ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Sat, 15 Aug 2020 06:13:54 GMT2020-08-15T06:13:54Z50931- Topological characterization of one-dimensional open fermionic systemshttps://scholarbank.nus.edu.sg/handle/10635/155026Title: Topological characterization of one-dimensional open fermionic systems
Authors: Zhang, Da-Jian; Gong, Jiangbin
Abstract: © 2018 American Physical Society. A topological measure characterizing symmetry-protected topological phases in one-dimensional open fermionic systems is proposed. It is built upon the kinematic approach to the geometric phase of mixed states and facilitates the extension of the notion of topological phases from zeroerature to nonzeroerature cases. In contrast to a previous finding that topological properties may not survive above a certain critical temperature, we find that topological properties of open systems, in the sense of the measure suggested here, can persist at any finite temperature and disappear only in the mathematical limit of infinite temperature. Our result is illustrated with two paradigmatic models of topological matter. The bulk topology at nonzero temperatures manifested as robust mixed edge state populations is examined via two figures of merit.
Thu, 01 Nov 2018 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1550262018-11-01T00:00:00Z
- Interband coherence induced correction to Thouless pumping: possible observation in cold-atom systemshttps://scholarbank.nus.edu.sg/handle/10635/167774Title: Interband coherence induced correction to Thouless pumping: possible observation in cold-atom systems
Authors: Raghava, Gudapati Naresh; Zhou, Longwen; Gong, Jiangbin
Abstract: © 2017, EDP Sciences, SIF, Springer-Verlag GmbH Germany. In Thouless pump, the charge transport in a one-dimensional insulator over an adiabatic cycle is topologically quantized. For nonequilibrium initial states, however, interband coherence will induce a previously unknown contribution to Thouless pumping. Though not geometric in nature, this contribution is independent of the time scale of the pumping protocol. In this work, we perform a detailed analysis of our previous finding [H.L. Wang et al., Phys. Rev. B 91, 085420 (2015)] in an already available cold-atom setup. We show that initial states with interband coherence can be obtained via a quench of the system’s Hamiltonian. Adiabatic pumping in the post-quench system are then examined both theoretically and numerically, in which the interband coherence is shown to play an important role and can hence be observed experimentally. By choosing adiabatic protocols with different switching-on rates, we also show that the contribution of interband coherence to adiabatic pumping can be tuned. It is further proposed that the interband coherence induced correction to Thouless pumping may be useful in capturing a topological phase transition point. All our results have direct experimental interests.
Tue, 08 Aug 2017 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1677742017-08-08T00:00:00Z
- Revealing many-body effects on interband coherence through adiabatic charge pumpinghttps://scholarbank.nus.edu.sg/handle/10635/168513Title: Revealing many-body effects on interband coherence through adiabatic charge pumping
Authors: Mu, Sen; Zhang, Da-Jian; Zhou, Longwen; Gong, Jiangbin
Abstract: © 2019 American Physical Society. The adiabatic charge pumping of a nonequilibrium state of spinless fermions in a one-dimensional lattice is investigated, with an emphasis placed on its usefulness in revealing many-body interaction effects on interband coherence. For a noninteracting system, the pumped charge per adiabatic cycle depends not only on the topology of the occupied bands but also on the interband coherence in the initial state. This insight leads to an interesting opportunity for quantitatively observing how quantum coherence is affected by many-body interaction that is switched on for a varying duration prior to adiabatic pumping. In particular, interband coherence effects can be clearly observed by adjusting the switch-on rates with different adiabatic pumping protocols and by scanning the duration of many-body interaction prior to adiabatic pumping. The time dependence of single-particle interband coherence in the presence of many-body interaction can then be examined in detail. As an interesting side result, for relatively weak interaction strength, it is found that the difference in the pumped charges between different pumping protocols vanishes if a coherence measure defined by the single-particle density matrix in the sublattice representation reaches its local minima. Our results hence provide an interesting means to quantitatively probe the dynamics of quantum coherence in the presence of many-body interaction (e.g., in a thermalization process).
Fri, 04 Oct 2019 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685132019-10-04T00:00:00Z
- Piecewise adiabatic following: General analysis and exactly solvable modelshttps://scholarbank.nus.edu.sg/handle/10635/168540Title: Piecewise adiabatic following: General analysis and exactly solvable models
Authors: Gong, Jiangbin; Wang, Qing-hai
Abstract: © 2019 American Physical Society. The dynamics of a periodically driven system whose time evolution is governed by the Schrödinger equation with non-Hermitian Hamiltonians can be perfectly stable. This finding was only obtained very recently and will be enhanced by many exact solutions discovered in this work. The main concern of this study is to investigate the adiabatic following dynamics in such non-Hermitian systems stabilized by periodic driving. We focus on the peculiar behavior of stable cyclic (Floquet) states in the slow-driving limit. It is found that the stable cyclic states can either behave as intuitively expected by following instantaneous eigenstates, or exhibit piecewise adiabatic following by sudden switching between instantaneous eigenstates. We aim to cover broad categories of non-Hermitian systems under a variety of different driving scenarios. We systematically analyze the sudden-switch behavior by a universal route. That is, the sign change of the critical exponent in our asymptotic analysis of the solutions is always found to be the underlying mechanism to determine if the adiabatic following dynamics is trivial or piecewise. This work thus considerably extends our early study on the same topic [Gong and Wang, Phys. Rev. A 97, 052126 (2018)2469-992610.1103/PhysRevA.97.052126] and shall motivate more interest in non-Hermitian systems.
Tue, 08 Jan 2019 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685402019-01-08T00:00:00Z
- Coupled-wire construction of static and Floquet second-order topological insulatorshttps://scholarbank.nus.edu.sg/handle/10635/168534Title: Coupled-wire construction of static and Floquet second-order topological insulators
Authors: Bomantara, Raditya Weda; Zhou, Longwen; Pan, Jiaxin; Gong, Jiangbin
Abstract: © 2019 American Physical Society. Second-order topological insulators (SOTI) exhibit protected gapless boundary states at their hinges or corners. In this paper, we propose a generic means to construct SOTIs in static and Floquet systems by coupling one-dimensional topological insulator wires along a second dimension through dimerized hopping amplitudes. The Hamiltonian of such SOTIs admits a Kronecker sum structure, making it possible for obtaining its features by analyzing two constituent one-dimensional lattice Hamiltonians defined separately in two orthogonal dimensions. The resulting topological corner states do not rely on any delicate spatial symmetries, but are solely protected by the chiral symmetry of the system. We further utilize our idea to construct Floquet SOTIs, whose number of topological corner states is arbitrarily tunable via changing the hopping amplitudes of the system. Finally, we propose to detect the topological invariants of static and Floquet SOTIs constructed with our approach in experiments by measuring the mean chiral displacements of wavepackets.
Tue, 29 Jan 2019 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685342019-01-29T00:00:00Z
- Discrete time crystals in many-body quantum chaoshttps://scholarbank.nus.edu.sg/handle/10635/168531Title: Discrete time crystals in many-body quantum chaos
Authors: Nurwantoro, P; Bomantara, RW; Gong, J
Abstract: © 2019 American Physical Society. Discrete time crystals (DTCs) are phases of matter characterized by the presence of an observable evolving with nT periodicity under a T-periodic Hamiltonian, where n>1 is an integer insensitive to small parameter variations. In particular, DTCs with n=2 have been extensively studied in periodically quenched and kicked spin systems in recent years. In this paper, we study the emergence of DTCs in a many-body system whose semiclassical mean-field dynamics is nonintegrable, using a rather simple model depicting a harmonically driven spin chain. We advocate to first employ a semiclassical approximation to arrive at a mean-field Hamiltonian and then identify the parameter regime at which DTCs exist, with standard tools borrowed from studies of classical chaos. Specifically, we seek symmetric-breaking solutions by examining the stable islands on the Poincaré surface of section of the mean-field Hamiltonian. We then turn to the actual many-body quantum system, evaluate the stroboscopic dynamics of the total magnetization in the full quantum limit, and verify the existence of DTCs. Our effective and straightforward approach indicates that in general DTCs are one natural aspect of many-body quantum chaos with mixed classical phase space structure. Our approach can also be applied to general time-periodic systems, which is thus promising for finding DTCs with n>2 and opening possibilities for exploring DTCs properties beyond their time-translational breaking features.
Mon, 23 Dec 2019 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685312019-12-23T00:00:00Z
- Non-Hermitian Floquet topological phases: Exceptional points, coalescent edge modes, and the skin effecthttps://scholarbank.nus.edu.sg/handle/10635/168515Title: Non-Hermitian Floquet topological phases: Exceptional points, coalescent edge modes, and the skin effect
Authors: Zhang, Xizheng; Gong, Jiangbin
Abstract: © 2020 American Physical Society. Periodically driven non-Hermitian systems can exhibit rich topological band structure and non-Hermitian skin effect, without analogs in their static or Hermitian counterparts. In this work we investigate the exceptional band-touching points in the Floquet quasienergy bands, the topological characterization of such exceptional points and the Floquet non-Hermitian skin effect (FNHSE). Specifically, we exploit the simplicity of periodically quenched two-band systems in one dimension or two dimensions to analytically obtain the Floquet effective Hamiltonian as well as locations of the many exceptional points possessed by the Floquet bulk bands. Two different types of topological winding numbers are used to characterize the topological features. Bulk-boundary correspondence (BBC) is naturally found to break down due to FNHSE, which can be drastically different among different bulk states. Remarkably, given the simple nature of our model systems, recovering the BBC is doable in practice only for certain parameter regime where a low-order truncation of the characteristic polynomial (which determines the Floquet band structure) becomes feasible. Furthermore, irrespective of which parameter regime we work with, we find a number of intriguing aspects of Floquet topological zero modes and π modes. For example, under the open boundary condition, zero edge modes and π edge modes can individually coalesce and localize at two different boundaries. These anomalous edge states can also switch their accumulation boundaries when a certain system parameter is tuned. These results indicate that non-Hermitian Floquet topological phases, though more challenging to understand than their Hermitian counterparts, can be extremely rich in the presence of FNHSE.
Mon, 13 Jan 2020 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685152020-01-13T00:00:00Z
- Geometric characterization of non-Hermitian topological systems through the singularity ring in pseudospin vector spacehttps://scholarbank.nus.edu.sg/handle/10635/168525Title: Geometric characterization of non-Hermitian topological systems through the singularity ring in pseudospin vector space
Authors: Li, Linhu; Lee, Ching Hua; Gong, Jiangbin
Abstract: © 2019 American Physical Society. This work unveils how geometric features of two-band non-Hermitian Hamiltonians can classify the topology of their eigenstates and energy manifolds. Our approach generalizes the Bloch sphere visualization of Hermitian systems to a "Bloch torus" picture for non-Hermitian systems, by extending the origin of the Bloch sphere to a singularity ring (SR) in the vector space of the real pseudospin. The SR captures the structure of generic spectral exceptional degeneracies, which emerge only if the real pseudospin vector actually falls on the SR. Applicable to non-Hermitian systems that may or may not have exceptional degeneracies, this SR picture affords convenient visualization of various symmetry constraints and reduces their topological characterization to the classification of simple intersection or winding behavior, as detailed by our explicit study of chiral, sublattice, particle-hole, and conjugated particle-hole symmetries. In 1D, the winding number about the SR corresponds to the band vorticity measurable through the Berry phase. In 2D, more complicated winding behavior leads to a variety of phases that illustrates the richness of the interplay between SR topology and geometry beyond mere Chern number classification. Through a normalization procedure that puts generic two-band non-Hermitian Hamiltonians on equal footing, our SR approach also allows for vivid visualization of the non-Hermitian skin effect.
Thu, 01 Aug 2019 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685252019-08-01T00:00:00Z
- Measurement-only quantum computation with Floquet Majorana corner modeshttps://scholarbank.nus.edu.sg/handle/10635/168517Title: Measurement-only quantum computation with Floquet Majorana corner modes
Authors: Bomantara, RW; Gong, J
Abstract: © 2020 American Physical Society. Majorana modes, typically arising at the edges of one-dimensional topological superconductors, are considered to be a promising candidate for encoding nonlocal qubits in fault-tolerant quantum computing. Here we exploit the two-dimensional geometry of Majorana corner modes in second-order topological superconductors to establish measurement-only quantum computation. It is shown that eight Majorana corner modes emerge when such systems are periodically driven, through which two nonlocal logical qubits and one nonlocal ancilla qubit can be constructed. Quantum gate operations can then be implemented by a designed series of parity measurements of topologically protected Majorana corner modes, accomplished via Mach-Zehnder type interference in the conductance between different corners of a second-order topological superconductor. Our theoretical proposal represents a scenario in which topologically protected single- A nd two-qubit gate operations can be carried out in a minimal setup, thus potentially establishing an efficient and low-cost building block for Majorana-based qubit architectures.
Sat, 15 Feb 2020 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685172020-02-15T00:00:00Z
- Time-dependent PT-symmetric quantum mechanics in generic non-Hermitian systemshttps://scholarbank.nus.edu.sg/handle/10635/168523Title: Time-dependent PT-symmetric quantum mechanics in generic non-Hermitian systems
Authors: Zhang, DJ; Wang, QH; Gong, J
Abstract: © 2019 American Physical Society. A conceptual framework extending (time-independent) PT-symmetric quantum mechanics into the time-dependent domain is presented. It is built upon a nontrivial time-dependent metric operator identified here and works for generic finite-dimensional non-Hermitian systems. All the ingredients of our framework, such as the time-dependent Hilbert space, the observable, and the measurement postulate, can be "realized" by means of dilating and reinterpreting the non-Hermitian system in question as a part of a larger Hermitian system. Aided by our metric operator, we formulate the concepts of stable and unstable phases for generic non-Hermitian systems and argue that they, respectively, generalize the notions of unbroken and broken phases in time-independent PT-symmetric systems. Possible applications of our framework are illustrated with well-known examples in quantum thermodynamics.
Fri, 20 Dec 2019 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685232019-12-20T00:00:00Z
- Floquet engineering with particle swarm optimization: Maximizing topological invariantshttps://scholarbank.nus.edu.sg/handle/10635/168518Title: Floquet engineering with particle swarm optimization: Maximizing topological invariants
Authors: Zhang, S; Gong, J
Abstract: © 2019 American Physical Society. It is of theoretical and experimental interest to engineer topological phases with very large topological invariants via periodic driving. As advocated by this paper, such Floquet engineering can be elegantly achieved by the particle swarm optimization (PSO) technique from the swarm intelligence family. With the recognition that conventional gradient-based optimization approaches are not suitable for directly optimizing topological invariants as integers, the highly effective PSO route yields new promises in the search for exotic topological phases, requiring limited physical resources. Our results are especially timely in view of two important insights from literature: Low-frequency driving may be beneficial in creating large topological invariants, but an open-ended low-frequency driving often leads to drastic fluctuations in the obtained topological invariants. Indeed, using a simple continuously driven Harper model with three quasienergy bands, we show that the Floquet-band Chern numbers can enjoy a many-fold increase compared with that using a simple harmonic driving of the same period, without demanding more energy cost of the driving field. It is also found that the resulting Floquet insulator bands are still well gapped with the maximized topological invariants in agreement with physical observations from Thouless pumping. The emergence of many edge modes under the open boundary condition is also consistent with the bulk-edge correspondence. Our results are expected to be highly useful towards the optimization of many different types of topological invariants in Floquet topological matter.
Thu, 26 Dec 2019 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685182019-12-26T00:00:00Z
- Hybrid Higher-Order Skin-Topological Modes in Nonreciprocal Systemshttps://scholarbank.nus.edu.sg/handle/10635/168528Title: Hybrid Higher-Order Skin-Topological Modes in Nonreciprocal Systems
Authors: Lee, Ching Hua; Li, Linhu; Gong, Jiangbin
Abstract: © 2019 American Physical Society. Higher-order phases are characterized by corner or hinge modes that arise due to the interesting interplay of localization mechanisms along two or more dimensions. In this work, we introduce and construct a novel class of "hybrid" higher-order skin-topological boundary modes in nonreciprocal systems with two or more open boundaries. Their existence crucially relies on nonreciprocal pumping in addition to topological localization. Unlike usual non-Hermitian "skin" modes, they can exist in lattices with vanishing net reciprocity due to the selective nature of nonreciprocal pumping: While the bulk modes remain extended due to the cancellation of nonreciprocity within each unit cell, boundary modes experience a curious spontaneous breaking of reciprocity in the presence of topological localization, thereby experiencing the non-Hermitian skin effect. The number of possible hybridization channels increases rapidly with dimensionality, leading to a proliferation of distinct phases. In addition, skin modes or hybrid skin-topological modes can restore unitarity and are hence stable, allowing for experimental observations and manipulations in non-Hermitian photonic and electrical metamaterials.
Tue, 02 Jul 2019 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685282019-07-02T00:00:00Z
- Floquet dynamical quantum phase transitionshttps://scholarbank.nus.edu.sg/handle/10635/168524Title: Floquet dynamical quantum phase transitions
Authors: Yang, Kai; Zhou, Longwen; Ma, Wenchao; Kong, Xi; Wang, Pengfei; Qin, Xi; Rong, Xing; Wang, Ya; Shi, Fazhan; Gong, Jiangbin; Du, Jiangfeng
Abstract: © 2019 American Physical Society. Dynamical quantum phase transitions (DQPTs) are manifested by time-domain nonanalytic behaviors of many-body systems. Introducing a quench is so far understood as a typical scenario to induce DQPTs. In this work, we discover a type of DQPTs, termed Floquet DQPTs, as intrinsic features of time-periodic systems. Floquet DQPTs occur within each period of continuous driving, without the need for any quenches. In particular, in a harmonically driven spin chain model, we find analytically the existence of Floquet DQPTs in and only in a parameter regime hosting a certain nontrivial Floquet topological phase. The Floquet DQPTs are further characterized by a dynamical topological invariant defined as the winding number of the Pancharatnam geometric phase versus quasimomentum. These findings are experimentally demonstrated with a single spin in diamond. This work thus opens a door for future studies of DQPTs in connection with topological matter.
Wed, 28 Aug 2019 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685242019-08-28T00:00:00Z
- Single-atom energy-conversion device with a quantum loadhttps://scholarbank.nus.edu.sg/handle/10635/168823Title: Single-atom energy-conversion device with a quantum load
Authors: Van Horne, N; Yum, D; Dutta, T; HANGGI,PETER; Gong, J; Poletti, D; Mukherjee, M
Abstract: © 2020, The Author(s). We report on a single-atom energy-conversion quantum device operating as an engine, or a refrigerator, coupled to a quantum load. The ‘working fluid’ consists of the two optical levels of an ion, while the load is one of its vibrational modes, cooled down to the quantum regime. We explore two important differences with classical engines: (1) the presence of a strong generic coupling interaction between engine and load, which can induce correlations between them and (2) the use of nonthermal baths. We examine the ergotropy of the load, which indicates the maximum amount of energy of the load extractable using solely unitary operations. We show that ergotropy rises with the number of engine cycles despite an increase in the information entropy of the load. The increase of ergotropy of the load points to the possibility of using the phonon distribution of a single atom as a form of quantum battery.
Tue, 01 Dec 2020 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1688232020-12-01T00:00:00Z
- Quantum geometric tensor in PT -symmetric quantum mechanicshttps://scholarbank.nus.edu.sg/handle/10635/155024Title: Quantum geometric tensor in PT -symmetric quantum mechanics
Authors: Zhang, DJ; Wang, QH; Gong, J
Abstract: © 2019 American Physical Society. A series of geometric concepts are formulated for PT-symmetric quantum mechanics and they are further unified into one entity, i.e., an extended quantum geometric tensor (QGT). The imaginary part of the extended QGT gives a Berry curvature whereas the real part induces a metric tensor on the system's parameter manifold. This results in a unified conceptual framework to understand and explore physical properties of PT-symmetric systems from a geometric perspective. To illustrate the usefulness of the extended QGT, we show how its real part, the metric tensor, can be exploited as a tool to detect quantum phase transitions as well as spontaneous PT symmetry breaking in PT-symmetric systems.
Thu, 04 Apr 2019 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1550242019-04-04T00:00:00Z
- Graph-theory treatment of one-dimensional strongly repulsive fermionshttps://scholarbank.nus.edu.sg/handle/10635/168818Title: Graph-theory treatment of one-dimensional strongly repulsive fermions
Authors: Decamp, Jean; Gong, Jiangbin; Loh, Huanqian; Miniatura Christian Pierre-Marie
Mon, 20 Apr 2020 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1688182020-04-20T00:00:00Z
- Quantum Hyperdiffusion in one-dimensional tight-binding latticeshttps://scholarbank.nus.edu.sg/handle/10635/97684Title: Quantum Hyperdiffusion in one-dimensional tight-binding lattices
Authors: Zhang, Z.; Tong, P.; Gong, J.; Li, B.
Abstract: Transient quantum hyperdiffusion, namely, faster-than-ballistic wave packet spreading for a certain time scale, is found to be a typical feature in tight-binding lattices if a sublattice with on-site potential is embedded in a uniform lattice without on-site potential. The strength of the sublattice on-site potential, which can be periodic, disordered, or quasiperiodic, must be below certain threshold values for quantum hyperdiffusion to occur. This is explained by an energy band mismatch between the sublattice and the rest uniform lattice and by the structure of the underlying eigenstates. Cases with a quasiperiodic sublattice can yield remarkable hyperdiffusion exponents that are beyond three. A phenomenological explanation of hyperdiffusion exponents is also discussed. © 2012 American Physical Society.
Wed, 15 Feb 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/976842012-02-15T00:00:00Z
- Converting Zitterbewegung oscillation to directed motionhttps://scholarbank.nus.edu.sg/handle/10635/112403Title: Converting Zitterbewegung oscillation to directed motion
Authors: Zhang, Q.; Gong, J.B.; Oh, C.H.
Abstract: Zitterbewegung (ZB) oscillation, namely, the jittering center-of-mass motion predicted by free-space Dirac (or Dirac-like) equations, has been studied in several different contexts. It is shown here that ZB can be converted to directed center-of-mass motion by a modulation of the Dirac-like equation, if the modulation is on resonance with the ZB frequency. Tailored modulation may also stop, re-launch or even reverse the directed motion of a wavepacket with negligible distortion. The predictions may be examined by current ZB experiments using trapped-ion systems. © 2011 Europhysics Letters Association.
Sat, 01 Oct 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1124032011-10-01T00:00:00Z
- A special section on chaos and transport at the nanoscalehttps://scholarbank.nus.edu.sg/handle/10635/98958Title: A special section on chaos and transport at the nanoscale
Authors: Gong, J.; Xie, J.R.H.
Mon, 01 Nov 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/989582010-11-01T00:00:00Z
- Localization behavior of Dirac particles in disordered graphene superlatticeshttps://scholarbank.nus.edu.sg/handle/10635/97084Title: Localization behavior of Dirac particles in disordered graphene superlattices
Authors: Zhao, Q.; Gong, J.; Müller, C.A.
Abstract: Graphene superlattices (GSLs), formed by subjecting a monolayer graphene sheet to a periodic potential, can be used to engineer band structures and, from there, charge transport properties, but these are sensitive to the presence of disorder. The localization behavior of massless 2D Dirac particles induced by weak disorder is studied for both scalar-potential and vector-potential GSLs, computationally as well as analytically by a weak-disorder expansion. In particular, it is investigated how the Lyapunov exponent (inverse localization length) depends on the incidence angle to a 1D GSL. Delocalization resonances are found for both scalar and vector GSLs. The sharp angular dependence of the Lyapunov exponent may be exploited to realize disorder-induced filtering, as verified by full 2D numerical wave packet simulations. © 2012 American Physical Society.
Fri, 02 Mar 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/970842012-03-02T00:00:00Z
- Preferred states of decoherence under intermediate system-environment couplinghttps://scholarbank.nus.edu.sg/handle/10635/97581Title: Preferred states of decoherence under intermediate system-environment coupling
Authors: Wang, W.-G.; He, L.; Gong, J.
Abstract: The notion that decoherence rapidly reduces a superposition state to an incoherent mixture implicitly adopts a special representation, namely, the representation of preferred (pointer) states (PS). For weak or strong system-envrionment interaction, the behavior of PS is well known. Via a simple dynamical model that simulates a two-level system interacting with few other degrees of freedom as its environment, it is shown that even for intermediate system-environment coupling, approximate PS may still emerge from the coherent quantum dynamics of the whole system in the absence of any thermal averaging. The found PS can also continuously deform to expected limits for weak or strong system-environment coupling. Computational results are also qualitatively explained. The findings should be useful towards further understanding of decoherence and quantum thermalization processes. © 2012 American Physical Society.
Fri, 17 Feb 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/975812012-02-17T00:00:00Z
- Quantum diffusion dynamics in nonlinear systems: A modified kicked-rotor modelhttps://scholarbank.nus.edu.sg/handle/10635/115250Title: Quantum diffusion dynamics in nonlinear systems: A modified kicked-rotor model
Authors: Gong, J.; Wang, J.
Abstract: Using a simple method analogous to a quantum rephasing technique, a simple modification to a paradigm of classical and quantum chaos is proposed. The interesting quantum maps thus obtained display remarkably rich quantum dynamics. Emphasis is placed on the destruction of dynamical localization without breaking periodicity, unbounded quantum anomalous diffusion in integrable systems, and transient dynamical localization. Experimental realizations of this work are also discussed. © 2007 The American Physical Society.
Thu, 27 Sep 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1152502007-09-27T00:00:00Z
- Interband coherence induced correction to adiabatic pumping in periodically driven systemshttps://scholarbank.nus.edu.sg/handle/10635/127996Title: Interband coherence induced correction to adiabatic pumping in periodically driven systems
Authors: Wang H.; Zhou L.; Gong J.
Thu, 01 Jan 2015 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1279962015-01-01T00:00:00Z
- Nonlinear Landau-Zener processes in a periodic driving fieldhttps://scholarbank.nus.edu.sg/handle/10635/115835Title: Nonlinear Landau-Zener processes in a periodic driving field
Authors: Zhang, Q.; Hänggi, P.; Gong, J.
Abstract: Effects of a periodic driving field on Landau-Zener (LZ) processes are studied using a nonlinear two-mode model that describes the mean-field dynamics of a many-body system. A variety of different dynamical phenomena in different parameter regimes of the driving field are discussed and analyzed. These include shifted, weakened, or enhanced phase dependence of nonlinear LZ (NLZ) processes, nonlinearity-enhanced population transfer in the adiabatic limit and Hamiltonian chaos on the mean-field level. The emphasis of this work is based on how the impact of a periodic driving field on LZ processes with self-interaction differs from those without self-interaction. Apart from gaining knowledge of driven NLZ processes, our findings can be used to gauge the strength of nonlinearity and for efficient manipulation of the mean-field dynamics of many-body systems. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Thu, 03 Jul 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1158352008-07-03T00:00:00Z
- Formation and transformation of vector solitons in two-species Bose-Einstein condensates with a tunable interactionhttps://scholarbank.nus.edu.sg/handle/10635/96666Title: Formation and transformation of vector solitons in two-species Bose-Einstein condensates with a tunable interaction
Authors: Liu, X.; Pu, H.; Xiong, B.; Liu, W.M.; Gong, J.
Abstract: Under a unified theory we investigate the formation of various types of vector solitons in two-species Bose-Einstein condensates with arbitrary scattering lengths. We then show that by tuning the interaction parameter via Feshbach resonance, transformation between different types of vector solitons is possible. Our results open up alternate ways in the quantum control of multispecies Bose-Einstein condensates. © 2009 The American Physical Society.
Mon, 05 Jan 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/966662009-01-05T00:00:00Z
- Fokker-Planck equation with arbitrary dc and ac fields: Continued fraction methodhttps://scholarbank.nus.edu.sg/handle/10635/52944Title: Fokker-Planck equation with arbitrary dc and ac fields: Continued fraction method
Authors: Lee, C.K.; Gong, J.
Abstract: The continued fraction method (CFM) is used to solve the Fokker-Planck equation with arbitrary dc and ac fields. With an appropriate choice of basis functions, the Fokker-Planck equation is converted into a set of linear algebraic equations with short-ranged coupling and then CFM is implemented to obtain numerical solutions with high efficiency. Both a proposed perturbative CFM and the numerically exact matrix CFM are used to study the nonlinear response of driven systems, with their results compared to assess the validity regime of the perturbative approach. The proposed perturbative CFM approach needs scalar quantities only and hence is more efficient within its validity regime. Two nonlinear systems of different nature are used as examples: molecular dipole (rotational Brownian motion) and particle in a periodic potential (translational Brownian motion). The associated full dynamics is presented in the compact form of hysteresis loops. It is observed that as the strength of an AC driving field increases, pronounced nonlinear effects are manifested in the deformation of the hysteresis loops. © 2011 American Physical Society.
Wed, 06 Jul 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/529442011-07-06T00:00:00Z
- Indistinguishability and interference in the coherent control of atomic and molecular processeshttps://scholarbank.nus.edu.sg/handle/10635/96905Title: Indistinguishability and interference in the coherent control of atomic and molecular processes
Authors: Gong, J.; Brumer, P.
Abstract: The subtle and fundamental issue of indistinguishability and interference between independent pathways to the same target state is examined in the context of coherent control of atomic and molecular processes, with emphasis placed on possible "which-way" information due to quantum entanglement established in the quantum dynamics. Because quantum interference between independent pathways to the same target state occurs only when the independent pathways are indistinguishable, it is first shown that creating useful coherence between nondegenerate states of a molecule for subsequent quantum interference manipulation cannot be achieved by collisions between atoms or molecules that are prepared in momentum and energy eigenstates. Coherence can, however, be transferred from light fields to atoms or molecules. Using a particular coherent control scenario, it is shown that this coherence transfer and the subsequent coherent phase control can be readily realized by the most classical states of light, i.e., coherent states of light. It is further demonstrated that quantum states of light may suppress the extent of phase-sensitive coherent control by leaking out some which-way information while "incoherent interference control" scenarios proposed in the literature have automatically ensured the indistinguishability of multiple excitation pathways. The possibility of quantum coherence in photodissociation product states is also understood in terms of the disentanglement between photodissociation fragments. Results offer deeper insights into quantum coherence generation in atomic and molecular processes. © 2010 American Institute of Physics.
Fri, 01 Jan 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/969052010-01-01T00:00:00Z
- Conductance properties of rough quantum wires with colored surface disorderhttps://scholarbank.nus.edu.sg/handle/10635/96061Title: Conductance properties of rough quantum wires with colored surface disorder
Authors: Akguc, G.B.; Gong, J.
Abstract: Effects of correlated disorder on wave localization have attracted considerable interest. Motivated by the importance of studies of quantum transport in rough nanowires, here we examine how colored surface roughness impacts the conductance of two-dimensional quantum waveguides, using direct-scattering calculations based on the reaction matrix approach. The computational results are analyzed in connection with a theoretical relation between the localization length and the structure factor of correlated disorder. We also examine and discuss several cases that have not been treated theoretically or are beyond the validity regime of available theories. Results indicate that conductance properties of quantum wires are controllable via colored surface disorder. ©2008 The American Physical Society.
Mon, 22 Sep 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/960612008-09-22T00:00:00Z
- Generating a fractal butterfly floquet spectrum in a class of driven SU(2) systemshttps://scholarbank.nus.edu.sg/handle/10635/117025Title: Generating a fractal butterfly floquet spectrum in a class of driven SU(2) systems
Authors: Wang, J.; Gong, J.
Abstract: A scheme for generating a fractal butterfly Floquet spectrum, first proposed by Wang and Gong [Phys. Rev. A 77, 031405(R) (2008)], is extended to driven SU(2) systems such as a driven two-mode Bose-Einstein condensate. A class of driven systems without a link with the Harper-model context is shown to have an intriguing butterfly Floquet spectrum. The found butterfly spectrum shows remarkable deviations from the known Hofstadter's butterfly. In addition, the level crossings between Floquet states of the same parity and between Floquet states of different parities are studied and highlighted. The results are relevant to studies of fractal statistics, quantum chaos, and coherent destruction of tunneling, as well as the validity of mean-field descriptions of Bose-Einstein condensates. © 2010 The American Physical Society.
Fri, 05 Feb 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1170252010-02-05T00:00:00Z
- Sensitive frequency dependence of the carrier-envelope phase effect on bound-bound transitions: An interference perspectivehttps://scholarbank.nus.edu.sg/handle/10635/97904Title: Sensitive frequency dependence of the carrier-envelope phase effect on bound-bound transitions: An interference perspective
Authors: Peng, D.; Wu, B.; Fu, P.; Wang, B.; Gong, J.; Yan, Z.-C.
Abstract: We investigate numerically with Hylleraas coordinates the frequency dependence of the carrier-envelope phase (CEP) effect on bound-bound transitions of helium induced by an ultrashort laser pulse of a few cycles. We find that the CEP effect is very sensitive to the carrier frequency of the laser pulse, occurring regularly even at far-offresonance frequencies. By analyzing a two-level model, we find that the CEP effect can be attributed to the quantum interference between neighboring multiphoton transition pathways, which is made possible by the broadened spectrum of the ultrashort laser pulse. A general picture is developed along this line to understand the sensitivity of the CEP effect to the laser's carrier frequency. Multilevel influence on the CEP effect is also discussed. © 2010 The American Physical Society.
Fri, 01 Jan 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/979042010-01-01T00:00:00Z
- Phase-space characterization of complexity in quantum many-body dynamicshttps://scholarbank.nus.edu.sg/handle/10635/112484Title: Phase-space characterization of complexity in quantum many-body dynamics
Authors: Balachandran, V.; Benenti, G.; Casati, G.; Gong, J.
Abstract: We propose a phase-space Wigner harmonics entropy measure for many-body quantum dynamical complexity. This measure, which reduces to the well-known measure of complexity in classical systems and which is valid for both pure and mixed states in single-particle and many-body systems, takes into account the combined role of chaos and entanglement in the realm of quantum mechanics. The effectiveness of the measure is illustrated in the example of the Ising chain in a homogeneous tilted magnetic field. We provide numerical evidence that the multipartite entanglement generation leads to a linear increase in entropy until saturation in both integrable and chaotic regimes, so that in both cases the number of harmonics of the Wigner function grows exponentially with time. The entropy growth rate can be used to detect quantum phase transitions. The proposed entropy measure can also distinguish between integrable and chaotic many-body dynamics by means of the size of long-term fluctuations which become smaller when quantum chaos sets in. © 2010 The American Physical Society.
Wed, 20 Oct 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1124842010-10-20T00:00:00Z
- Chaos and correspondence in classical and quantum Hamiltonian ratchets: A Heisenberg approachhttps://scholarbank.nus.edu.sg/handle/10635/95946Title: Chaos and correspondence in classical and quantum Hamiltonian ratchets: A Heisenberg approach
Authors: Pelc, J.; Gong, J.; Brumer, P.
Abstract: Previous work motivates this study as to how asymmetry-driven quantum ratchet effects can persist despite a corresponding fully chaotic classical phase space. A simple perspective of ratchet dynamics, based on the Heisenberg picture, is introduced. We show that ratchet effects are in principle of common origin in classical and quantum mechanics, although full chaos suppresses these effects in the former but not necessarily the latter. The relationship between ratchet effects and coherent dynamical control is noted. © 2009 The American Physical Society.
Tue, 16 Jun 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/959462009-06-16T00:00:00Z
- Quantum control of ultra-cold atoms: Uncovering a novel connection between two paradigms of quantum nonlinear dynamicshttps://scholarbank.nus.edu.sg/handle/10635/117128Title: Quantum control of ultra-cold atoms: Uncovering a novel connection between two paradigms of quantum nonlinear dynamics
Authors: Wang, J.; Mouritzen, A.S.; Gong, J.
Abstract: Controlling the translational motion of cold atoms using optical lattice potentials is of both theoretical and experimental interest. By designing two on-resonance time sequences of kicking optical lattice potentials, a novel connection between two paradigms of nonlinear mapping systems, i.e. the kicked rotor model and the kicked Harper model, is established. In particular, it is shown that Hofstadter's butterfly quasi-energy spectrum in periodically driven quantum systems may soon be realized experimentally, with the effective Planck constant tunable by varying the time delay between two sequences of control fields. Extensions of this study are also discussed. The results are intended to open up a new generation of cold-atom experiments of quantum nonlinear dynamics.
Sun, 01 Mar 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1171282009-03-01T00:00:00Z
- The effect of state preparation in a many-body systemhttps://scholarbank.nus.edu.sg/handle/10635/98261Title: The effect of state preparation in a many-body system
Authors: Chaudhry, A.Z.; Gong, J.
Abstract: For a quantum system interacting with its environment, the role of state preparation is nontrivial. The reason is that before the state preparation procedure, the system and the environment are correlated. Consequently, the state preparation procedure (which acts on the system) indirectly influences the state of the environment depending on the state preparation. In this paper, we use an experimentally realizable model describing a collection of N two-level atoms coupled to a common environment to investigate the influence of the state preparation procedure. We show that the dynamical map describing the evolution of the open quantum system can depend appreciably on the state preparation procedure. Moreover, this effect can be enhanced by increasing N. Our results should be useful for quantum control and quantum tomography. © 2013 Published by NRC Research Press.
Sat, 01 Feb 2014 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/982612014-02-01T00:00:00Z
- Boosting work characteristics and overall heat-engine performance via shortcuts to adiabaticity: Quantum and classical systemshttps://scholarbank.nus.edu.sg/handle/10635/95880Title: Boosting work characteristics and overall heat-engine performance via shortcuts to adiabaticity: Quantum and classical systems
Authors: Deng, J.; Wang, Q.-H.; Liu, Z.; Hänggi, P.; Gong, J.
Abstract: Under a general framework, shortcuts to adiabatic processes are shown to be possible in classical systems. We study the distribution function of the work done on a small system initially prepared at thermal equilibrium. We find that the work fluctuations can be significantly reduced via shortcuts to adiabatic processes. For example, in the classical case, probabilities of having very large or almost zero work values are suppressed. In the quantum case, negative work may be totally removed from the otherwise non-positive-definite work values. We also apply our findings to a micro Otto-cycle-based heat engine. It is shown that the use of shortcuts, which directly enhances the engine output power, can also increase the heat-engine efficiency substantially, in both quantum and classical regimes. © 2013 American Physical Society.
Thu, 12 Dec 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/958802013-12-12T00:00:00Z
- Synthetic spin-orbit coupling in two-level cold atomshttps://scholarbank.nus.edu.sg/handle/10635/53212Title: Synthetic spin-orbit coupling in two-level cold atoms
Authors: Zhang, Q.; Gong, J.-B.; Oh, C.-H.
Abstract: We theoretically and computationally show the simplest realization of SOC using two-level cold atoms interacting with only one laser beam. The underlying mechanism is based on the non-adiabatic nature of laser-atom interaction, with the Rabi frequency being not much larger than the kinetic energy of the atom. We use Zitterbewegung oscillation to further illustrate the effects of the synthesized SOC on the quantum dynamics of the two-level cold atoms. We expect our proposal to be of experimental interest in the quantum simulation of SOC-related physics. © 2013 Chinese Physical Society and IOP Publishing Ltd.
Thu, 01 Aug 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/532122013-08-01T00:00:00Z
- Controlled measurement processes: Simple spin-chain model of controlled quantum-state amplificationhttps://scholarbank.nus.edu.sg/handle/10635/96090Title: Controlled measurement processes: Simple spin-chain model of controlled quantum-state amplification
Authors: Balachandran, V.; Gong, J.
Abstract: Controlled measurement processes with high fidelity and robustness will be of substantial interest to fundamental quantum studies as well as quantum-information processing. This work proposes a simple scheme to realize controlled measurements of the state of a single spin, via controlled quantum signal amplification in a one-dimensional spin-chain model. It is shown that, by adiabatically moving an external field applied to a spin chain that is also irradiated with a transverse driving field, a robust spin amplifier may be realized with little dispersion. Our results may find applications in spin-based quantum-information processing and in controlled growth of "Schrö dinger cat" states. © 2009 The American Physical Society.
Mon, 05 Jan 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/960902009-01-05T00:00:00Z
- Decoherence induced by a composite environmenthttps://scholarbank.nus.edu.sg/handle/10635/96151Title: Decoherence induced by a composite environment
Authors: Chaudhry, A.Z.; Gong, J.
Abstract: It is not always justified to model the environment of a realistic quantum system as a collection of harmonic oscillators or two-level systems. To explore interesting physics associated with a composite environment, here we study the dynamics of a qubit coupled to a second two-level system, which is in turn coupled to a harmonic-oscillator bath. That is, the qubit of interest is in the presence of a composite environment consisting of the second two-level system and a conventional harmonic-oscillator bath. We investigate the issue of pointer states of decoherence for the qubit for different coupling strengths between the qubit and such a composite environment. It is shown that for weak-, intermediate-, or strong-coupling strengths, the qubit's reduced density matrix has a coupling-strength-dependent diagonal representation, thus yielding coupling-strength-dependent computational pointer states. Interestingly, the associated decoherence rate can decrease as the coupling strength increases. © 2014 American Physical Society.
Fri, 31 Jan 2014 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/961512014-01-31T00:00:00Z
- Explicit designs of spin chains for perfect quantum state transferhttps://scholarbank.nus.edu.sg/handle/10635/96573Title: Explicit designs of spin chains for perfect quantum state transfer
Authors: Xi, X.Q.; Gong, J.B.; Zhang, T.; Yue, R.H.; Liu, W.M.
Abstract: Simple and explicit designs of short isotropic "XY" spin chains for perfect quantum state transfer are obtained analytically. Detailed results indicate that many qualitatively different designs exist, thus establishing more flexibility in designing quantum channels for high-fidelity quantum information transfer. © 2008 Springer.
Mon, 01 Dec 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/965732008-12-01T00:00:00Z
- Kicked-Harper model versus on-resonance double-kicked rotor model: From spectral difference to topological equivalencehttps://scholarbank.nus.edu.sg/handle/10635/97018Title: Kicked-Harper model versus on-resonance double-kicked rotor model: From spectral difference to topological equivalence
Authors: Wang, H.; Ho, D.Y.H.; Lawton, W.; Wang, J.; Gong, J.
Abstract: Recent studies have established that, in addition to the well-known kicked-Harper model (KHM), an on-resonance double-kicked rotor (ORDKR) model also has Hofstadter's butterfly Floquet spectrum, with strong resemblance to the standard Hofstadter spectrum that is a paradigm in studies of the integer quantum Hall effect. Earlier it was shown that the quasienergy spectra of these two dynamical models (i) can exactly overlap with each other if an effective Planck constant takes irrational multiples of 2π and (ii) will be different if the same parameter takes rational multiples of 2π. This work makes detailed comparisons between these two models, with an effective Planck constant given by 2πM/N, where M and N are coprime and odd integers. It is found that the ORDKR spectrum (with two periodic kicking sequences having the same kick strength) has one flat band and N-1 nonflat bands with the largest bandwidth decaying in a power law as ∼KN+2, where K is a kick strength parameter. The existence of a flat band is strictly proven and the power-law scaling, numerically checked for a number of cases, is also analytically proven for a three-band case. By contrast, the KHM does not have any flat band and its bandwidths scale linearly with K. This is shown to result in dramatic differences in dynamical behavior, such as transient (but extremely long) dynamical localization in ORDKR, which is absent in the KHM. Finally, we show that despite these differences, there exist simple extensions of the KHM and ORDKR model (upon introducing an additional periodic phase parameter) such that the resulting extended KHM and ORDKR model are actually topologically equivalent, i.e., they yield exactly the same Floquet-band Chern numbers and display topological phase transitions at the same kick strengths. A theoretical derivation of this topological equivalence is provided. These results are also of interest to our current understanding of quantum-classical correspondence considering that the KHM and ORDKR model have exactly the same classical limit after a simple canonical transformation. © 2013 American Physical Society.
Wed, 27 Nov 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/970182013-11-27T00:00:00Z
- Control of tripod-scheme cold-atom wavepackets by manipulating a non-Abelian vector potentialhttps://scholarbank.nus.edu.sg/handle/10635/116965Title: Control of tripod-scheme cold-atom wavepackets by manipulating a non-Abelian vector potential
Authors: Zhang, Q.; Gong, J.; Oh, C.H.
Abstract: Tripod-scheme cold atoms interacting with laser beams have attracted considerable interest for their role in synthesizing effective non-Abelian vector potentials. Such effective vector potentials can be exploited to realize an all-optical imprinting of geometric phases onto matter waves. By working on carefully designed extensions of our previous work, we show that coherent lattice structure of cold-atom sub-wavepackets can be formed and that the non-Abelian Aharonov-Bohm effect can be easily manifested via the translational motion of cold atoms. We also show that by changing the frame of reference, effects due to a non-Abelian vector potential may be connected with a simple dynamical phase effect, and that under certain conditions it can be understood as an Abelian geometric phase in a different frame of reference. Results should help design better schemes for the control of cold-atom matter waves. © 2010 Elsevier Inc.
Tue, 01 Jun 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1169652010-06-01T00:00:00Z
- Universal dynamical decoupling: Two-qubit states and beyondhttps://scholarbank.nus.edu.sg/handle/10635/98525Title: Universal dynamical decoupling: Two-qubit states and beyond
Authors: Mukhtar, M.; Saw, T.B.; Soh, W.T.; Gong, J.
Abstract: Uhrig's dynamical decoupling pulse sequence has emerged as a universal and highly promising approach to decoherence suppression. So far, both the theoretical and experimental studies have examined single-qubit decoherence only. This work extends Uhrig's universal dynamical decoupling from one-qubit to two-qubit systems and even to general multilevel quantum systems. In particular, we show that by designing appropriate control Hamiltonians for a two-qubit or a multilevel system, Uhrig's pulse sequence can also preserve a generalized quantum coherence measure to the order of 1+O(TN+1) with only N pulses. Our results lead to a very useful scheme for efficiently locking two-qubit entangled states. Future important applications of Uhrig's pulse sequence in preserving the quantum coherence of multilevel quantum systems can also be anticipated. © 2010 The American Physical Society.
Fri, 29 Jan 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/985252010-01-29T00:00:00Z
- Controlling the ratchet effect for cold atomshttps://scholarbank.nus.edu.sg/handle/10635/96101Title: Controlling the ratchet effect for cold atoms
Authors: Kenfack, A.; Gong, J.; Pattanayak, A.K.
Abstract: Low-order quantum resonances manifested by directed currents have been realized with cold atoms. Here we show that by increasing the strength of an experimentally achievable delta-kicking ratchet potential, quantum resonances of a very high order may naturally emerge and can induce larger ratchet currents than low-order resonances, with the underlying classical limit being fully chaotic. The results offer a means of controlling quantum transport of cold atoms. © 2008 The American Physical Society.
Thu, 31 Jan 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/961012008-01-31T00:00:00Z
- Role of initial system-environment correlations: A master equation approachhttps://scholarbank.nus.edu.sg/handle/10635/97834Title: Role of initial system-environment correlations: A master equation approach
Authors: Chaudhry, A.Z.; Gong, J.
Abstract: In order to achieve practical implementations of emerging quantum technologies, it is important to have a firm understanding of the dynamics of realistic quantum open systems. Master equations provide a widely used tool in this regard. In this work we first construct a master equation, valid for weak system-environment coupling, which explicitly takes into account the impact of preparing an initial system state from an equilibrium system-environment state that has system-environment correlations. We then investigate the role of initial system-environment correlations using this master equation for a system consisting of many two-level atoms interacting with a common environment. We show that, in general, due to the initial system-environment correlations before a state preparation, the quantum state of the system can evolve at a faster time scale. Moreover, we also consider different initial state preparations, and demonstrate that the influence of state preparations depends on the initial states prepared. Our results can be of interest to many topics based on quantum open systems where system-environment correlation effects have been neglected before. © 2013 American Physical Society.
Mon, 11 Nov 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/978342013-11-11T00:00:00Z
- Reexamination of measurement-induced chaos in entanglement-purification protocolshttps://scholarbank.nus.edu.sg/handle/10635/97786Title: Reexamination of measurement-induced chaos in entanglement-purification protocols
Authors: Guan, Y.; Nguyen, D.Q.; Xu, J.; Gong, J.
Abstract: Entanglement-purification protocols, developed for the sake of high-fidelity communication through noisy quantum channels, are highly nonlinear quantum operations and can offer a very useful context to studies of nonlinear complex maps. Recently it was demonstrated that the feedback mechanism used in a typical purification protocol can cause the evolution dynamics of qubits to exhibit chaos. In this work we extend the investigation by considering the natural time evolution of qubits during a purification process, leading to a number of interesting findings that reflect the competition between the natural unitary evolution of qubits and nonlinear purification operations. As a result, the overall evolution dynamics of entanglement can be much richer. Possible applications are also proposed. © 2013 American Physical Society.
Mon, 20 May 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/977862013-05-20T00:00:00Z
- Adiabatic quantum transport in a spin chain with a moving potentialhttps://scholarbank.nus.edu.sg/handle/10635/95730Title: Adiabatic quantum transport in a spin chain with a moving potential
Authors: Balachandran, V.; Gong, J.
Abstract: Many schemes to realize quantum state transfer in spin chains are not robust to random fluctuations in the spin-spin coupling strength. In efforts to achieve robust quantum state transfer, an adiabatic quantum population transfer scheme is proposed in this study. The proposed scheme makes use of a slowly moving external parabolic potential and is qualitatively explained in terms of the adiabatic following of a quantum state with a moving separatrix structure in the classical phase space of a pendulum analogy. Detailed aspects of our adiabatic population transfer scheme, including its robustness, is studied computationally. Applications of our adiabatic scheme in quantum information transfer are also discussed, with emphasis placed on the usage of a dual spin chain to encode quantum phases. The results should also be useful for the control of electron tunneling in an array of quantum dots. © 2008 The American Physical Society.
Fri, 04 Jan 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/957302008-01-04T00:00:00Z
- Exponential quantum spreading in a class of kicked rotor systems near high-order resonanceshttps://scholarbank.nus.edu.sg/handle/10635/96575Title: Exponential quantum spreading in a class of kicked rotor systems near high-order resonances
Authors: Wang, H.; Wang, J.; Guarneri, I.; Casati, G.; Gong, J.
Abstract: Long-lasting exponential quantum spreading was recently found in a simple but very rich dynamical model, namely, an on-resonance double-kicked rotor model. The underlying mechanism, unrelated to the chaotic motion in the classical limit but resting on quasi-integrable motion in a pseudoclassical limit, is identified for one special case. By presenting a detailed study of the same model, this work offers a framework to explain long-lasting exponential quantum spreading under much more general conditions. In particular, we adopt the so-called "spinor" representation to treat the kicked-rotor dynamics under high-order resonance conditions and then exploit the Born-Oppenheimer approximation to understand the dynamical evolution. It is found that the existence of a flat band (or an effectively flat band) is one important feature behind why and how the exponential dynamics emerges. It is also found that a quantitative prediction of the exponential spreading rate based on an interesting and simple pseudoclassical map may be inaccurate. In addition to general interests regarding the question of how exponential behavior in quantum systems may persist for a long time scale, our results should motivate further studies toward a better understanding of high-order resonance behavior in δ-kicked quantum systems. © 2013 American Physical Society.
Wed, 27 Nov 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/965752013-11-27T00:00:00Z
- Two-mode Bose-Einstein condensate in a high-frequency driving field that directly couples the two modeshttps://scholarbank.nus.edu.sg/handle/10635/116013Title: Two-mode Bose-Einstein condensate in a high-frequency driving field that directly couples the two modes
Authors: Zhang, Q.; Hänggi, P.; Gong, J.
Abstract: A two-mode Bose-Einstein condensate coupled by a high-frequency modulation field is found to display rich features. An effective stationary Hamiltonian approach reveals the emergence of additional degenerate eigenstates as well as additional topological structures of the spectrum. Possible applications, such as the suppression of nonlinear Landau-Zener tunneling, are discussed. An interesting phenomenon, which we call "deterministic symmetry-breaking trapping" associated with separatrix crossing, is also found in an adiabatic process. © 2008 The American Physical Society.
Tue, 13 May 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1160132008-05-13T00:00:00Z
- Quantum ratchet control - Harvesting on Landau-Zener transitionshttps://scholarbank.nus.edu.sg/handle/10635/97703Title: Quantum ratchet control - Harvesting on Landau-Zener transitions
Authors: Morales-Molina, L.; Flach, S.; Gong, J.B.
Abstract: We control the current of a single-particle quantum ratchet by designing ramping schemes for experimentally accessible control parameters. We harvest on Landau-Zener transitions between Floquet states. Adiabatic and diabatic ramping allow to control the resulting directed transport. We find strong changes of the current in the adiabatic regime. Simple loops in control parameter space with alternating adiabatic and diabatic ramping are proposed. We obtain also current reversal. Full desymmetrization of the quantum ratchet increases the critical ramping speed which separates the adiabatic from the diabatic regime. © 2008 Europhysics Letters Association.
Fri, 01 Aug 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/977032008-08-01T00:00:00Z
- Controlled quantum-state transfer in a spin chainhttps://scholarbank.nus.edu.sg/handle/10635/96092Title: Controlled quantum-state transfer in a spin chain
Authors: Gong, J.; Brumer, P.
Abstract: Control of the transfer of quantum information encoded in quantum wave packets moving along a spin chain is demonstrated. Specifically, based on a relationship with control in a paradigm of quantum chaos, it is shown that wave packets with slow dispersion can automatically emerge from a class of initial superposition states involving only a few spins, and that arbitrary unspecified traveling wave packets can be nondestructively stopped and later relaunched with perfection. The results establish an interesting application of quantum chaos studies in quantum information science. © 2007 The American Physical Society.
Fri, 23 Mar 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/960922007-03-23T00:00:00Z