The study of information scrambling in many-body systems has sharpened our understanding of quantum chaos, complexity and gravity. Here, we extend the framework for exploring information scrambling to infinite dimensional continuous variable (CV) systems. Unlike their discrete variable cousins, continuous variable systems exhibit two complementary domains of information scrambling: i) scrambling in the phase space of a single mode and ii) scrambling across multiple modes of a many-body system. Moreover, for each of these domains, we identify two distinct `types' of scrambling; genuine scrambling, where an initial operator localized in phase space spreads out and quasi scrambling, where a local ensemble of operators distorts but the overall phase space volume remains fixed. To characterize these behaviors, we introduce a CV out-of-time-order correlation (OTOC) function based upon displacement operators and offer a number of results regarding the CV analog for unitary designs. Finally, we investigate operator spreading and entanglement growth in random local Gaussian circuits; to explain the observed behavior, we propose a simple hydrodynamical model that relates the butterfly velocity, the growth exponent and the diffusion constant. Experimental realizations of continuous variable scrambling as well as its characterization using CV OTOCs will be discussed.

Feb 14

arXiv:1902.04599 [pdf, other]

Topological Phase Transition in Superconductors with Mirror Symmetry

Adam Lowe, Miguel Ortuno, Igor V Yurkevich Subjects: Superconductivity (cond-mat.supr-con)

We provide analytical and numerical evidence that the attractive two-dimensional Kitaev model on a lattice with mirror symmetry demonstrates unusual 'intrinsic' phase at half filling. This phase emerges in the phase diagram at the boundary separating two topological superconductors with opposite Chern numbers and exists due to condensation of non-zero momentum Cooper pairs. Unlike Fulde-Ferrell-Larkin-Ovchinnikov superconductivity, the Cooper pairs momenta are lying along two lines in the Brillouin zone meaning simultaneous condensation of a continuum of Cooper pairs.

arXiv:1902.04769 [pdf, other]

Interacting non-Hermitian ultracold bosonic systems in three-dimensional harmonic trap: two-body exact solutions and high-order exceptional points

Lei Pan, Shu Chen, Xiaoling Cui

Comments: 11 pages, 6 figures and 1 table Subjects: Quantum Gases (cond-mat.quant-gas)

We study interacting ultracold atoms in a three-dimensional (3D) harmonic trap with spin-selective dissipations, which can be effectively described by non-Hermitian parity-time (PT) symmetric Hamiltonians. By solving the non-Hermitian two-body problem of spin-1/2 (spin-1) bosons in a 3D harmonic trap exactly, we find that the system can exhibit third-order (fifth-order) exceptional point (EP) with ultra-sensitive cube-root (fifth-root) spectral response due to interaction anisotropies in spin channels. We also present the general principle for the creation of high-order EPs and their spectral sensitivities with arbitrary particle number N and arbitrary spin s. What is even more interesting is that with spin-independent interactions, the EP order of bosons can be as high as 2Ns+1, and the spectral response around EP can be as sensitive as ∼ϵ1/(2ks+1) under a k-body interaction anisotropy. These results could provide a convenient route towards more powerful sensor devices in spinor cold atomic systems.

arXiv:1902.04874 [pdf, other]

Symmetry Imbalance in a Shaking Triangular Optical Lattice

Xinxin Guo, Wenjun Zhang, Zhihan Li, Hongmian Shui, Xuzong Chen, Xiaoji Zhou

Subjects: Quantum Gases (cond-mat.quant-gas)

Lots of interesting phenomena can be investigated under symmetry imbalance condition, such as symmetry protected topological orders, simulation of frustrated quantum anti-ferromagnetism and so on. In this work, we demonstrated a simple but effective method to realize desired symmetry imbalance of momentum distribution of the atoms in an optical triangular lattice, by shaking the lattice and controlling the driven signal. A quasi-momentum oscillation along the shaking direction in lattice frame of reference causes the formation of the mix of ground energy band and first excited band in laboratory frame of reference and thus the symmetry imbalance, within the regime that the shaking frequency is far less than the coupling frequency between ground band and higher energy bands. We experimentally investigate the influence of the initial phase, frequency, amplitude of the shaking signal on the symmetry imbalance, and observe good agreement with our theoretical model. This supplies a useful tool to study atomic behavior in the non-zero quasi-momentum.

arXiv:1902.05041 (cross-list from quant-ph) [pdf, other]

Detection of quantum phases via out-of-time-order correlators

Ceren B. Dağ, Kai Sun, L.-M. Duan

Comments: 4 pages, 3 figures with supplementary Subjects: Quantum Physics (quant-ph); Statistical Mechanics (cond-mat.stat-mech)

We elucidate the relation between out-of-time-order correlators (OTOCs) and the phase transitions via analytically studying the OTOC dynamics both in non-degenerate and degenerate spectra. Our method points to key ingredients to dynamically detect quantum phases as well as their symmetry breaking patterns via out-of-time-order correlators for a wide range of quantum phase transitions. We apply our method to a critical model, XXZ model that numerically confirms our predictions. We further discuss how our method could be useful to understand the dynamical features of the OTOCs.