arXiv subjects

X. X. Yi

Publications and source records attributed to X. X. Yi.

At least 55 records · Page 3Linked to original sources

arXiv2021

Open dynamics in the Aubry-André-Harper model coupled to a finite bath: the influence of localization in the system and dimensionality of bath

The population evolution of single excitation is studied in the Aubry- André- Harper (AAH) model coupled to a $d (=1,2,3)$-dimensional simple lattices bath with a focus on the effect of localization in the system and the dimensionality of bath. By performing a precise evaluation of time-independent Schrödinger equation, the reduced energy levels of the system can be determined. It is found that the reduce energy levels show significant relevance for the bath dimensions. Subsequently, the time evolution of excitation is studied in both the system and bath. It is found that excitation in the system can decay super-exponentially when $d=1$ or exponentially when $d=2,3$. Regarding the finite nature of bath, the spreading of excitation in the lattices bath is also studied. We find that, depending on the dimensions of bath and the initial state, the spreading of excitation in the bath is diffusive or behaves localization.

quant-ph↗

arXiv2021

Two-level systems with periodic $N$-step driving fields: Exact dynamics and quantum state manipulations

In this work, we derive exact solutions of a dynamical equation, which can represent all two-level Hermitian systems driven by periodic $N$-step driving fields. For different physical parameters, this dynamical equation displays various phenomena for periodic $N$-step driven systems. The time-dependent transition probability can be expressed by a general formula that consists of cosine functions with discrete frequencies, and, remarkably, this formula is suitable for arbitrary parameter regimes. Moreover, only a few cosine functions (i.e., one to three main frequencies) are sufficient to describe the actual dynamics of the periodic $N$-step driven system. {Furthermore}, we find that a beating in the transition probability emerges when two (or three) main frequencies are similar. Some applications are also demonstrated in quantum state manipulations by periodic $N$-step driving fields.

quant-ph↗

arXiv2021

Robust single-qubit gates by composite pulses in three-level systems

Composite pulses are an efficient tool for robust quantum control. In this work, we derive the form of the composite pulse sequence to implement robust single-qubit gates in a three-level system, where two low-energy levels act as a qubit. The composite pulses can efficiently cancel the systematic errors up to a certain order. We find that the three-pulse sequence cannot completely eliminate the first order of systematic errors, but still availably makes the fidelity resistant to variations in a specific direction. When employing more pulses in the sequence ($N>3$), the fidelity can be insensitive to the variations in all directions and the robustness region becomes much wider. Finally we demonstrate the applications of composite pulses in quantum information processing, e.g., robust quantum information transfer between two qubits.

quant-ph↗

arXiv2021

Hall conductance of a non-Hermitian two-band system with k-dependent decay rates

Two-band model works well for Hall effect in topological insulators. It turns out to be non-Hermitian when the system is subjected to environments, and its topology characterized by Chern numbers has received extensive studies in the past decades. However, how a non-Hermitian system responses to an electric field and what is the connection of the response to the Chern number defined via the non-Hermitian Hamiltonian remain barely explored. In this paper, focusing on a k-dependent decay rate, we address this issue by studying the response of such a non-Hermitian Chern insulator to an external electric field. To this aim, we first derive an effective non-Hermitian Hamiltonian to describe the system and give a specific form of k-dependent decay rate. Then we calculate the response of the non-Hermitian system to a constant electric field. We observe that the environment leads the Hall conductance to be a weighted integration of curvature of the ground band and hence the conductance is no longer quantized in general. And the environment induces a delay in the response of the system to the electric field. A discussion on the validity of the non-Hermitian model compared with the master equation description is also presented.

cond-mat.mes-hall↗

arXiv2020

Dynamics of discrete-time quantum walk with time-correlated unitary noise

We investigate the dynamics of discrete-time quantum walk subject to time correlated noise. Noise is described as an unitary coin-type operator before each step, and attention is focused on the noise generated by a Gaussian Ornstein Uhlenbeck process, going beyond the usual telegraph noise, where the random variables are consist of only -1 and 1. Under the first-order approximation of BCH formula, the master equation of noisy discrete-time quantum walk is derived. The dynamics given by the master equation are in good agreement with those given by numerical simulations within a certain period of steps, which is controlled by noise parameters. Two remarker behaviors of long time noisy dynamics are observed in numerical simulations, corresponding to two opposite noise regimes: in slow noise regime, with the increase of the noise amplitude, the quantum coherence is suppressed, and the dynamics of noisy discrete-time quantum walk gradually transits to that of classical random walk. In fast noise regime, the walker is confined into few lattice sites, and the width of wave packet is much narrower compared with that in slow noise regime.

quant-ph↗

arXiv2020

Nonlinear Dirac Points and topological invariant in coupled hexagonal lattices

Topological phases and materials have attracted much attention in recent years. Though many progress has been made, the effect of nonlinearity on such system remains untouched. In this paper, by considering the mean-field approximation in a coupled boson-hexagonal lattice system, we obtain a different type of Dirac cone. Due to its special structure of the cone, the Berry phase (two-dimensional Zak phase) of this new Dirac cone is quantized differently, i.e., it has been modified due to the interactions and the critical line between different topological phases has moved, depending on the type of interactions. Furthermore, the new Berry phase is found to be quantized, offering a possible nonlinear topological invariant which can supply as a criterion of topological classification for that interacting system. The quantum phase transition in terms of this criterion in the system has also been examined.

quant-ph↗

arXiv2020

Quantum $φ$-synchronization in coupled optomechanical system with periodic modulation

Based on the concepts of quantum synchronization and quantum phase synchronization proposed by A. Mari \textit{et al.} in Phys. Rev. Lett. 111, 103605 (2013), we introduce and characterize the measure of a more generalized quantum synchronization called quantum $φ$-synchronization under which the pairs of variables have the same amplitude and possess the same $φ$ phase shift. Naturally, quantum synchronization and quantum anti-synchronization become special cases of quantum $φ$-synchronization. Their relations and differences are also discussed. To illustrate these theories, we investigate the quantum $φ$-synchronization and quantum phase synchronization phenomena of two coupled optomechanical systems with periodic modulation and show that quantum $φ$-synchronization is more general as a measure of synchronization. We also show the phenomenon of quantum anti-synchronization when $φ=π$.

quant-ph↗

arXiv2020

Discrete time-crystalline order in Bose-Hubbard model with dissipation

Periodically driven quantum systems manifest various non-equilibrium features which are absent at equilibrium. For example, discrete time-translation symmetry can be broken in periodically driven quantum systems leading to an exotic phase of matter, called discrete time crystal(DTC). For open quantum systems, previous studies showed that DTC can be found only when there exists a meta-stable state in the undriven system. However, by investigating the simplest Bose-Hubbard model with dissipation and time periodically tunneling, we find in this paper that a $2T$ DTC can appear even when the meta-stable state is absent in the undriven system. This observation extends the understanding of DTC and shed more light on the physics behind the DTC. Besides, by the detailed analysis of simplest two-sites model, we show further that the two-sites model can be used as basic building blocks to construct large rings in which a $nT$ DTC might appear. These results might find applications into engineering exotic phases in driven open quantum systems.

quant-ph↗

arXiv2020

Edge state, bound state and anomalous dynamics in the Aubry-André-Haper system coupled to non-Markovian baths

Bound states and their influence on the dynamics of an one-dimensional tight-binding system subject to environments are studied in this paper. We identify specifically three kinds of bound states. The first is a discrete bound state (DBS), of which the energy level exhibits a gap from the continuum. The DBS exhibits the similar features of localization as the edge states in the system and thus can suppress the decay of system. The second is a bound state in the continuum (BIC), which can suppress the system decay too. It is found that the BIC is intimately connected to the edge mode of the system since both of them show almost the same features of localization and energy. The third one displays a large gap from the continuum and behaves extendible (not localized). Moreover the population of the system on this state decays partly but not all of them does. This is different from the two former bound states. The time evolution of a single excitation in the system is studied in order to illustrate the influence of the bound states. We found that both DBS and BIC play an important role in the time evolution, for example, the excitation becomes localized and not decay depending on the overlap between the initial state and the DBS or BIC. Furthermore we observe that the single excitation takes a long-range hopping when the system falls into the regime of strong localizations. This feature can be understood as the interplay of system localizations and the bath-induced long-range correlation.

quant-ph↗

arXiv2019

Quantum metrology enhanced by coherence-induced-driving in a cavity QED setup

We propose a quantum metrology scheme in a cavity QED setup to achieve the Heisenberg limit. In our scheme, a series of identical two-level atoms randomly pass through and interact with a dissipative single-mode cavity. Different from the entanglement based Heisenberg limit metrology scheme, we do not need to prepare the atomic entangled states before they enter into the cavity. We show that the initial atomic coherence will induce an effective driving to the cavity field, whose steady state is an incoherent superposition of orthogonal states, with the superposition probabilities being dependent on the atom-cavity coupling strength. By measuring the average photon number of the cavity in the steady state, we demonstrate that the root-mean-square of the fluctuation of the atom-cavity coupling strength is proportional to $1/N_c^2$ ($N_c$ is the effective atom number interacting with the photon in the cavity during its lifetime). It implies that we have achieved the Heisenberg limit in our quantum metrology process. We also discuss the experimental feasibility of our theoretical proposal. Our findings may find potential applications in quantum metrology technology.

quant-ph↗

arXiv2019

Topological Phase Transition of A Non-Hermitian Crosslinked Chain

Non-Hermiticity enriches the contents of topological classification of matter including exceptional points, bulk-edge correspondence and skin effect. Gain and loss can be described by imaginary diagonal elements in Hamiltonians and the topological phase transition for a crosslinked chain in the presence of such non-Hermiticity is investigated in this work. We obtain the phase diagram in term of a winding number analytically. The boundaries of the phases coincide with the surfaces of exceptional points in the parameter space. The topologically original edge states locating mainly at the joints between domains of different phases hold on even for the long chain. The non-Hermitian topological feature can also be reflected by vortex structures in the vector fields of complex eigenenergies and expected values of Pauli matrices or the trajectories of these quantities. This model can be implemented in coupled waveguides or photonic crystals. And the edge states are immune to various kinds of disorders until the topological phase transition occurs. This work benefits our insight into the influence of gain and loss on the topological phase of matter.

cond-mat.mes-hall↗

arXiv2018

Quantum state engineering by periodical two-step modulation in atomic system

By periodical two-step modulation, we demonstrate that the dynamics of multilevel system can still evolve even in multiple large detunings regime, and provide the effective Hamiltonian (of interest) for this system. We then illustrate this periodical modulation in quantum state engineering, including achieving direct transition from the ground state to the Rydberg state or the desired superposition of two Rydberg states without satisfying two-photon resonance condition, switching between Rydberg blockade regime and Rydberg antiblockade regime, stimulating distinct atomic transitions by the same laser field, and implementing selective transitions in the same multilevel system. Particularly, it is robust against perturbation of control parameters. Another advantage is that the waveform of laser field has simple square-wave form which is readily implemented in experiments. Thus, it offers us a novel method of quantum state engineering in quantum information processing.

quant-ph↗

arXiv2018

Single photon transmission in strong three-mode optomechanical circulatory system

In the few-photon regime, we theoretically propose a feasible scheme to realize strong coupling optomechanical cycle in a three-mode optomechanical circulatory system (OMCS) comprising of cross-Kerr (CK) type and linear coupling between the corresponding two bosonic modes, meanwhile, where one of the bosonic modes is strongly coherently driven and the rest are weakly driven. A Langevin equation for strong three-mode OMCS is derived by adjusting the parameters of the appropriate strong coherent laser. We show that this strong coupling of optomechanical originates from the strong coherently driven the CK coupling modes, thereby, in coherent state displacement representation enhance the optomechanical coupling and present the strengthen of the nonolinearity. We obtain a set of optimal parameters by full numerical simulation of dynamics matrix, under this condition, minimal strong OMCS that exhibit a single photon perfect non-reciprocal transmission between the two optical modes, and unidirectional transportation inter the optomechanical modes. For a detection field, the system presents optomechanical induced transparency (OMIT) behavior. The presented results can be widely applied to quantum devices such as circulators, diodes, and transistors. Moreover, we brief analysis that a system which consisting of dual mechanical modes and one optical mode, further extend the system to a generalized coupled one-dimensional optomechanical array and arbitrary modes optomechanical loop network. Experimentally, the system has been demonstrated to be achieved through a variety of experimental protocols and configurations, for example optomechanical crystal and integrated quantum superconducting circuit.

quant-ph↗

arXiv2018

Quantum synchronization of two mechanical oscillators in coupled optomechanical systems with Kerr nonlinearity

We investigate the quantum synchronization phenomena of two mechanical oscillators of different frequencies in two optomechanical systems under periodically modulating cavity detunings or driving amplitudes, which can interact mutually through an optical fiber or a phonon tunneling. The cavities are filled with Kerr-type nonlinear medium. It is found that, no matter which the coupling and periodically modulation we choose, both of the quantum synchronization of nonlinear optomechanical system are more appealing than the linear optomechanical system. It is easier to observe greatly enhanced quantum synchronization with Kerr nonlinearity. In addition, the different influences on the quantum synchronization between the two coupling ways and the two modulating ways are compared and discussed.

quant-ph↗

arXiv2018

Quantum Lyapunov control with machine learning

Quantum state engineering is a central task in Lyapunov-based quantum control. Given different initial states, better performance may be achieved if the control parameters, such as the Lyapunov function, are individually optimized for each initial state, however, at the expense of computing resources. To tackle this issue, we propose an initial-state-adaptive Lyapunov control strategy with machine learning, specifically, artificial neural networks trained through supervised learning. Two designs are presented and illustrated where the feedforward neural network and the general regression neural network are used to select control schemes and design Lyapunov functions, respectively. Since the sample generation and the training of neural networks are carried out in advance, the initial-state-adaptive Lyapunov control can be implemented without much increase of computational resources.

math.OC↗

arXiv2018

Noise-induced distributed entanglement in atom-cavity-fiber system

The distributed quantum computation plays an important role in large-scale quantum information processing. In the atom-cavity-fiber system, we put forward two efficient proposals to prepare the steady entanglement of two distant atoms with dissipation. The atomic spontaneous emission and the loss of fiber are exploited actively as powerful resources, while the effect of cavity decay is inhibited by quantum Zeno dynamics and quantum-jump-based feedback control. These proposals do not require precisely tailored Rabi frequencies or coupling strength between cavity and fiber. Furthermore, we discuss the feasibility of extending the present schemes into the systems consisting of two atoms at the opposite ends of the $n$ cavities connected by $(n-1)$ fibers, and the corresponding numerical simulation reveals that a high fidelity remains achievable with current experimental parameters.

quant-ph↗

arXiv2018

Engineering steady Knill-Laflamme-Milburn state of Rydberg atoms by dissipation

The Knill-Laflamme-Milburn (KLM) states have been proved to be a useful resource for quantum information processing [Nature 409, 46 (2001)]. For atomic KLM states, several schemes have been put forward based on the time-dependent unitary dynamics, but the dissipative generation of these states has not been reported. This work discusses the possibility for creating different forms of bipartite KLM states in neutral atom system, where the spontaneous emission of excited Rydberg states, combined with the Rydberg antiblockade mechanism, is actively exploited to engineer a steady KLM state from an arbitrary initial state. The numerical simulation of the master equation signifies that a fidelity above 99\% is available with the current experimental parameters.

quant-ph↗

arXiv2018

Engineering steady-state entanglement via dissipation and quantum Zeno dynamics in optical cavity

A new mechanism is proposed for dissipatively preparing maximal Bell entangled state of two atoms in an optical cavity. This scheme integrates the spontaneous emission, the light shift of atoms in the presence of dispersive microwave field, and the quantum Zeno dynamics induced by continuous coupling, to obtain a unique steady state irrespective of initial state. Even for a large cavity decay, a high-fidelity entangled state is achievable at a short convergence time, since the occupation of cavity mode is inhibited by the Zeno requirement. Therefore, a low single-atom cooperativity $C=g^2/(κγ)$ is good enough for realizing a high fidelity of entanglement in a wide range of decoherence parameters. As a straightforward extension, the feasibility for preparation of two-atom Knill-Laflamme-Milburn state with the same mechanism is also discussed.

quant-ph↗