Jan+2017

= Jan 2-Jan 6 Max, Jan 9-Jan 13 Biao Huang, Jan 16-Jan 20 Haiyuan Zou, Jan 23-Jan 27 Ahmet Keles =

= = = Jan 20 = = = =  [|arXiv:1701.05152] (cross-list from quant-ph) [ [|pdf], [|ps] , [|other] ]  = Quantum enhanced measurements without entanglement [|Daniel Braun], [|Gerardo Adesso] , [|Fabio Benatti] , [|Roberto Floreanini] , [|Ugo Marzolino] , [|Morgan W. Mitchell] , [|Stefano Pirandola]   Comments: 44 pages, including 7 pages of references  Subjects: Quantum Physics (quant-ph) ; Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas); Optics (physics.optics) Quantum-enhanced measurements exploit quantum mechanical effects for increasing the sensitivity of measurements of certain physical parameters and have great potential for both fundamental science and concrete applications. Most of the research has so far focused on using highly entangled states, which are, however, difficult to produce and to stabilize for a large number of constituents. In the following we review alternative mechanisms, notably the use of more general quantum correlations such as quantum discord, identical particles, or non-trivial hamiltonians; the estimation of thermodynamical parameters or parameters characterizing non-equilibrium states; and the use of quantum phase transitions. We describe both theoretically achievable enhancements and enhanced sensitivities, not primarily based on entanglement, that have already been demonstrated experimentally, and indicate some possible future research directions. = =

= Jan 19 = [|arXiv:1701.04844] [ [|pdf], [|other] ] Quantum Entanglement in Neural Network States [|Dong-Ling Deng], [|Xiaopeng Li] , [|S. Das Sarma]   Comments: 14 pages, 6 figures  Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn) ; Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph) Machine learning, one of today's most rapidly growing interdisciplinary fields, promises an unprecedented perspective for solving intricate quantum many-body problems. Understanding the physical aspects of the representative artificial neural-network states is recently becoming highly desirable in the applications of machine learning techniques to quantum many-body physics. Here, we study the quantum entanglement properties of neural-network states, with a focus on the restricted-Boltzmann-machine (RBM) architecture. We prove that the entanglement of all short-range RBM states satisfies an area law for arbitrary dimensions and bipartition geometry. For long-range RBM states we show by using an exact construction that such states could exhibit volume-law entanglement, implying a notable capability of RBM in representing quantum states with massive entanglement. Strikingly, the neural-network representation for these states is remarkably efficient, in a sense that the number of nonzero parameters scales only linearly with the system size. We further examine generic RBM states with random weight parameters. We find that their averaged entanglement entropy obeys volume-law scaling and meantime strongly deviates from the Page-entropy of the completely random pure states. We show that their entanglement spectrum has no universal part associated with random matrix theory and bears a Poisson-type level statistics. We show, through a concrete example of the one-dimensional symmetry-protected topological cluster states, that the RBM representation may also be used as a tool to analytically compute the entanglement spectrum. Our results uncover the unparalleled power of artificial neural networks in representing quantum many-body states, which paves a novel way to bridge computer science based machine learning techniques to outstanding quantum condensed matter physics problems.

 [|arXiv:1701.04831] [ [|pdf], [|other] ] On the Equivalence of Restricted Boltzmann Machines and Tensor Network States [|Jing Chen], [|Song Cheng] , [|Haidong Xie] , [|Lei Wang] , [|Tao Xiang]   Comments: 10 pages + 3 appendices; Code implementations at [|this https URL] Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Quantum Physics (quant-ph); Machine Learning (stat.ML)Restricted Boltzmann machine (RBM) is one of the fundamental building blocks of deep learning. RBM finds wide applications in dimensional reduction, feature extraction, and recommender systems via modeling the probability distributions of a variety of input data including natural images, speech signals, and customer ratings, etc. We build a bridge between RBM and tensor network states (TNS) widely used in quantum many-body physics research. We devise efficient algorithms to translate an RBM into the commonly used TNS. Conversely, we give sufficient and necessary conditions to determine whether a TNS can be transformed into an RBM of given architectures. Revealing these general and constructive connections can cross-fertilize both deep learning and quantum-many body physics. Notably, by exploiting the entanglement entropy bound of TNS, we can rigorously quantify the expressive power of RBM on complex datasets. Insights into TNS and its entanglement capacity can guide the design of more powerful deep learning architectures. On the other hand, RBM can represent quantum many-body states with fewer parameters compared to TNS, which may allow more efficient classical simulations.

= Jan 18 = [|arXiv:1701.04702] [ [|pdf], [|other] ] Fermi-Bose mixture in mixed dimensions [|M. A. Caracanhas], [|F. Schreck] , [|C. Morais Smith]   Comments: 11 pages, 6 figures, 2 tables and supplementary material  Subjects: Quantum Gases (cond-mat.quant-gas)  One of the challenging goals in the studies of many-body physics with ultracold atoms is the creation of a topological  p x + ip y   superfluid for identical fermions in two dimensions (2D). The expectations of reaching the critical temperature  T c  through p-wave Feshbach resonance in spin-polarized fermionic gases have soon faded away because on approaching the resonance, the system becomes unstable due to inelastic-collision processes. Here, we consider an alternative scenario in which a single-component degenerate gas of fermions in 2D is paired via phonon-mediated interactions provided by a 3D BEC background. Within the weak-coupling regime, we calculate the critical temperature  T c  for the fermionic pair formation, using Bethe-Salpeter formalism, and show that it is significantly boosted by higher-order diagramatic terms, such as phonon dressing and vertex corrections. We describe in detail an experimental scheme to implement our proposal, and show that the long-sought p-wave superfluid is at reach with state-of-the-art experiments.

= Jan 17 = [|arXiv:1701.04296] [ [|pdf], [|ps] , [|other] ] Topological Fulde-Ferrell Superfluids in Triangular Lattices [|Long-Fei Guo], [|Peng Li] , [|Su Yi]   Comments: 6 pages, 5figures  Subjects: Quantum Gases (cond-mat.quant-gas)  Fulde-Ferrell (FF) Larkin-Ovchinnikov (LO) phases were proposed for superconductors or superfluids in strong magnetic field. With the experimental progresses in ultracold atomic systems, topological FFLO phases has also been put forward, since it is a natural consequence of realizable spin-orbital coupling (SOC).In this work, we theoretically investigate a triangular lattice model with SOC and in-plane field. By constructing the phase diagram, we show that it can produce topological FF states with Chern numbers,  C =± 1  and  C =− 2  . We get the phase boundaries by the change of the sign of Pfaffian. The chiral edge states for different topological FF phases are also elucidated.

= Jan 16 =

[|arXiv:1701.03690] [ [|pdf], [|other] ] Hybrid-space density matrix renormalization group study of the doped two-dimensional Hubbard model [|G. Ehlers], [|S. R. White] , [|R. M. Noack]   Comments: 16 pages, 13 figures  Subjects: Strongly Correlated Electrons (cond-mat.str-el) The performance of the density matrix renormalization group (DMRG) is strongly influenced by the choice of the local basis of the underlying physical lattice. We demonstrate that, for the two-dimensional Hubbard model, the hybrid real-momentum space formulation of the DMRG is computationally more efficient than the standard real-space formulation. In particular, we show that the computational cost for fixed bond dimension of the hybrid-space DMRG is approximately independent of the width of the lattice, in contrast to the real-space DMRG, for which it is proportional to the width squared. We apply the hybrid-space algorithm to calculate the ground state of the doped two-dimensional Hubbard model on cylinders of width four and six sites; at n = 0.875  filling, the ground state exhibits a striped charge-density distribution with a wavelength of eight sites for both  U / t = 4.0  and  U / t = 8.0. We find that the strength of the charge ordering depends on U / t  and on the boundary conditions.Furthermore, we investigate the magnetic ordering as well as the decay of the static spin, charge, and pair-field correlation functions.

= Jan 10 = =Two-component quantum Hall effects in topological flat bands = <span style="background-color: #ffffff; display: block; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: medium;"> [|Tian-Sheng Zeng], [|W. Zhu] , [|D. N. Sheng] <span style="background-color: #ffffff; display: block; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: small;">(Submitted on 10 Jan 2017) > We study quantum Hall states for two-component particles (hardcore bosons and fermions) loading in topological lattice models. By tuning the interplay of interspecies and intraspecies interactions, we demonstrate that two-component fractional quantum Hall states emerge at certain fractional filling factors <span class="mi" style="font-family: MathJax_Math-italic; font-size: 17.28px; vertical-align: 0px;">ν <span class="mo" style="font-family: MathJax_Main; font-size: 17.28px; vertical-align: 0px;">= <span class="mn" style="font-family: MathJax_Main; font-size: 17.28px; vertical-align: 0px;">1 <span class="mo" style="font-family: MathJax_Main; font-size: 17.28px; vertical-align: 0px;">/ <span class="mn" style="font-family: MathJax_Main; font-size: 17.28px; vertical-align: 0px;">2 for fermions ( <span class="mi" style="font-family: MathJax_Math-italic; font-size: 17.28px; vertical-align: 0px;">ν <span class="mo" style="font-family: MathJax_Main; font-size: 17.28px; vertical-align: 0px;">= <span class="mn" style="font-family: MathJax_Main; font-size: 17.28px; vertical-align: 0px;">2 <span class="mo" style="font-family: MathJax_Main; font-size: 17.28px; vertical-align: 0px;">/ <span class="mn" style="font-family: MathJax_Main; font-size: 17.28px; vertical-align: 0px;">3  for bosons) in the lowest Chern band, classified by features from ground states including the unique Chern number matrix (inverse of <span class="mi" style="font-family: MathJax_Main-bold; font-size: 17.28px; vertical-align: 0px;">K  -matrix), the fractional charge and spin pumpings, and two parallel propagating edge modes. Moreover, we also apply our strategy to two-component fermions at integer filling factor <span class="mi" style="font-family: MathJax_Math-italic; font-size: 17.28px; vertical-align: 0px;">ν <span class="mo" style="font-family: MathJax_Main; font-size: 17.28px; vertical-align: 0px;">= <span class="mn" style="font-family: MathJax_Main; font-size: 17.28px; vertical-align: 0px;">2, where a possible topological Neel antiferromagnetic phase is under intense debate very recently. By tuning the onsite Hubbard repulsion, we establish a first-order phase transition directly from a two-component fermionic <span class="mi" style="font-family: MathJax_Math-italic; font-size: 17.28px; vertical-align: 0px;">ν <span class="mo" style="font-family: MathJax_Main; font-size: 17.28px; vertical-align: 0px;">= <span class="mn" style="font-family: MathJax_Main; font-size: 17.28px; vertical-align: 0px;">2 quantum Hall state at weak interaction to a topologically trivial antiferromagnetic insulator at strong interaction, therefore exclude the possibility of an intermediate topological phase for our system. =<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: x-large;">Dynamical control of electron-phonon interactions with high-frequency light = <span style="background-color: #ffffff; display: block; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: medium;"> [|C. Dutreix], [|M. I. Katsnelson] <span style="background-color: #ffffff; display: block; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: small;">(Submitted on 15 Nov 2016) > This work addresses the one-dimensional problem of Bloch electrons when they are rapidly driven by a homogeneous time-periodic light and linearly coupled to vibrational modes. Starting from a generic time-periodic electron-phonon Hamiltonian, we derive a time-independent effective Hamiltonian that describes the stroboscopic dynamics up to the third order in the high-frequency limit. This yields nonequilibrium corrections to the electron-phonon coupling that are controllable dynamically via the driving strength. This shows in particular that local Holstein interactions in equilibrium are corrected by nonlocal Peierls interactions out of equilibrium, as well as by phonon-assisted hopping processes that make the dynamical Wannier-Stark localization of Bloch electrons impossible. Subsequently, we revisit the Holstein polaron problem out of equilibrium in terms of effective Green functions, and specify explicitly how the binding energy and effective mass of the polaron can be controlled dynamically. These tunable properties are reported within the weak- and strong-coupling regimes since both can be visited within the same material when varying the driving strength. This work provides some insight into controllable microscopic mechanisms that may be involved during the multicycle laser irradiations of organic molecular crystals in ultrafast pump-probe experiments, although it should also be suitable for realizations in shaken optical lattices of ultracold atoms.

= Jan 09 =

=<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: x-large;">Radical chiral Floquet phases in a periodically driven Kitaev model and beyond = <span style="background-color: #ffffff; display: block; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: medium;"> [|Hoi Chun Po], [|Lukasz Fidkowski] , [|Ashvin Vishwanath] , [|Andrew C. Potter] <span style="background-color: #ffffff; display: block; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: small;">(Submitted on 5 Jan 2017) > Time periodic driving serves not only as a convenient way to engineer effective Hamiltonians, but also as a means to produce intrinsically dynamical phases that do not exist in the static limit. A recent example of the latter are 2D chiral Floquet (CF) phases exhibiting anomalous edge dynamics that pump discrete packets of quantum information along one direction. In non-fractionalized systems with only bosonic excitations, this pumping is quantified by a dynamical topological index that is a rational number -- highlighting its difference from the integer valued invariant underlying equilibrium chiral phases (e.g. quantum Hall systems). Here, we explore CF phases in systems with emergent anyon excitations that have fractional statistics (Abelian topological order). Despite the absence of mobile non-Abelian particles in these systems, external driving can supply the energy to pump otherwise immobile non-Abelian defects (sometimes called twist defects or genons) around the boundary, thereby transporting an irrational fractional number of quantum bits along the edge during each drive period. This enables new CF phases with chiral indices that are square roots of rational numbers, inspiring the label: "radical CF phases". We demonstrate an unexpected bulk-boundary correspondence, in which the radical CF edge is tied to bulk dynamics that exchange electric and magnetic anyon excitations during each period. We construct solvable, stroboscopically driven versions of Kitaev's honeycomb spin model that realize these radical CF phases, and discuss their stability against heating in strongly disordered many-body localized settings or in the limit of rapid driving as an exponentially long-lived pre-thermal phenomena.