Arxiv Selection Aug 2018
Jul 30 - Aug 5 Biao Huang, Aug 6- Aug 12 Xuguang Yue, Aug 13- Aug 19 Haiyuan Zou, Aug 20- Aug 26 Zehan Li, Aug 27-Sep 2 Jiansong Pan
Contents
Aug. 30
Emulating Topological Currents Arisen from Dipolar Parity Anomaly in Two-Dimensional Optical Lattices
Zhi Lin, Xian-Jia Huang, Dan-Wei Zhang, Shi-Liang Zhu, Z. D. Wang
Comments: 5 pages, 4 figures
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas)
We reveal topological currents arising from dipolar parity anomaly in the presence of spatiotemporally weak-dependent energy-momentum separation of paired Dirac points in two-dimensional space-time inversion symmetric semimetals. A corresponding lattice model is proposed to emulate the topological currents by using two-component ultracold atoms in a two-dimensional optical Raman lattice. In our scheme, the topological currents can be generated by varying in-site coupling between the two atomic components in time and tuned via the laser fields. Moreover, we show that the topological particle currents can directly be detected from measuring the drift of the center-of-mass of the atomic gases.
Aug. 29
Topological non-linear σ-model, higher gauge theory, and a realization of all 3+1D topological orders for boson systems
Chenchang Zhu, Tian Lan, Xiao-Gang Wen
Comments: 32 pages, 14 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
A discrete non-linear σ-model is obtained by triangulate both the space-time Md+1 and the target space K. If the path integral is given by the sum of all the complex homomorphisms ϕ:Md+1→K, with an partition function that is independent of space-time triangulation, then the corresponding non-linear σ-model will be called topological non-linear σ-model which is exactly soluble. Those exactly soluble models suggest that phase transitions induced by fluctuations with no topological defects (i.e. fluctuations described by homomorphisms ϕ) usually produce a topologically ordered state and are topological phase transitions, while phase transitions induced by fluctuations with all the topological defects give rise to trivial product states and are not topological phase transitions. If K is a space with only non-trivial first homotopy group G which is finite, those topological non-linear σ-models can realize all 3+1D bosonic topological orders without emergent fermions, which are described by Dijkgraaf-Witten theory with gauge group π1(K)=G. Here, we show that the 3+1D bosonic topological orders with emergent fermions can be realized by topological non-linear σ-models with π1(K)= finite groups, π2(K)=Z2, and πn>2(K)=0. A subset of those topological non-linear σ-models corresponds to 2-gauge theories, which realize and classify bosonic topological orders with emergent fermions that have no emergent Majorana zero modes at triple string intersections. The classification of 3+1D bosonic topological orders may correspond to a classification of unitary fully dualizable fully extended topological quantum field theories in 4-dimensions.
Aug. 27
Dynamical quantum phase transitions in U(1) quantum link models
Yi-Ping Huang, Debasish Banerjee, Markus Heyl
Comments: 8 pages, 8 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Lattice (hep-lat); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)
Quantum link models are extensions of Wilson-type lattice gauge theories which realize exact gauge invariance with finite-dimensional Hilbert spaces. Quantum link models not only reproduce the standard features of Wilson's lattice gauge theories, but also host new phenomena such as crystalline confined phases. We study the non-equilibrium quench dynamics for two representative cases, U(1) quantum link models in (1+1)d and (2+1)d, through the lens of dynamical quantum phase transitions. Finally, we discuss the connection to the high-energy perspective and the experimental feasibility to observe the discussed phenomena in recent quantum simulator settings such as trapped ions, ultra-cold atoms, and Rydberg atoms.
Aug. 24
Fermion-Mediated Interactions Between Bosonic Atoms
B.J. DeSalvo, Krutik Patel, Geyue Cai, Cheng Chin
Comments: 7 pages, 6 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
In high energy and condensed matter physics, particle exchange plays an essential role in the understanding of long-range interactions. For example, the exchange of massive bosons leads to the Yukawa potential [1, 2]. Phonon exchange between electrons gives rise to Cooper pairing in superconductors [3]. When a Bose-Einstein condensate (BEC) of Cs is embedded in a degenerate Fermi gas of Li, we show that interspecies interactions can give rise to an effective trapping potential, damping, and attractive boson-boson interactions mediated by fermions. The latter, related to the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism [4], results from a coherent three-body scattering process. Such mediated interactions are expected to form novel magnetic phases [5] and supersolids [6]. We show that for suitable conditions, the mediated interactions can convert a stable BEC into a train of “Bose-Fermi solitons” [7, 8].
Aug. 23
Quantum tunneling dynamics of an interacting Bose-Einstein Condensate through a Gaussian barrier
P. Manju, K. S. Hardman, M. A. Sooriyabandara, P. B. Wigley, J. D. Close, N. P. Robins, M. R. Hush, S. S. Szigeti
Comments: 10 pages, 7 figures
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas)
The transmission of an interacting Bose-Einstein condensate incident on a repulsive Gaussian barrier is investigated through numerical simulation. The dynamics associated with interatomic interactions are studied across a broad parameter range not previously explored. Effective 1D Gross-Pitaevskii equation (GPE) simulations are compared to classical Boltzmann-Vlasov equation (BVE) simulations in order to isolate purely coherent matterwave effects. Quantum tunneling is then defined as the portion of the GPE transmission not described by the classical BVE. An exponential dependence of transmission on barrier height is observed in the purely classical simulation, suggesting that observing such exponential dependence is not a sufficient condition for quantum tunneling. Furthermore, the transmission is found to be predominately described by classical effects, although interatomic interactions are shown to modify the magnitude of the quantum tunneling. Interactions are also seen to affect the amount of classical transmission, producing transmission in regions where the non-interacting equivalent has none. This theoretical investigation clarifies the contribution quantum tunneling makes to overall transmission in many-particle interacting systems, potentially informing future tunneling experiments with ultracold atoms.
Aug. 22
Equations of state from individual one-dimensional Bose gases
F. Salces-Carcoba, C. J. Billington, A. Putra, Y. Yue, S. Sugawa, I. B. Spielman
Comments: 19 pages including supplementary material, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph)
We trap individual 1D Bose gases and obtain the associated equation of state by combining calibrated confining potentials with in-situ density profiles. Our observations agree well with the exact Yang–Yang 1D thermodynamic solutions under the local density approximation. We find that our final 1D system undergoes inefficient evaporative cooling that decreases the absolute temperature, but monotonically reduces a degeneracy parameter.
Aug. 21
Interacting bosons in two-dimensional lattices with localized dissipation
Arko Roy, Kush Saha
Comments: 7 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
Motivated by the recent experiment [Takafumi Tomita et al., Sci. Adv. 3, (2017)], we study the dynamics of interacting bosons in a two-dimensional optical lattice with local dissipation. Together with the Gutzwiller mean-field theory for density matrices and Lindblad master equation, we show how the onsite interaction be- tween bosons affects the particle loss for various strengths of dissipation. For moderate dissipation, the trend in particle loss differs significantly near the superfluid-Mott boundary than the deep superfluid regime. While the loss is suppressed for stronger dissipation in the deep superfluid regime, revealing the typical quantum Zeno effect, the loss near the phase boundary shows non-monotonic dependence on the dissipation strength. We fur- thermore show that close to the phase boundary, the long-time dynamics is well contrasted with the dissipative dynamics deep into the superfluid regime. Thus the loss of particle due to dissipation may act as a probe to differentiate strongly-correlated superfluid regime from its weakly-correlated counterpart.
Aug. 20
Quantum FFLO state in clean layered superconductors
Kok Wee Song and Alexei E. Koshelev
Comments: 14 pages, 8 figures
Subjects: Superconductivity (cond-mat.supr-con); Strongly Correlated Electrons (cond-mat.str-el)
We investigate the influence of Landau quantization on the superconducting instability for a pure layered superconductor in the magnetic field directed perpendicular to the layers. We demonstrate that the quantization corrections to the Cooper-pairing kernel with finite Zeeman spin splitting promote the formation of the nonuniform state in which the order parameter is periodically mod- ulated along the magnetic field, i.e., between the layers (Fulde-Ferrell-Larkin-Ovchinnikov [FFLO] state). The conventional uniform state experiences such quantization-induced FFLO instability at low temperatures even in a common case of predominantly orbital suppression of superconductivity when the Zeeman spin splitting is expected to have a relatively weak effect. The maximum relative FFLO temperature is given by the ratio of the superconducting transition temperature and the Fermi energy. This maximum is realized when the ratio of the spin-spitting energy and the Landau- level separation is half-integer. These results imply that the FFLO states may exist not only in the Pauli-limited superconductors but also in very clean materials with small Zeeman spin-splitting energy.
Aug. 17
1.arXiv:1808.05220 [pdf, other]
Configuration-Controlled Many-Body Localization and the Mobility Emulsion
Michael Schecter, Thomas Iadecola, Sankar Das Sarma
Comments: 19 pages, 12 figures
Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech); Strongly Correlated Electrons (cond-mat.str-el)
We uncover a new non-ergodic phase, distinct from the many-body localized (MBL) phase, in a disordered two-leg ladder of interacting spinless fermions. The dynamics of this emergent phase, which has no single particle analog and exists only for strong disorder and finite interaction, is determined by the many-body configuration of the initial state. Remarkably, this phase features the coexistence of localized and extended many-body states at fixed energy density and thus does not exhibit a many-body mobility edge, nor does it reduce to a model with a single-particle mobility edge in the noninteracting limit. We show that eigenstates in this phase can be described in terms of interacting emergent Ising spin degrees of freedom ("singlons") suspended in a mixture with inert charge degrees of freedom ("doublons" and "holons"), and thus dub it a mobility emulsion (ME). We argue that grouping eigenstates by their doublon/holon density reveals a transition between localized and extended states that is invisible as a function of energy density. We further demonstrate that the dynamics of the system following a quench may exhibit either thermalizing or localized behavior depending on the doublon/holon density of the initial product state. Intriguingly, the ergodicity of the ME is thus tuned by the initial state of the many-body system. These results establish a new paradigm for using many-body configurations as a tool to study and control the MBL transition. The ME phase may be observable in suitably prepared cold atom optical lattices.
2. arXiv:1808.05250 [pdf, ps, other]
Origin of Magic Angles in Twisted Bilayer Graphene
Grigory Tarnopolsky, Alex J. Kruchkov, Ashvin Vishwanath
Comments: 7 pages, 4 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
Twisted Bilayer graphene (TBG) is known to feature isolated and relatively flat bands near charge neutrality, when tuned to special magic angles. However, different criteria for the magic angle such as the vanishing of Dirac speed, minimal bandwidth or maximal band gap to higher bands typically give different results. Here we study a modified continuum model for TBG which has an infinite sequence of magic angles θ at which, we simultaneously find that (i) the Dirac speed vanishes (ii) absolutely flat bands appear at neutrality and (iii) bandgaps to the excited bands are maximized. When parameterized in terms of α∼1/θ, they recur with the simple periodicity of Δα≃3/2 , which, beyond the first magic angle, differs from earlier calculations. Further, in this model we prove that the vanishing of the Dirac velocity ensures the exact flatness of the band and show that the flat band wave functions are related to doubly-periodic functions composed of ratios of theta functions. Also, using perturbation theory up to α 8 we capture important features of the first magic angle θ≈ 1.09 ∘ ( α≈0.586 ), which precisely explains the numerical results. Finally, based on our model we discuss the prospects for observing the second magic angle in TBG.
3. arXiv:1808.05478 [pdf, other]
Static and dynamic phases of a Tonks-Girardeau gas in an optical lattice
Mathias Mikkelsen, Thomás Fogarty, Thomas Busch
Comments: 24 pages, 12 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We investigate the properties of a Tonks-Girardeau gas in the presence of a one-dimensional lattice potential. Such a system is known to exhibit a pinning transition when the lattice is commensurate with the particle density, leading to the formation of an insulating state even at infinitesimally small lattice depths. Here we examine the properties of the gas at all lattices depths and, in addition to the static properties, also consider the non-adiabatic dynamics induced by the sudden motion of the lattice potential with a constant speed. Our work provides a continuum counterpart to the work done in discrete lattice models.
Aug. 16
1. arXiv:1808.04826 [pdf, other]
Orbital currents in insulating and doped antiferromagnets
Mathias S. Scheurer, Subir Sachdev
Comments: 24 pages. 4 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We describe square lattice spin liquids which break time-reversal symmetry, while preserving translational symmetry. The states are distinguished by the manner in which they transform under mirror symmetries. All the states have spontaneous orbital charge currents in the bulk (even in the insulator), but in some cases, orbital currents are non-zero only in a formulation with three orbitals per unit cell. The states are formulated using both the bosonic and fermionic spinon approaches. We describe states with Z_2 and U(1) bulk topological order, and the chiral spin liquid with semionic excitations. The chiral spin liquid has no orbital currents in the one-band formulation, but does have orbital currents in the three-band formulation. We discuss application to the cuprate superconductors, after postulating that the broken time-reversal and mirror symmetries persist into confining phases which may also break other symmetries. In particular, the broken symmetries of the chiral spin liquid could persist into the N\'eel state.
Aug. 15
1. arXiv:1808.04793 [pdf, ps, other]
Self-bound Bose-Fermi liquids in lower dimensions
Debraj Rakshit, Tomasz Karpiuk, Mirosław Brewczyk, Maciej Lewenstein, Mariusz Gajda
Comments: 5 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We study weakly interacting mixtures of ultracold atoms composed of bosonic and fermionic species in 2D and 1D space. When interactions between particles are appropriately tuned, self-bound quantum liquids can be formed. Formation of these droplets is due to the higher order correction terms contributing to the total energy and originating in quantum fluctuations. The fluctuations depend drastically on the dimensionality of the system. We concentrate here on low dimensional systems because they are the most promising from experimental point of view due to significant reduction of three-body losses. We analyse stability conditions for 2D and 1D systems and predict values of equilibrium densities of droplets.
2. arXiv:1808.04599 [pdf, other]
Experimental Determination of Bose-Hubbard Energies
Yosuke Takasu, Yusuke Nakamura, Jun Kobayashi, Hiroto Asaka, Yoshiaki Fukushima, Kensuke Inaba, Makoto Yamashita, Yoshiro Takahashi
Comments: 21 pages, 19 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We present the first experimental measurement of the ensemble averages of both the kinetic and interaction energies of the three-dimensional Bose--Hubbard model at finite temperature and various optical lattice depths across weakly to strongly interacting regimes, for an almost unit filling factor. The kinetic energy is obtained through Fourier transformation of a time-of-flight signal, and the interaction energy is measured using a newly developed atom-number-projection spectroscopy technique, by exploiting an ultra-narrow optical transition of two-electron atoms. The obtained experimental results can be used as benchmarks for state-of-the-art numerical methods of quantum many-body theory. As an illustrative example, we compare the measured energies with numerical calculations involving the Gutzwiller and cluster-Gutzwiller approximations, assuming realistic trap potentials and particle numbers at nonzero entropy (finite temperature); we obtain good agreement without fitting parameters. We also discuss the possible application of this method to temperature estimations for atoms in optical lattices using the thermodynamic relation. This study offers a unique advantage of cold atom system for `quantum simulators', because, to the best of our knowledge, it is the first experimental determination of both the kinetic and interaction energies of quantum many-body system.
Aug. 14
1.arXiv:1808.04129 [pdf, other]
Monitoring currents of cold atoms in atomtronic circuits
S. Safaei, L.-C. Kwek, R. Dumke, L. Amico
Subjects: Quantum Gases (cond-mat.quant-gas)
We push forward the idea that complex atomtronic circuits can be exploited to devise new protocols for the diagnostics of the cold atoms systems. Specifically, we study the quench dynamics of a condensate confined in a ring-shaped potential coupled with a rectilinear guide of finite size. We find that the dynamics of the atoms inside the guide is distinctive of states with different winding numbers in the ring condensate. The depletion of the density that is localized around the tunneling region of the ring condensate can decay in a pair of excitations experiencing a Sagnac effect. With our approach, the current states of the condensate in the ring can be read-out by inspection of the rectilinear guide only, leaving the ring condensate minimally affected by the measurement. In addition, our results set the basis for the definition of new quantum rotation sensors. At the same time, our scheme can be employed to explore fundamental questions of the dynamics of bosonic condensates.
2. arXiv:1808.03966 [pdf, other]
Quantum Phases in Resonantly Driven Fermi Hubbard Model
Ning Sun, Pengfei Zhang, Hui Zhai
Comments: 5 pages, 5 figures in main text
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
In this letter we consider quantum phases and the phase diagram of a Fermi Hubbard model under periodic driving that has been realized in recent cold atom experiments, in particular, when the driving frequency is resonant with the interaction energy. The effective Hamiltonian describing this situation contains a correlated hopping term where the density occupation strongly modifies the hopping strength. Focusing on half filling, in addition to the charge and spin density wave phases, large regions of ferromagnetic phase and phase separation are discovered in the weakly interacting regime. The mechanism of this ferromagnetism is attributed to the correlated hopping because in this regime the hopping strength within a ferromagnetic domain is normalized to a larger value than the hopping strength across the domain. Thus, the kinetic energy favors a large ferromagnetic domain and consequently drives the system into a ferromagnetic phase. We note that this is a different mechanism in contrast to the well-known Stoner mechanism for ferromagnetism where the ferromagnetism is driven by interaction energy.
Aug. 13
1.arXiv:1808.03634 [pdf, other]
Minimum model for the electronic structure of twisted bilayer graphene and related structures
Xianqing Lin, David Tománek
Comments: 5 pages, 3 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
We introduce a minimum tight-binding model with only three parameters extracted from graphene and untwisted bilayer graphene. This model reproduces quantitatively the electronic structure of not only these two systems and bulk graphite near the Fermi level, but also that of twisted bilayer graphene including the value of the magic angle, at which bands at E_F flatten without overlap and two gaps open, one above and one below E_F. The Hamiltonian is sufficiently transparent and flexible to be adopted to other twisted layered systems.
Aug. 10
1. arXiv:1808.03132 (cross-list from quant-ph) [pdf, other]
Optical bistability and nonlinear dynamics by saturation of cold Yb atoms in a cavity
Hannes Gothe, Tristan Valenzuela, Matteo Cristiani, Jürgen Eschner
Subjects: Quantum Physics (quant-ph); Atomic Physics (physics.atom-ph); Optics (physics.optics)
We observed optical bistability as well as oscillations in the upper bistable branch when cold ytterbium atoms are dispersively coupled to a high finesse optical cavity. Comparable previous observations were explained by atomic density oscillations in case of a Bose-Einstein-Condensate or optical pumping in case of a cold cloud of cesium. Both explanations do not apply in our case of thermal atoms with only a single ground state. We propose a simple two-level model including saturation to describe our experimental results. This paper introduces the experimental setup, derives the mentioned model and compares it to our experimental data. Good quantitative agreement supports our model.
2. arXiv:1808.02089 (cross-list from hep-ph) [pdf, ps, other]
Gravitational Effects in g Factor Measurements and High-Precision Spectroscopy: Limits of Einstein's Equivalence Principle
Ulrich D. Jentschura
Comments: 19 pages; RevTeX
Subjects: High Energy Physics - Phenomenology (hep-ph); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th); Atomic Physics (physics.atom-ph)
We study the interplay of general relativity, the equivalence principle, and high-precision experiments involving atomic transitions and g factor measurements. In particular, we derive a generalized Dirac Hamiltonian, which describes both the gravitational coupling for weak fields, as well as the electromagnetic coupling, e.g., to a central Coulomb field. An approximate form of this Hamiltonian is used to derive the leading gravitational corrections to transition frequencies and g factors. The position-dependence of atomic transitions is shown to be compatible with the equivalence principle, up to a very good approximation. The compatibility of g factor measurements requires a deeper, subtle analysis, in order to eventually restore the compliance of g factor measurements with the equivalence principle. Finally, we analyze small, but important limitations of Einstein's equivalence principle due to quantum effects, within high-precision experiments. We also study the relation of these effects to a conceivable gravitationally induced collapse of a quantum mechanical wave function (Penrose conjecture), and space-time noncommutativity, and find that the competing effects should not preclude the measurability of the higher-order gravitational corrections. Surprisingly large higher-order gravitational effects are obtained for transitions in diatomic molecules.
3. arXiv:1808.03125 [pdf, ps, other]
Analogue Hawking Radiation and Sine-Gordon Soliton in a Superconducting Circuit
Zehua Tian, Jiangfeng Du
Comments: 6 pages, 2 figures
Subjects: Quantum Physics (quant-ph); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
We propose the use of a waveguide-like transmission line based on direct-current superconducting quantum interference devices (dc-SQUID) and study the sine-Gordon (SG) equation which characterises the dynamical behavior of the superconducting phase in this transmission line. Guided by the duality between black holes in Jackiw-Teitelboim (JT) dilaton gravity and solitons in sine-Gordon field theory, we show how to, in our setup, realize 1 + 1 dimensional black holes as solitons of the sine-Gordon equation. We also study the analogue Hawking radiation in terms of the quantum soliton evaporation, and analyze its feasibility within current circuit quantum electrodynamics (cQED) technology. Our results may not only facilitate experimentally understanding the relation between Jackiw-Teitelboim dilaton gravity and sine-Gordon field theory, but also pave a new way, in principle, for the exploration of analogue quantum gravitational effects.
4. arXiv:1808.03131 [pdf, ps, other]
Non-commutative measure of quantum correlations under local operations
D. G. Bussandri, A. P. Majtey, A. Valdés-Hernández
Subjects: Quantum Physics (quant-ph)
We study some desirable properties of recently introduced measures of quantum correlations based on the amount of non-commutativity quantified by the Hilbert-Schmidt norm (Sci Rep 6:25241, 2016, and Quantum Inf. Process. 16:226, 2017). Specifically, we show that: 1) for any bipartite (A+B) state, the measures of quantum correlations with respect to subsystem A are non-increasing under any Local Commutative Preserving Operation on subsystem A, and 2) for Bell diagonal states, the measures are non-increasing under arbitrary local operations on B. Our results accentuate the potentialities of such measures, and exhibit them as valid monotones in a resource theory of quantum correlations with free operations restricted to the appropriate local channels.
5. arXiv:1808.03176 [pdf, other]
Floquet engineering in superconducting circuits: from arbitrary spin-spin interactions to the Kitaev honeycomb model
Mahdi Sameti, Michael J. Hartmann
Comments: 21 pages, 9 figures
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Superconductivity (cond-mat.supr-con)
We derive a theory for the generation of arbitrary spin-spin interactions in superconducting circuits via periodic time modulation of the individual qubits or the qubit-qubit interactions. The modulation frequencies in our approach are in the microwave or radio frequency regime so that the required fields can be generated with standard generators. Among others, our approach is suitable for generating spin lattices that exhibit quantum spin liquid behavior such as Kitaev's honeycomb model.
6. arXiv:1808.03193 [pdf, other]
Critical phenomena in an extended Dicke model
J. P. J. Rodriguez, S. A. Chilingaryan, B. M. Rodríguez-Lara
Comments: 15 pages, 6 figures
Subjects: Quantum Physics (quant-ph)
Spectral characterization is a fundamental step in the development of useful quantum technology platforms. Here, we study an ensemble of interacting qubits coupled to a single quantized field mode, an extended Dicke model that might be at the heart of Bose-Einstein condensate in a cavity or circuit-QED experiments for large and small ensemble sizes, respectively. We present a semi-classical and quantum analysis of the model. In the semi-classical regime, we show analytic results that reveal the existence of a third regime, in addition of the two characteristic of the standard Dicke model, characterized by one logarithmic and two jump discontinuities in the derivative of the density of states. We show that the finite quantum system shows two different types of clustering at the jump discontinuities, signaling a precursor of two excited quantum phase transitions. These are confirmed using Peres lattices where unexpected order arises around the new precursor. Interestingly, Peres conjecture regarding the relation between spectral characteristics of the quantum model and the onset of chaos in its semi-classical equivalent is valid in this model as a revival of order in the semi-classical dynamics occurs around the new phase transition.
Aug. 9
1. arXiv:1808.02795 (cross-list from cond-mat.dis-nn) [pdf, other]
Weighted models for level statistics across the many--body localization transition
Piotr Sierant, Jakub Zakrzewski
Comments: 13 fascinating pp., comments welcome
Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Quantum Gases (cond-mat.quant-gas)
Level statistics across the many-body localization transition are studied in detail for a random disorder. The gap ratio statistics reveals characteristic inter-sample randomness reflecting fluctuations in localization properties of the system, in particular Griffiths-like rare events. Defining a mean gap ratio for a single realization of disorder we show that it has a broad, system specific distribution across the whole transition. That explains the necessity of introducing weighted random matrix ensembles that correctly grasp the sample-to-sample variation of system properties including the rare events. We consider two such approaches. One is a weighted short-range plasma model, the other a weighted power--law random banded matrix model. Treating the single sample gap ratio distribution as input, the considered weighted models yield a very good agreement both for spacing distribution including its exponential tail and the number variance up to tens of level spacings. We show explicitly that our weighted models describe the level statistics across the whole ergodic to many-body localized transition much more faithfully than earlier predictions. The remaining deviations for long-range spectral correlations are discussed and attributed mainly to the intricacies of level unfolding.
2. arXiv:1808.02503 (cross-list from quant-ph) [pdf, other]
Fast and scalable quantum information processing with two-electron atoms in optical tweezer arrays
G. Pagano, F. Scazza, M. Foss-Feig
Comments: 10+4 pages, 6+1 figures, 69 references
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph)
Atomic systems, ranging from trapped ions to ultracold and Rydberg atoms, offer unprecedented control over both internal and external degrees of freedom at the single-particle level. They are considered among the foremost candidates for realizing quantum simulation and computation platforms that can outperform classical computers at specific tasks. In this work, we describe a realistic experimental toolbox for quantum information processing with neutral alkaline-earth-like atoms in optical tweezer arrays. In particular, we propose a comprehensive and scalable architecture based on a programmable array of alkaline-earth-like atoms, exploiting their electronic clock states as a precise and robust auxiliary degree of freedom, and thus allowing for efficient all-optical one- and two-qubit operations between nuclear spin qubits. The proposed platform promises excellent performance thanks to high-fidelity register initialization, rapid spin-exchange gates and error detection in readout. As a benchmark and application example, we compute the expected fidelity of an increasing number of subsequent SWAP gates for optimal parameters, which can be used to distribute entanglement between remote atoms within the array.
Aug. 8
1. arXiv:1808.02062 [pdf, other]
Atomtronics with a spin: statistics of spin transport and non-equilibrium orthogonality catastrophe in cold quantum gases
Jhih-Shih You, Richard Schmidt, Dmitri A. Ivanov, Michael Knap, Eugene Demler
Comments: 9+11 pages, 10 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)
We propose to investigate the full counting statistics of nonequilibrium spin transport with an ultracold atomic quantum gas. The setup makes use of the spin control available in atomic systems to generate spin transport induced by an impurity atom immersed in a spin-imbalanced two-component Fermi gas. In contrast to solid-state realizations, in ultracold atoms spin relaxation and the decoherence from external sources is largely suppressed. As a consequence, once the spin current is turned off by manipulating the internal spin degrees of freedom of the Fermi system, the nonequilibrium spin population remains constant. Thus one can directly count the number of spins in each reservoir to investigate the full counting statistics of spin flips, which is notoriously challenging in solid state devices. Moreover, using Ramsey interferometry, the dynamical impurity response can be measured. Since the impurity interacts with a many-body environment that is out of equilibrium, our setup provides a way to realize the non-equilibrium orthogonality catastrophe. Here, even for spin reservoirs initially prepared in a zero-temperature state, the Ramsey response exhibits an exponential decay, which is in contrast to the conventional power-law decay of Anderson's orthogonality catastrophe. By mapping our system to a multi-step Fermi sea, we are able to derive analytical expressions for the impurity response at late times. This allows us to reveal an intimate connection of the decay rate of the Ramsey contrast and the full counting statistics of spin flips.
2. arXiv:1808.02065 (cross-list from quant-ph) [pdf, ps, other]
Retrieval of free-Majorana wavefunctions for the finite Kitaev chain using an appropriate momentum representation
Karen Rodríguez, Angélica Alejandra Pérez-Losada, Arturo Argüelles
Comments: 7 pages, 7 figures
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas)
In quantum mechanics, the spaces of momentum and its conjugate, the position, are related via Fourier transforms and thus the properties are interwoven with their structure. In particular, for lattice systems possessing an underlying discrete position space, the momentum becomes finite. Moreover, if the lattice length is finite, L<∞, the momentum space also becomes both finite and discrete breaking altogether the continuity of the dispersion relation. This aspect is relevant in new systems such as the topological materials. We address this point paving the path for new ways to the observation of Majorana quasi-particles. Furthermore, the Kitaev model, which is the simplest Hamiltonian supporting Majorana fermions, is therefore taken as a starting point for the theoretical description of the work. Our study focuses on finding the zero-energy modes in an artificial arrangement of a non-interacting superconducting finite nanowire by using a discrete-sine transform with the purpose of going from position to momentum space considering hard-wall boundary conditions in the process. Here, a new L-dimensional Nambu operator to diagonalize the system in the Bogoliubov-de Gennes formalism is proposed and, further, we arrive to a new space with a considerable reduced dimension allowing the treatment of larger system sizes. We also present a comparison between the numerical and third order perturbation theory for the weak coupling regime results presenting an excellent agreement. Finally, we analyze the wavefunctions of the retrieved zero-energy modes showing that they are indeed edge states and thus they have an exponential decay.
3. arXiv:1808.02038 (cross-list from cond-mat.stat-mech) [pdf, other]
Scrambling in the Dicke model
Yahya Alavirad, Ali Lavasani
Comments: 10 pages, 10 figures, 2 pages of appendix
Subjects: Statistical Mechanics (cond-mat.stat-mech); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
The scrambling rate λL associated with the exponential growth of out-of-time-ordered correlators can be used to characterize quantum chaos. Here we use the Majorana Fermion representation of spin 1/2 systems to study quantum chaos in the Dicke model. We take the system to be in thermal equilibrium and compute λL throughout the phase diagram to leading order in 1/N. We find that the chaotic behavior is strongest close to the critical point. At high temperatures λL is nonzero over an extended region that includes both the normal and super-radiant phases. At low temperatures λL is nonzero in (a) close vicinity of the critical point and (b) a region within the super-radiant phase. In the process we also derive a new effective theory for the super-radiant phase at finite temperatures. Our formalism does not rely on the assumption of total spin conservation.
Aug. 7
1. arXiv:1808.01314 [pdf, other]
Impenetrable SU(N) fermions in one-dimensional lattices
Yicheng Zhang, Lev Vidmar, Marcos Rigol
Comments: 12 pages, 13 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech); Strongly Correlated Electrons (cond-mat.str-el)
We study SU(N) fermions in the limit of infinite onsite repulsion between all species. We focus on states in which every pair of consecutive fermions carries a different spin flavor. Since the particle order cannot be changed (because of the infinite onsite repulsion) and contiguous fermions have a different spin flavor, we refer to the corresponding constrained model as a model of distinguishable quantum particles. We introduce an exact numerical method to calculate equilibrium one-body correlations of distinguishable quantum particles based on a mapping onto noninteracting spinless fermions. In contrast to most many-body systems in one dimension, which usually exhibit either power-law or exponential decay of off-diagonal one-body correlations with distance, distinguishable quantum particles exhibit a Gaussian decay of one-body correlations in the ground state, while finite temperature correlations are well described by a stretched exponential decay.
2. arXiv:1808.01993 (cross-list from cond-mat.stat-mech) [pdf, other]
Quantum XX-model with competing short- and long-range interactions: Phases and phase transitions in and out of equilibrium
Ferenc Iglói, Benjamin Blaß, Gergö Roósz, Heiko Rieger
Comments: 17 pages, 12 figure, revtex
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We consider the quantum XX-model in the presence of competing nearest-neighbour and global-range interactions, which is equivalent to a Bose-Hubbard model with cavity mediated global range interactions in the hard core boson limit. Using fermionic techniques the problem is solved exactly in one dimension in the thermodynamic limit. The ground state phase diagram consists of two ordered phases: ferromagnetic (F) and antiferromagnetic (AF), as well as an XY-phase having quasi-long-range order. We have also studied quantum relaxation after sudden quenches. Quenching from the AF phase to the XY region remanent AF order is observed below a dynamical transition line. In the opposite quench, from the XY region to the AF-phase beyond a static metastability line AF order arises on top of remanent XY quasi-long-range order, which corresponds to dynamically generated supersolid state in the equivalent Bose-Hubbard model with hard-core bosons.
Aug. 6
1. arXiv:1808.01015[pdf, other]
Spontaneous formation and relaxation of spin domains in antiferromagnetic spin-1 quasi-condensates
K. Jiménez-García, A. Invernizzi, B. Evrard, C. Frapolli, J. Dalibard, F. Gerbier
Comments: Supplementary material available as ancillary file
Subjects: Quantum Gases (cond-mat.quant-gas)
Quantum systems of many interacting particles at low temperatures generally organize themselves into ordered phases of matter, whose nature and symmetries are captured by an order parameter. In the simplest cases, this order parameter is spatially uniform. For example, a system of localized spins with ferromagnetic interactions align themselves to a common direction and build up a macroscopic magnetization on large distances. However, non-uniform situations also exist in nature, for instance in antiferromagnetism where the magnetization alternates in space. The situation becomes even richer when the spin-carrying particles are mobile, for instance in the so-called stripe phases emerging for itinerant electrons in strongly-correlated materials. Understanding such inhomogeneously ordered states is of central importance in many-body physics. In this work, we study experimentally the magnetic ordering of itinerant spin-1 bosons in inhomegeneous spin domains at nano-Kelvin temperatures. We demonstrate that spin domains form spontaneously after a phase separation transition, \textit{i.e.} in the absence of external magnetic force, purely because of the antiferromagnetic interactions between the atoms. Furthermore, we explore how the equilibrium domain configuration emerges from an initial state prepared far-from-equilibrium.
2. arXiv:1808.00975 [pdf, other]
Rotating atomic quantum gases with light-induced azimuthal gauge potentials and the observation of Hess-Fairbank effect
P. -K. Chen, L. -R. Liu, M. -J. Tsai, N. -C. Chiu, Y. Kawaguchi, S. -K. Yip, M. -S. Chang, Y. -J. Lin
Subjects: Quantum Gases (cond-mat.quant-gas)
We demonstrate synthetic azimuthal gauge potentials for Bose-Einstein condensates using atom-light coupling. The gauge potential is created by adiabatically loading the condensate into the lowest energy Raman-dressed state-a coreless vortex. Here one Raman beam carries orbital-angular-momentum. The azimuthal gauge potentials act as effective rotations and are tunable by the Raman coupling and detuning. We characterize the spin texture of the dressed states, which agree with the theory. The lowest energy dressed state is stable with the Raman beams on, and the half-atom-number lifetime is about 4.5 s. Finally, we employ the azimuthal gauge potential to demonstrate the Hess-Fairbank effect, i.e., we produce dressed atoms in the absolute ground state which have zero quasi-angular momentum when the synthetic magnetic flux is below a critical value. Above the critical flux, a transition into a polar-core vortex is observed. Both types of SO(3) vortices in the |⟨F⃗ ⟩|=1 manifold are created, the coreless vortex and the polar-core vortex. We thus present a paradigm of creating topological excitations by tailoring atom-light interactions. The gauge field in the stationary Hamiltonian opens the door to investigate rotation properties of atomic superfluid under thermal equilibrium.
3. arXiv:1808.01250 (cross-list from cond-mat.dis-nn) [pdf, other]
Stark many-body localization
M. Schulz, C.A. Hooley, R. Moessner, F. Pollmann
Comments: 6 pages, 4 figures
Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
We consider spinless fermions on a finite one-dimensional lattice, interacting via nearest-neighbor repulsion and subject to a strong electric field. In the non-interacting case, due to Wannier-Stark localization, the single-particle wave functions are exponentially localized even though the model has no quenched disorder. We show that this system remains localized in the presence of interactions and exhibits physics analogous to models of conventional many-body localization (MBL). In particular, the entanglement entropy grows logarithmically with time after a quench, albeit with a slightly different functional form from the MBL case, and the level statistics of the many-body energy spectrum are Poissonian. We moreover predict that a quench experiment starting from a charge-density wave state would show results similar to those of Schreiber et al. [Science 349, 842 (2015)].
4. arXiv:1808.00978 (cross-list from cond-mat.str-el) [pdf, other]
Temperature-driven gapless topological insulator
Miguel Gonçalves, Pedro Ribeiro, Rubem Mondaini, Eduardo V. Castro
Comments: 5 pages, 4 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas)
We investigate the phase diagram of the Haldane-Falicov-Kimball model -- a model combining topology, interactions and spontaneous disorder at finite temperatures. Using an unbiased numerical method, we map out the phase diagram on the interaction--temperature plane. Along with known phases, we unveil an \textit{insulating charge ordered state with gapless excitations }and a temperature-driven \textit{gapless topological insulating} phase. Intrinsic -- temperature generated -- disorder, is the key ingredient explaining the unexpected behavior. Our findings support the possibility of having temperature-driven topological phase transitions into gapped and gapless topological insulating phases in systems with a large mass unbalance in fermionic species.
Aug. 2
arXiv:1808.00028 (cross-list from physics.atom-ph) [pdf, other]
A Fermi Degenerate Gas of Polar Molecules
Luigi De Marco, Giacomo Valtolina, Kyle Matsuda, William G. Tobias, Jacob P. Covey, Jun Ye (Submitted on 31 Jul 2018) It has long been expected that quantum degenerate gases of molecules would open access to a wide range of phenomena in molecular and quantum sciences. However, the very complexity that makes ultracold molecules so enticing has made reaching degeneracy an outstanding experimental challenge over the past decade. We now report the production of a Fermi degenerate gas of ultracold polar molecules of potassium--rubidium (KRb). Through coherent adiabatic association in a deeply degenerate mixture of a rubidium Bose-Einstein condensate and a potassium Fermi gas, we produce molecules at temperatures below 0.3 times the Fermi temperature. We explore the properties of this reactive gas and demonstrate how degeneracy suppresses chemical reactions, making a long-lived degenerate gas of polar molecules a reality.
Aug. 1
arXiv:1808.00506 [pdf, other]
Floquet dynamics in driven Fermi-Hubbard systems Michael Messer, Kilian Sandholzer, Frederik Görg, Joaquín Minguzzi, Rémi Desbuquois, Tilman Esslinger (Submitted on 1 Aug 2018) We study the dynamics and timescales of a periodically driven Fermi-Hubbard model in a three-dimensional hexagonal lattice. The evolution of the Floquet many-body state is analyzed by comparing it to an equivalent implementation in undriven systems. The dynamics of double occupancies for the near- and off-resonant driving regime indicate that the effective Hamiltonian picture is valid for several orders of magnitude in modulation time. Furthermore, we show that driving a hexagonal lattice compared to a simple cubic lattice allows to modulate the system up to 1~s, corresponding to hundreds of tunneling times, with only minor atom loss. Here, driving at a frequency close to the interaction energy does not introduce resonant features to the atom loss.
