ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Thu, 09 Dec 2021 14:37:23 GMT2021-12-09T14:37:23Z501101- 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
- Symmetry breaking and self-trapping of a dipolar Bose-Einstein condensate in a double-well potentialhttps://scholarbank.nus.edu.sg/handle/10635/53203Title: Symmetry breaking and self-trapping of a dipolar Bose-Einstein condensate in a double-well potential
Authors: Xiong, B.; Gong, J.; Pu, H.; Bao, W.; Li, B.
Abstract: The quantum self-trapping phenomenon of a Bose-Einstein condensate (BEC) represents a remarkable nonlinear effect of wide interest. By considering a purely dipolar BEC in a double-well potential, we study how the dipole orientation affects the ground-state structure and the transition between self-trapping and Josephson oscillations in dynamics. Three-dimensional numerical results and an effective two-mode model demonstrate that the onset of self-trapping of a dipolar BEC can be radically modified by the dipole orientation. We also analyze the failure of the two-mode model in predicting the rate of Josephson oscillations. We hope that our results can motivate experimental work as well as future studies of self-trapping of ultracold dipolar gases in optical lattices. © 2009 The American Physical Society.
Mon, 05 Jan 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/532032009-01-05T00:00:00Z
- Statistical properties of power-law random banded unitary matrices in the delocalization-localization transition regimehttps://scholarbank.nus.edu.sg/handle/10635/53190Title: Statistical properties of power-law random banded unitary matrices in the delocalization-localization transition regime
Authors: Bandyopadhyay, J.N.; Gong, J.
Abstract: Power-law random banded unitary matrices (PRBUM), whose matrix elements decay in a powerlaw fashion, were recently proposed to model the critical statistics of the Floquet eigenstates of periodically driven quantum systems. In this work, we numerically study in detail the statistical properties of PRBUM ensembles in the delocalization-localization transition regime. In particular, implications of the delocalization-localization transition for the fractal dimension of the eigenvectors, for the distribution function of the eigenvector components, and for the nearest neighbor spacing statistics of the eigenphases are examined. On the one hand, our results further indicate that a PRBUM ensemble can serve as a unitary analog of the power-law random Hermitian matrix model for Anderson transition. On the other hand, some statistical features unseen before are found from PRBUM. For example, the dependence of the fractal dimension of the eigenvectors of PRBUM upon one ensemble parameter displays features that are quite different from that for the power-law random Hermitian matrix model. Furthermore, in the timereversal symmetric case the nearest neighbor spacing distribution of PRBUM eigenphases is found to obey a semi-Poisson distribution for a broad range, but display an anomalous level repulsion in the absence of time-reversal symmetry. © 2012 EDP Sciences, Società Italiana di Fisica, Springer-Verlag.
Mon, 01 Oct 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/531902012-10-01T00:00:00Z
- Optimized dynamical decoupling sequences in protecting two-qubit stateshttps://scholarbank.nus.edu.sg/handle/10635/97464Title: Optimized dynamical decoupling sequences in protecting two-qubit states
Authors: Pan, Y.; Xi, Z.-R.; Gong, J.
Abstract: Aperiodic dynamical decoupling (DD) sequences of π pulses are of great interest to decoherence control and have been recently extended from single-qubit to two-qubit systems. If the environmental noise power spectrum is made available, then one may further optimize aperiodic DD sequences to reach higher efficiency of decoherence suppression than known universal schemes. This possibility is investigated in this work for the protection of two-qubit states, using an exactly solvable pure dephasing model including both local and nonlocal noise. The performance of optimized DD sequences in protecting two-qubit states is compared with that achieved by nested Uhrig's DD (nested-UDD) sequences, for several different types of noise spectrum. Except for the cases with noise spectrum decaying slowly in the high-frequency regime, optimized DD sequences with tens of control pulses can perform orders of magnitude better than that of nested-UDD. A two-qubit system with highly unbalanced local noise is also examined to shed more light on a recent experiment. Possible experiments that may be motivated by this work are discussed. © 2011 IOP Publishing Ltd.
Wed, 14 Sep 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/974642011-09-14T00:00:00Z
- 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
- 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
- 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
- Deformed Jarzynski equalityhttps://scholarbank.nus.edu.sg/handle/10635/175155Title: Deformed Jarzynski equality
Authors: Deng J.; Jaramillo J.D.; Hänggi P.; Gong J.
Abstract: The well-known Jarzynski equality, often written in the form e-??F = ?e-?W?, provides a non-equilibrium means to measure the free energy difference DF of a system at the same inverse temperature ? based on an ensemble average of non-equilibrium workW. The accuracy of Jarzynski's measurement scheme was known to be determined by the variance of exponential work, denoted as var (e-?W) . However, it was recently found that var (e-?W) can systematically diverge in both classical and quantum cases. Such divergence will necessarily pose a challenge in the applications of Jarzynski equality because it may dramatically reduce the efficiency in determining ?F. In this work, we present a deformed Jarzynski equality for both classical and quantum non-equilibrium statistics, in efforts to reuse experimental data that already suffers from a diverging var (e-?W) . The main feature of our deformed Jarzynski equality is that it connects free energies at different temperatures and it may still work efficiently subject to a diverging var (e-?W) . The conditions for applying our deformed Jarzynski equality may be met in experimental and computational situations. If so, then there is no need to redesign experimental or simulation methods. Furthermore, using the deformed Jarzynski equality, we exemplify the distinct behaviors of classical and quantum work fluctuations for the case of a time-dependent driven harmonic oscillator dynamics and provide insights into the essential performance differences between classical and quantum Jarzynski equalities. © 2017 by the authors.
Sun, 01 Jan 2017 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1751552017-01-01T00:00:00Z
- Critical non-Hermitian skin effecthttps://scholarbank.nus.edu.sg/handle/10635/198117Title: Critical non-Hermitian skin effect
Authors: Li, L.; Lee, C.H.; Mu, S.; Gong, J.
Abstract: Critical systems represent physical boundaries between different phases of matter and have been intensely studied for their universality and rich physics. Yet, with the rise of non-Hermitian studies, fundamental concepts underpinning critical systems - like band gaps and locality - are increasingly called into question. This work uncovers a new class of criticality where eigenenergies and eigenstates of non-Hermitian lattice systems jump discontinuously across a critical point in the thermodynamic limit, unlike established critical scenarios with spectrum remaining continuous across a transition. Such critical behavior, dubbed the “critical non-Hermitian skin effect”, arises whenever subsystems with dissimilar non-reciprocal accumulations are coupled, however weakly. This indicates, as elaborated with the generalized Brillouin zone approach, that the thermodynamic and zero-coupling limits are not exchangeable, and that even a large system can be qualitatively different from its thermodynamic limit. Examples with anomalous scaling behavior are presented as manifestations of the critical non-Hermitian skin effect in finite-size systems. More spectacularly, topological in-gap modes can even be induced by changing the system size. We provide an explicit proposal for detecting the critical non-Hermitian skin effect in an RLC circuit setup, which also directly carries over to established setups in non-Hermitian optics and mechanics. © 2020, The Author(s).
Wed, 01 Jan 2020 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1981172020-01-01T00:00:00Z
- 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
- 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
- Dual topological characterization of non-Hermitian Floquet phaseshttps://scholarbank.nus.edu.sg/handle/10635/200714Title: Dual topological characterization of non-Hermitian Floquet phases
Authors: Zhou, Longwen; Gu, Yongjian; Gong, Jiangbin
Abstract: Non-Hermiticity is expected to add far more physical features to the already rich Floquet topological phases of matter. Nevertheless, a systematic approach to characterize non-Hermitian Floquet topological matter is still lacking. In this work we introduce a dual scheme to characterize the topology of non-Hermitian Floquet systems in momentum space and in real space using a piecewise quenched nonreciprocal Su-Schrieffer-Heeger model for our case studies. Under the periodic boundary condition, topological phases are characterized by a pair of experimentally accessible winding numbers that make jumps between integers and half integers. Under the open boundary condition, a Floquet version of the so-called open boundary winding number is found to be integers and can predict the number of pairs of zero and π Floquet edge modes coexisting with the non-Hermitian skin effect. Our results indicate that a dual characterization of non-Hermitian Floquet topological matter is necessary and also feasible because the formidable task of constructing the celebrated generalized Brillouin zone for non-Hermitian Floquet systems with multiple hopping length scales can be avoided. This work hence paves a way for further studies of non-Hermitian physics in nonequilibrium systems.
Thu, 21 Jan 2021 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/2007142021-01-21T00:00:00Z
- Impurity induced scale-free localizationhttps://scholarbank.nus.edu.sg/handle/10635/200713Title: Impurity induced scale-free localization
Authors: Li, Linhu; Lee, Ching Hua; Gong, Jiangbin
Abstract: Non-Hermitian systems have been shown to have a dramatic sensitivity to their boundary conditions. In particular, the non-Hermitian skin effect induces collective boundary localization upon turning off boundary coupling, a feature very distinct from that under periodic boundary conditions. Here we develop a full framework for non-Hermitian impurity physics in a non-reciprocal lattice, with periodic/open boundary conditions and even their interpolations being special cases across a whole range of boundary impurity strengths. We uncover steady states with scale-free localization along or even against the direction of non-reciprocity in various impurity strength regimes. Also present are Bloch-like states that survive albeit broken translational invariance. We further explore the co-existence of non-Hermitian skin effect and scale-free localization, where even qualitative aspects of the system’s spectrum can be extremely sensitive to impurity strength. Specific circuit setups are also proposed for experimentally detecting the scale-free accumulation, with simulation results confirming our main findings.
Wed, 03 Mar 2021 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/2007132021-03-03T00:00:00Z
- Floquet higher-order topological insulator in a periodically driven bipartite latticehttps://scholarbank.nus.edu.sg/handle/10635/200715Title: Floquet higher-order topological insulator in a periodically driven bipartite lattice
Authors: Zhu, Weiwei; Chong, YD; Gong, Jiangbin
Abstract: Floquet higher-order topological insulators (FHOTIs) are a novel topological phase that can occur in periodically driven lattices. An appropriate experimental platform to realize FHOTIs has not yet been identified. We introduce a periodically driven bipartite (two-band) system that hosts FHOTI phases without static counterparts, and predict that this lattice can be realized in experimentally realistic optical waveguide arrays, similar to those previously used to study anomalous Floquet insulators. The model exhibits interesting phase transitions from first-order to second-order topological matter by tuning a coupling strength parameter, without breaking lattice symmetry. In the FHOTI phase, the lattice hosts corner modes at eigenphase 0 or π, which are robust against disorder in the individual couplings.
Wed, 06 Jan 2021 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/2007152021-01-06T00:00:00Z
- Nonlinearity induced topological physics in momentum space and real spacehttps://scholarbank.nus.edu.sg/handle/10635/200716Title: Nonlinearity induced topological physics in momentum space and real space
Authors: Tuloup, Thomas; Bomantara, Raditya Weda; Lee, Ching Hua; Gong, Jiangbin
Abstract: Nonlinearity induced topological properties in nonlinear lattice systems are studied in both momentum space and real space. Experimentally realizable through the Kerr effect on photonic waveguide systems, our working model depicts onsite nonlinearity added to the Su-Schrieffer-Heeger (SSH) model plus a chiral-symmetry-breaking term. Under the periodic boundary condition, two of the nonlinear energy bands approach the energy bands of the chiral-symmetric SSH model as nonlinearity strength increases. Further, we account for a correction to the Zak phase and obtain a general expression for nonlinear Zak phases. For sufficiently strong nonlinearity, the sum of all nonlinear Zak phases (not the sum of all conventional Zak phases) is found to be quantized. In real space, it is discovered that there is a strong interplay between nonlinear solitons and the topologically protected edge states of the associated chiral-symmetric linear system. Nonlinearity can recover the degeneracy between two edge soliton states, albeit a chiral-symmetry-breaking term. We also reveal the topological origin of in-gap solitons even when the associated linear system is in the topological trivial regime. These momentum-space and real-space results have clearly demonstrated new topological features induced by nonlinearity, indicating that topological physics in nonlinear lattice systems is far richer than previously thought.
Fri, 11 Sep 2020 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/2007162020-09-11T00:00:00Z
- Hierarchical theory of quantum adiabatic evolutionhttps://scholarbank.nus.edu.sg/handle/10635/180130Title: Hierarchical theory of quantum adiabatic evolution
Authors: Zhang, Q; Gong, J; Wu, B
Abstract: Quantum adiabatic evolution is a dynamical evolution of a quantum system under slow external driving. According to the quantum adiabatic theorem, no transitions occur between nondegenerate instantaneous energy eigenstates in such a dynamical evolution. However, this is true only when the driving rate is infinitesimally small. For a small nonzero driving rate, there are generally small transition probabilities between the energy eigenstates. We develop a classical mechanics framework to address the small deviations from the quantum adiabatic theorem order by order. A hierarchy of Hamiltonians is constructed iteratively with the zeroth-order Hamiltonian being determined by the original system Hamiltonian. The kth-order deviations are governed by a kth-order Hamiltonian, which depends on the time derivatives of the adiabatic parameters up to the kth-order. Two simple examples, the Landau-Zener model and a spin-1/2 particle in a rotating magnetic field, are used to illustrate our hierarchical theory. Our analysis also exposes a deep, previously unknown connection between classical adiabatic theory and quantum adiabatic theory. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Wed, 01 Jan 2014 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1801302014-01-01T00:00:00Z
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- Emergent Fermi surface in a many-body non-Hermitian fermionic chainhttps://scholarbank.nus.edu.sg/handle/10635/200718Title: Emergent Fermi surface in a many-body non-Hermitian fermionic chain
Authors: Mu, Sen; Lee, Ching Hua; Li, Linhu; Gong, Jiangbin
Abstract: Quantum degeneracy pressure (QDP) underscores the stability of matter and is arguably the most ubiquitous many-body effect. The associated Fermi surface (FS) has broad implications for physical phenomena, ranging from electromagnetic responses to entanglement entropy (EE) area law violations. Given recent fruitful studies in condensed-matter physics under effectively non-Hermitian descriptions, it becomes urgent to study how QDP and many-body interactions interplay with non-Hermitian effects. Through a prototypical critical one-dimensional fermionic lattice with asymmetric gain/loss, a real space FS is shown to naturally emerge, in addition to any existing momentum space FS. We carefully characterize such real space FS with the EE, by a renormalized temperature that encapsulates the interplay of thermal excitations and non-Hermiticity. Nearest-neighbor repulsion is also found to induce a competing charge density wave (CDW) that may erode the real space FS. The underlying physics surrounding criticality and localization is further analyzed with complex flux spectral flows. Our findings can be experimentally demonstrated with ultracold fermions in a suitably designed optical lattice.
Wed, 19 Aug 2020 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/2007182020-08-19T00:00:00Z
- Enhanced higher harmonic generation from nodal topologyhttps://scholarbank.nus.edu.sg/handle/10635/200720Title: Enhanced higher harmonic generation from nodal topology
Authors: Lee, Ching Hua; Yap, Han Hoe; Tai, Tommy; Xu, Gang; Zhang, Xiao; Gong, Jiangbin
Abstract: Among topological materials, nodal loop semimetals are arguably the most topologically sophisticated, with their valence and conduction bands intersecting along arbitrarily intertwined nodes. But unlike the well-known topological band insulators with quantized edge conductivities, nodal loop materials possess topologically nontrivial Fermi surfaces, not bands. Hence an important question arises: Are there also directly measurable or even technologically useful physical properties characterizing nontrivial nodal loop topology? In this paper, we provide an affirmative answer by showing that nodal linkages protect the higher harmonic generation (HHG) of electromagnetic signals. Specifically, nodal linkages enforce nonmonotonicity in the intraband semiclassical response of nodal materials, which will be robust against perturbations preserving the nodal topology. These nonlinearities distort incident radiation and produce higher frequency peaks in the teraHertz (THz) regime, as we quantitatively demonstrate for a few known nodal materials. Since THz sources are not yet ubiquitous, our new mechanism for HHG will greatly aid applications like material characterization and nonionizing imaging of object interiors.
Tue, 21 Jul 2020 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/2007202020-07-21T00:00:00Z
- Unraveling non-Hermitian pumping: Emergent spectral singularities and anomalous responseshttps://scholarbank.nus.edu.sg/handle/10635/200717Title: Unraveling non-Hermitian pumping: Emergent spectral singularities and anomalous responses
Authors: Lee, Ching Hua; Li, Linhu; Thomale, Ronny; Gong, Jiangbin
Abstract: Within the expanding field of non-Hermitian physics, non-Hermitian pumping has emerged as a key phenomenon, epitomized through the skin effect via extensive boundary mode accumulation modifying the conventional Bloch picture. Beyond redefining bulk-boundary correspondences, we show that non-Hermitian pumping induces an unprecedented type of spectral topology: It admits a classification in terms of graph topology, which is distinct from conventional topological classifications of the eigenstate or energy Riemann surface. Each topological class is characterized by a conformally invariant configuration of spectral branching singularities, with gap-preserving transitions giving rise to emergent band geometry and Berry curvature discontinuities physically manifested as anomalous response kinks. By placing all Hermitian and non-Hermitian lattice Hamiltonians on equal footing, our comprehensive framework also enables the first analytic construction of topological phase diagrams in the presence of multiple nonreciprocal coupling scales, as prototypically demonstrated for the extended non-Hermitian Chern and Kitaev models. Based on general algebraic geometry properties of the energy dispersion, our framework can be directly generalized to multiple bands, dimensions, and even interacting systems. Overall, it reveals the conspiracy of band representations, spectral topology, and complex geometry as it unfolds in directly measurable quantities.
Thu, 27 Aug 2020 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/2007172020-08-27T00:00:00Z
- Emergence and full 3D-imaging of nodal boundary Seifert surfaces in 4D topological matterhttps://scholarbank.nus.edu.sg/handle/10635/200722Title: Emergence and full 3D-imaging of nodal boundary Seifert surfaces in 4D topological matter
Authors: Li, Linhu; Lee, Ching Hua; Gong, Jiangbin
Abstract: The topological classification of nodal links and knot has enamored physicists and mathematicians alike, both for its mathematical elegance and implications on optical and transport phenomena. Central to this pursuit is the Seifert surface bounding the link/knot, which has for long remained a mathematical abstraction. Here we propose an experimentally realistic setup where Seifert surfaces emerge as boundary states of 4D topological systems constructed by stacking 3D nodal line systems along a 4th quasimomentum. We provide an explicit realization with 4D circuit lattices, which are freed from symmetry constraints and are readily tunable due to the dimension and distance agnostic nature of circuit connections. Importantly, their Seifert surfaces can be imaged in 3D via their pronounced impedance peaks, and are directly related to knot invariants like the Alexander polynomial and knot Signature. This work thus unleashes the great potential of Seifert surfaces as sophisticated yet accessible tools in exotic bandstructure studies.
Tue, 29 Oct 2019 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/2007222019-10-29T00:00:00Z
- Quantized classical response from spectral winding topology.https://scholarbank.nus.edu.sg/handle/10635/202780Title: Quantized classical response from spectral winding topology.
Authors: Li, Linhu; Mu, Sen; Lee, Ching Hua; Gong, Jiangbin
Abstract: Topologically quantized response is one of the focal points of contemporary condensed matter physics. While it directly results in quantized response coefficients in quantum systems, there has been no notion of quantized response in classical systems thus far. This is because quantized response has always been connected to topology via linear response theory that assumes a quantum mechanical ground state. Yet, classical systems can carry arbitrarily amounts of energy in each mode, even while possessing the same number of measurable edge states as their topological winding. In this work, we discover the totally new paradigm of quantized classical response, which is based on the spectral winding number in the complex spectral plane, rather than the winding of eigenstates in momentum space. Such quantized response is classical insofar as it applies to phenomenological non-Hermitian setting, arises from fundamental mathematical properties of the Green's function, and shows up in steady-state response, without invoking a conventional linear response theory. Specifically, the ratio of the change in one quantity depicting signal amplification to the variation in one imaginary flux-like parameter is found to display fascinating plateaus, with their quantized values given by the spectral winding numbers as the topological invariants.
Mon, 06 Sep 2021 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/2027802021-09-06T00:00:00Z
- Topological and dynamical features of periodically driven spin laddershttps://scholarbank.nus.edu.sg/handle/10635/202784Title: Topological and dynamical features of periodically driven spin ladders
Authors: Bomantara, Raditya Weda; Mu, Sen; Gong, Jiangbin
Abstract: Studies of periodically driven one-dimensional many-body systems have advanced our understanding of complex systems and stimulated promising developments in quantum simulation. It is hence of interest to go one step further, by investigating the topological and dynamical aspects of periodically driven spin ladders as clean quasi-one-dimensional systems with spin-spin interaction in the rung direction. Specifically, we find that such systems display subharmonic magnetization dynamics reminiscent to that of discrete time crystals (DTCs) at finite system sizes. Through the use of generalized Jordan-Wigner transformation, this feature can be attributed to the presence of corner Majorana π modes (MPMs), which are of topological origin, in the systems' equivalent Majorana lattice. Special emphasis is placed on how the coupling in the rung direction of the ladder prevents degeneracy from occurring between states differing by a single spin excitation, thus preserving the MPM-induced π/T quasienergy spacing of the Floquet eigenstates in the presence of parameter imperfection. This feature, which is absent in their strict one-dimensional counterparts, may yield fascinating consequences in future studies of higher dimensional Floquet many-body systems.
Wed, 02 Jun 2021 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/2027842021-06-02T00:00:00Z
- Point-gap topology with complete bulk-boundary correspondence and anomalous amplification in the Fock space of dissipative quantum systemshttps://scholarbank.nus.edu.sg/handle/10635/202785Title: Point-gap topology with complete bulk-boundary correspondence and anomalous amplification in the Fock space of dissipative quantum systems
Authors: Pan, Jian-Song; Li, Linhu; Gong, Jiangbin
Abstract: The spectral and dynamical properties of dissipative quantum systems, as modeled by a damped oscillator in the Fock space, are investigated from a topological point of view. Unlike a physical lattice system that is naturally under the open boundary condition, the bounded-from-below nature of the Fock space offers a unique setting for understanding and verifying non-Hermitian skin modes under semi-infinity boundary conditions that are elusive in actual physical lattices. A topological characterization based on the complex spectra of the Liouvillian superoperator is proposed and the associated complete set of topologically protected skin modes can be identified, thus reflecting the complete bulk-boundary correspondence of point-gap topology generally absent in realistic materials. Moreover, we discover anomalous skin modes with exponential amplification even though the quantum system is purely dissipative, a counterintuitive dynamical phenomenon that can be accessed in actual experiments. Our results indicate that current studies of non-Hermitian topological matter can greatly benefit research on quantum open systems and vice versa.
Mon, 24 May 2021 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/2027852021-05-24T00:00:00Z
- Delocalization of topological edge stateshttps://scholarbank.nus.edu.sg/handle/10635/202786Title: Delocalization of topological edge states
Authors: Zhu, Weiwei; Teo, Wei Xin; Li, Linhu; Gong, Jiangbin
Abstract: The non-Hermitian skin effect (NHSE) in non-Hermitian lattice systems depicts the exponential localization of eigenstates at the system's boundaries. It has led to a number of counterintuitive phenomena and challenged our understanding of bulk-boundary correspondence in topological systems. This work aims to investigate how the NHSE localization and topological localization of in-gap edge states compete with each other, with several representative static and periodically driven 1D models, whose topological properties are protected by different symmetries. The emerging insight is that at critical system parameters, even topologically protected edge states can be perfectly delocalized. In particular, it is discovered that this intriguing delocalization occurs if the real spectrum of the system's edge states falls on the same system's complex spectral loop obtained under the periodic boundary condition. We have also performed sample numerical simulation to show that such delocalized topological edge states can be safely reconstructed from time-evolving states. One particular application of delocalized topological edge states in assisting adiabatic edge state pumping is also computationally demonstrated.
Wed, 12 May 2021 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/2027862021-05-12T00:00:00Z
- 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
- Topological pumping assisted by Bloch oscillationshttps://scholarbank.nus.edu.sg/handle/10635/200719Title: Topological pumping assisted by Bloch oscillations
Authors: Ke, Yongguan; Hu, Shi; Zhu, Bo; Gong, Jiangbin; Kivshar, Yuri; Lee, Chaohong
Abstract: Adiabatic quantum pumping in one-dimensional lattices is extended by adding a
tilted potential to probe better topologically nontrivial bands. This extension
leads to almost perfectly quantized pumping for an arbitrary initial state
selected in a band of interest, including Bloch states. In this approach, the
time variable offers not only a synthetic dimension as in the case of the
Thouless pumping, but it assists also in the uniform sampling of all momenta
due to the Bloch oscillations induced by the tilt. The quantized drift of Bloch
oscillations is determined by a one-dimensional time integral of the Berry
curvature, being effectively an integer multiple of the topological Chern
number in the Thouless pumping. Our study offers a straightforward approach to
yield quantized pumping, and it is useful for probing topological phase
transitions.
Wed, 01 Jan 2020 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/2007192020-01-01T00:00:00Z
- Dissipative adiabatic measurements: Beating the quantum Cramér-Rao boundhttps://scholarbank.nus.edu.sg/handle/10635/200721Title: Dissipative adiabatic measurements: Beating the quantum Cramér-Rao bound
Authors: Zhang, Da-Jian; Gong, Jiangbin
Abstract: It is challenged only recently that the precision attainable in any
measurement of a physical parameter is fundamentally limited by the quantum
Cram\'{e}r-Rao Bound (QCRB). Here, targeting at measuring parameters in
strongly dissipative systems, we propose an innovative measurement scheme
called {\it dissipative adiabatic measurement} and theoretically show that it
can beat the QCRB. Unlike projective measurements, our measurement scheme,
though consuming more time, does not collapse the measured state and, more
importantly, yields the expectation value of an observable as its measurement
outcome, which is directly connected to the parameter of interest. Such a
direct connection {allows to extract} the value of the parameter from the
measurement outcomes in a straightforward manner, with no fundamental
limitation on precision in principle. Our findings not only provide a marked
insight into quantum metrology but also are highly useful in dissipative
quantum information processing.
Wed, 01 Jan 2020 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/2007212020-01-01T00:00:00Z
- Controlling the population imbalance of a Bose-Einstein condensate by a symmetry-breaking driving fieldhttps://scholarbank.nus.edu.sg/handle/10635/96100Title: Controlling the population imbalance of a Bose-Einstein condensate by a symmetry-breaking driving field
Authors: Morales-Molina, L.; Gong, J.
Abstract: Nonlinear Floquet states associated with a symmetry-breaking driving field are exploited to control the dynamics of a Bose-Einstein condensate in a double-well potential. The population imbalance between the two wells is shown to be controllable by slowly tuning system parameters along a closed path. The results extend symmetry-breaking-based quantum control to many-body systems on the mean-field level and extend navigation of linear Floquet states to navigation of nonlinear Floquet states. © 2008 The American Physical Society.
Mon, 27 Oct 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/961002008-10-27T00:00:00Z
- 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
- 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
- Wave-scattering formalism for thermal conductance in thin wires with surface disorderhttps://scholarbank.nus.edu.sg/handle/10635/98576Title: Wave-scattering formalism for thermal conductance in thin wires with surface disorder
Authors: Akguc, G.B.; Gong, J.
Abstract: Elastic wave characteristics of the heat conduction in low-temperature thin wires can be studied via a wave scattering formalism. A reaction matrix formulation of heat conductance modeled by elastic wave scattering is advocated. This formulation allows us to treat thin wires with arbitrary surface disorder. It is found that the correlation in the surface disorder may significantly affect the temperature dependence of the heat conductance. © 2009 The American Physical Society.
Fri, 06 Nov 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/985762009-11-06T00:00:00Z
- Simulation of chemical isomerization reaction dynamics on a NMR quantum simulatorhttps://scholarbank.nus.edu.sg/handle/10635/97929Title: Simulation of chemical isomerization reaction dynamics on a NMR quantum simulator
Authors: Lu, D.; Xu, N.; Xu, R.; Chen, H.; Gong, J.; Peng, X.; Du, J.
Abstract: Quantum simulation can beat current classical computers with minimally a few tens of qubits. Here we report an experimental demonstration that a small nuclear-magnetic-resonance quantum simulator is already able to simulate the dynamics of a prototype laser-driven isomerization reaction using engineered quantum control pulses. The experimental results agree well with classical simulations. We conclude that the quantum simulation of chemical reaction dynamics not computable on current classical computers is feasible in the near future. © 2011 American Physical Society.
Tue, 05 Jul 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/979292011-07-05T00:00:00Z
- Amplification and suppression of system-bath-correlation effects in an open many-body systemhttps://scholarbank.nus.edu.sg/handle/10635/95750Title: Amplification and suppression of system-bath-correlation effects in an open many-body system
Authors: Chaudhry, A.Z.; Gong, J.
Abstract: Understanding the rich dynamics of open quantum systems is of fundamental interest to quantum control and quantum information processing. By considering an open system of many identical two-level atoms interacting with a common bath, we show that the effects of system-bath correlations are amplified in a many-body system via the generation of a bath-dependent short time scale (inversely proportional to the number of atoms) in the system dynamics. The effects of system-bath correlations are therefore considerable even when each individual atom interacts with the bath weakly. We further show that the correlation-induced dynamical effects may still be suppressed via the dynamical decoupling approach, but they present a challenge for quantum state protection as the number of atom increases. © 2013 American Physical Society.
Wed, 30 Jan 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/957502013-01-30T00:00:00Z
- 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
- 2 × 2 random matrix ensembles with reduced symmetry: From Hermitian to PT-symmetric matriceshttps://scholarbank.nus.edu.sg/handle/10635/95593Title: 2 × 2 random matrix ensembles with reduced symmetry: From Hermitian to PT-symmetric matrices
Authors: Gong, J.; Wang, Q.-H.
Abstract: A possibly fruitful extension of conventional random matrix ensembles is proposed by imposing symmetry constraints on conventional Hermitian matrices or paritytime (PT )-symmetric matrices. To illustrate the main idea, we first study 2 × 2 complex Hermitian matrix ensembles with O(2)-invariant constraints, yielding novel level-spacing statistics such as singular distributions, the half-Gaussian distribution, distributions interpolating between the GOE (Gaussian orthogonal ensemble) distribution and half-Gaussian distributions, as well as the gapped-GOE distribution. Such a symmetry-reduction strategy is then used to explore 2 × 2 PT -symmetric matrix ensembles with real eigenvalues. In particular, PT -symmetric random matrix ensembles with U(2) invariance can be constructed, with the conventional complex Hermitian random matrix ensemble being a special case. In two examples of PT - symmetric random matrix ensembles, the level-spacing distributions are found to be the standard GUE (Gaussian unitary ensemble) statistics or the truncated- GUE statistics. © 2012 IOP Publishing Ltd.
Fri, 09 Nov 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/955932012-11-09T00:00:00Z
- 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
- Entanglement-induced decoherence and energy eigenstateshttps://scholarbank.nus.edu.sg/handle/10635/96497Title: Entanglement-induced decoherence and energy eigenstates
Authors: Wang, W.-G.; Gong, J.; Casati, G.; Li, B.
Abstract: Using recent results in the field of quantum chaos we derive explicit expressions for the time scale of decoherence induced by the system-environment entanglement. For a generic system-environment interaction and for a generic quantum chaotic system as environment, conditions are derived for energy eigenstates to be preferred states in the weak coupling regime. A simple model is introduced to numerically confirm our predictions. The results presented here may also help with understanding the dynamics of quantum entanglement generation in chaotic quantum systems. © 2008 The American Physical Society.
Tue, 22 Jan 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/964972008-01-22T00:00:00Z
- Reply to "comment on 'Wave-scattering formalism for thermal conductance in thin wires with surface disorder'"https://scholarbank.nus.edu.sg/handle/10635/97805Title: Reply to "comment on 'Wave-scattering formalism for thermal conductance in thin wires with surface disorder'"
Authors: Akguc, G.B.; Gong, J.
Abstract: We confirm the analysis given by Menezes in their comment. Hence, one heat conductance expression derived by us is actually equivalent to the one in a previous paper by Rego and Kirczenow. In addition, a rather detailed derivation of our heat conductance formula for a one-mode case is now given. © 2010 The American Physical Society.
Mon, 15 Mar 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/978052010-03-15T00:00:00Z
- Dynamical fluctuations in classical adiabatic processes: General description and their implicationshttps://scholarbank.nus.edu.sg/handle/10635/112421Title: Dynamical fluctuations in classical adiabatic processes: General description and their implications
Authors: Zhang, Q.; Gong, J.; Oh, C.H.
Abstract: Dynamical fluctuations in classical adiabatic processes are not considered by the conventional classical adiabatic theorem. In this work a general result is derived to describe the intrinsic dynamical fluctuations in classical adiabatic processes. Interesting implications of our general result are discussed via two subtopics, namely, an intriguing adiabatic geometric phase in a dynamical model with an adiabatically moving fixed-point solution, and the possible "pollution" to Hannay's angle or to other adiabatic phase objects for adiabatic processes involving non-fixed-point solutions. © 2011 Elsevier Inc..
Sun, 01 Apr 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1124212012-04-01T00:00:00Z
- 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