Spin-chain Environment Research Articles

Quantum speed limit and a signal of quantum criticality.

We study the quantum speed limit time (QSLT) of a coupled system consisting of a central spin and its surrounding environment, and the environment is described by a general XY spin-chain model. For initial pure state, we find that the local anomalous enhancement of the QSLT occurs near the critical point. In addition, we investigate the QSLT for arbitrary time-evolution state in the whole dynamics process and find that the QSLT will decay monotonously and rapidly at a large size of environment near the quantum critical point. These anomalous behaviors in the critical vicinity of XY spin-chain environment can be used to indicate the quantum phase transition point. Especially for the XX spin-chain environment, we find that the QSLT displays a sudden transition from discontinuous segmented values to a steady value at the critical point. In this case, the non-Makovianity and the Loschmidt echo are incapable of signaling the critical value of the transverse field, while the QSLT can still witness the quantum phase transition. So, the QSLT provides a further insight and sharper identification of quantum criticality.

Dynamical decoupling assisted acceleration of two-spin evolution in XY spin-chain environment

We study the speed-up role of dynamical decoupling in an open system, which is modeled as two central spins coupled to their own XY spin-chain environment. We show that the fast bang–bang pulses can suppress the system evolution, which manifests the quantum Zeno effect. In contrast, with the increasing of the pulse interval time, the bang–bang pulses can enhance the decay of the quantum speed limit time and induce the speed-up process, which displays the quantum anti-Zeno effect. In addition, we show that the random pulses can also induce the speed-up of quantum evolution.

One-norm geometric quantum discord of two-qubit state in spin chain environment at finite temperature

We study the dynamics of quantum correlations measured by quantum discord (QD) and one-norm geometric quantum discord (GQD) in a two-qubit system coupled to an XY spin chain with finite temperature. It is shown that one-norm GQD is more robust than QD against the decoherence induced by the spin chain with finite temperature, and one-norm GQD is superior to QD in detecting the critical point of quantum phase transition (QPT) of the spin chain. Considering the effects of the state parameters, the temperature of the spin chain and the asymmetric coupling parameter, we analyze the phenomenons of sudden transition and double sudden transitions occurred in the evolutions of QD and one-norm GQD. Besides, the influences of the Dzyaloshinsky–Moriya (DM) interaction and the three-site interaction in the spin chain are also taken into account.

Non-Markovianity of the Heisenberg XY spin environment with Dzyaloshinskii—Moriya interaction

Using the effective non-Markovian measure proposed by Breuer et al. recently, we study the memory effect of a central qubit system coupled to a spin chain environment with Dzyaloshinskii—Moriya interaction in a transverse field. It is discovered that the central qubit system presents different memory effects in different environment phases with the different oscillatory behaviors of the decoherence factor. Moreover, it is revealed that the Dzyaloshinskii—Moriya interaction has a prominent influence on the memory effect of a central qubit system via modifying the amplitude and period of the decoherence factor under certain conditions.

Sudden transitions and scaling behavior of geometric quantum correlation for two qubits in quantum critical environments at finite temperature

We investigate the phenomenon of sudden transitions in geometric quantum correlation of two qubits in spin chain environments at finite temperature. It is shown that when only one qubit is coupled to the spin environment, the geometric discord exhibits a double sudden transition behavior, which is closely related to the quantum criticality of the spin chain environment. When two qubits are uniformly coupled to a common spin chain environment, the geometric discord is found to display a sudden transition behavior whereby the system transits from pure classical decoherence to pure quantum decoherence. Moreover, an interesting scaling behavior is revealed for the frozen time, and we also present a scheme to prolong the time during which the discord remains constant by applying bang–bang pulses.

Dzyaloshinsky-Moriya作用影响下的qubit-qutrit系统关联动力学

本文分析了与各自自旋链环境相互耦合的qubit和qutrit中的negativity和几何量子失协动力学,讨论了系统和环境间的耦合强度g、各向异性参数γ、粒子数N、初始参数p以及Dzyaloshinsky-Moriya相互作用对系统演化的影响,得到一些重要结论。另外发现,依赖于各参数的选取,negativity不总是大于几何量子失协,且几何量子失协存在突然改变点。

Correlation dynamics of a qubit—qutrit system in a spin-chain environment with Dzyaloshinsky—Moriya interaction

We study the dynamics of correlations for a hybrid qubit—qutrit system in an XY spin-chain environment with Dzyaloshinsky—Moriya interaction. Our discussion involves a comparative analysis of negativity, quantum discord, and measurement-induced disturbance. It is found that the quantum discord is optimal of the three quantum correlations to detect the critical point of quantum phase transition. Only when the qubit interacts with the environment, is the phenomenon of sudden transition between the classical correlation and the quantum discord observed. Moreover, the Dzyaloshinsky—Moriya interaction enhances the decay of quantum correlations.

Quantum Discord for a Central Two-Qubit System Coupled to an XY Spin Chain with Energy Current

We investigate the quantum discord dynamics of a two-qubit system coupled to an XY spin-chain environment with energy current. We compared dynamical behaviors of quantum discord under different system-environment couplings, the size of the degrees of freedom of the environment, the anisotropy parameter, and the energy current. The results indicate that the energy current can strongly suppress the quantum discord in the weak-coupling region, while it has no obvious effect on the quantum discord in the strong-coupling regime.

Induced modification of the geometric phase of a qubit coupled to an XY spin chain by Dzyaloshinsky—Moriya interaction

We consider a qubit symmetrically and transversely coupled to an XY spin chain with Dzyaloshinsky—Moriya (DM) interaction in the presence of a transverse magnetic field. An analytical expression for the geometric phase of the qubit is obtained in the weak coupling limit. We find that the modification of the geometrical phase induced by the spin chain environment is greatly enhanced by DM interaction in the weak coupling limit around the quantum phase transition point of the spin chain.

Dynamics of correlations and scaling behaviors in a spin-chain environment

In this study, we investigate the dynamics of classical and quantum correlations for a two-qubit system in an XY spin-chain environment. Moreover, we focus on two coupling cases. We find that the quantum discord (QD) decay rates for the two cases differ in sudden change behavior and strongly depend on environmental parameters. An interesting scaling behavior for the QD is given under the weak-coupling condition. In the strong-coupling condition, the classical correlation (CC) and the QD oscillate quickly with time, but present regular distributions under a specific time interval. Moreover, the sudden change for the QD is presented by choosing the proper initial time and time interval. Finally, we find that the decay parts of the envelopes for the CC and QD satisfy a Gauss-related feature.

Fermionic stochastic Schrödinger equation and master equation: An open-system model

This paper considers the extension of the non-Markovian stochastic approach for quantum open systems strongly coupled to a fermionic bath, to the models in which the system operators commute with the fermion bath. This technique can also be a useful tool for studying open quantum systems coupled to a spin-chain environment, which can be further transformed into an effective fermionic bath. We derive an exact stochastic Schr\"{o}dinger equation (SSE), called fermionic quantum state diffusion (QSD) equation, from the first principle by using the fermionic coherent state representation. The reduced density operator for the open system can be recovered from the average of the solutions to the QSD equation over the Grassmann-type noise. By employing the exact fermionic QSD equation, we can derive the corresponding exact master equation. The power of our approach is illustrated by the applications of our stochastic approach to several models of interest including the one-qubit dissipative model, the coupled two-qubit dissipative model, the quantum Brownian motion model and the N-fermion model coupled to a fermionic bath. Different effects caused by the fermionic and bosonic baths on the dynamics of open systems are also discussed.

Discord under the influence of a quantum phase transition

This paper studies the discord of a bipartite two-level system coupling to an XY spin-chain environment in a transverse field and investigates the relationship between the discord property and the environment's quantum phase transition. The results show that the quantum discord is also able to characterize the quantum phase transitions. We also discuss the difference between discord and entanglement, and show that quantum discord may reveal more general information than quantum entanglement for characterizing the environment's quantum phase transition.

Quantum discord for a central two-qubit system coupled to anXY-spin-chain environment

We investigate the dynamic behaviors of quantum discord for a central two-qubit system coupled to an $\mathit{XY}$-spin-chain environment. In the weak-coupling regime, we show that the quantum discord for the two central qubits can become minimized rapidly close to the critical point of a quantum phase transition. By considering the two qubits that are initially prepared in the Werner state, we study the evolution of the quantum discord and that of entanglement under the same conditions. Our results imply that entanglement can disappear completely after a finite time, while the quantum discord decreases and tends to be a stable value according to the initial-state parameter for a very-long-time interval. In this sense, the quantum discord is more robust than entanglement for the quantum system exposed to the environment. The relation between the quantum correlations and the classical correlation is also shown for two particular cases.

Disentanglement dynamics of interacting two qubits and two qutrits in an XY spin-chain environment with the Dzyaloshinsky–Moriya interaction

We investigate disentanglement dynamics of two coupled qubits and qutrits which interact uniformly to a general XY spin-chain environment with the Dzyaloshinsky–Moriya (DM) interaction. We obtained exact expression of the time evolution operator and analyzed the dynamical process of the decoherence factors. Through explicitly calculating the concurrence and the negativity, we examined disentanglement behaviors of two coupled central spins evolve from different initial pure states, which are found to be nontrivially different from those of the uncorrelated ones, in particular, the enhanced decay of the entanglement induced by quantum criticality of the surrounding environment may be broken by introducing self-Hamiltonian of the central spin system. Moreover, the DM interaction may have different influences on decay of the entanglement depending on the strength of the system–environment coupling, the anisotropy of the environmental spin chain and the intensity of the transverse magnetic field, as well as the explicit form of the initial states of the central spin system.

Zeno and anti-Zeno effects in the presence of spin-chain environment

We investigate the quantum Zeno and anti-Zeno effects in a two-state system coupling to a spin chain environment in transverse magnetic fields. The effective decay rate affected by the environment has been calculated and analyzed. The results show that the Zeno-anti-Zeno transitions can occur in such system, which has a connection with the quantum phase transition in the environment.

Entanglement dynamics of three-qubit states under an XY spin-chain environment

The entanglement dynamics of three-qubit states under a general XY spin-chain environment which can exhibit a quantum phase transition is investigated by using negativity as entanglement measure. Our results imply that the entanglement evolution depends not only on the states of concern but also on the system-environment coupling, the anisotropy parameter, the size of the environment, and the strength of the external field applied to the environment. For the cases under study, we find that the entanglement decay is enhanced by quantum phase transition under weak coupling. The conditions to identify quantum decoherence-free subspaces have been discussed.

Disentanglement of two qubits coupled to anXYspin chain: Role of quantum phase transition

We study the disentanglement evolution of two spin qubits which interact with a general $XY$ spin-chain environment. The dynamical process of the disentanglement is numerically and analytically investigated in the vicinity of a quantum phase transition (QPT) of the spin chain in both weak and strong coupling cases. We find that the disentanglement of the two spin qubits may be greatly enhanced by the quantum critical behavior of the environmental spin chain. We give a detailed analysis to facilitate the understanding of the QPT-enhanced decaying behavior of the coherence factor. Furthermore, the scaling behavior in the disentanglement dynamics is also revealed and analyzed.

Decoherence Induced by Spin Chain Environment

We study the decoherence process of an exact solvable modelthat consists of a central spin-1/2 coupling to the surrounding anisotropyspin-1/2 chain in transverse fields. The Loschmidt echo is calculated tostudy the character of decoherence with different degree of anisotropy.Our results show that the degree of anisotropy γ greatly affectsthe decoherence process of the central spin-system when the spin chainis in weak transverse fields, but it gives weak effect in the strongtransverse field. The decoherence process of the central system changeddramatically along the line of the critical points, and this may beexplained as the reflection of quantum phase transitions.