Dynamics of geometric discord and measurement-induced nonlocality at finite temperature

By using geometric measure of discord (GMOD) [B. Dakić, V. Vedral, Č. Brukner, Phys. Rev. Lett. 105, 109502 (2010)] and measurement-induced nonlocality (MIN) [S. Luo, S. Fu, Phys. Rev. Lett. 106, 120401 (2011)], we investigate quantum correlation of a pair of two-level systems, each of which is interacting with a reservoir at finite temperature T. We show that, for a broad class of states of the system, GMOD and MIN can endure sudden death, and there is no asymptotic decay for MIN while asymptotic decay exists for GMOD. We also give the dynamics of GMOD and MIN with respect to the temperature and illustrate their different characteristics.     

Dynamics of Measurement-Induced Non-Locality and Geometric Measure of Discord in a Two-Qubit Heisenberg XY Chain

By taking into account the Dzyaloshinsky-Moriya (DM) interaction under uniform magnetic field, quantum correlation behaviors measured by the measurement-induced nonlocality (MIN) and the geometric measure of discord (GMOD) in a two-qubit XY model are investigated in detail. Turning the different parameters can lead the two kinds of measurements to present different properties. For example, increasing the parameter B(uniform magnetic field), the existing region of MIN is larger than GMOD; MIN can appear the phenomenon of monotonous reduction when the parameter D(Dzyaloshinsky-Moriya interaction) is smaller than one threshold value, while GMOD cannot; MIN monotonously reduces with enhancive value of T(temperature), while GMOD initial experiences a slightly increasing and then decreases. One interesting point is that the more obvious and complicated difference between them are shown from the initial values. This property is both true for the zero temperature and the finite temperature. Through analyzing the limit case of the temperature approaching zero, the analytic solutions give the detailed reasons why have different effect on the initial values. Moreover, from the analytic solutions, we know the initial value of MIN is always larger than or equal to GMOD.     

Non-Markovianity and initial correlations for the generalized Lindblad master equation

Taking a two-level system interacting with a two-band environment as an example, we discuss the non-Markovianity and initial system-environment correlations for the generalized Lindblad master equation by using a recently introduced measure given by Breuer et al. Our results show that the initial states of the environment and initial system-environment correlations have strong effects on the non-Markovianity of the reduced system. Moreover, we also show that information and energy can flow in different directions, i.e., energy flows to the system while information flows from the system, which is quite different from previous results.     

Dynamics of entropic measurement-induced nonlocality in structured reservoirs

We propose the entropic measurement-induced nonlocality (MIN) as the maximal increment of von Neumann entropy induced by the locally non-disturbing measurement, and study its behaviors in both the independent and common structured reservoirs. We present schemes for preserving the MIN, and show that for certain initial states the MIN, including the quantum correlations, can even be enhanced by the common reservoir. Additionally, we also show that the different measures of MIN may give different qualitative characterizations of nonlocal properties, i.e., it is rather measure dependent than state dependent.     

Detecting nonclassical system-environment correlations by local operations

We develop a general strategy for the detection of nonclassical system-environment correlations in the initial states of an open quantum system. The method employs a dephasing map which operates locally on the open system and leads to an experimentally accessible witness for genuine quantum correlations, measuring the Hilbert-Schmidt distance between pairs of open system states. We further derive the expectation value of the witness for various random matrix ensembles modeling generic features of complex quantum systems. This expectation value is shown to be proportional to a measure for the quantum discord which reduces to the concurrence for pure initial states.     

Monogamy relations of nonclassical correlations for multi-qubit states

Nonclassical correlations have been found useful in many quantum information processing tasks, and various measures have been proposed to quantify these correlations. In this work, we mainly study one of nonclassical correlations, called measurement-induced nonlocality (MIN). First, we establish a close connection between this nonlocal effect and the Bell nonlocality for two-qubit states. Then, we derive a tight monogamy relation of MIN for any pure three-qubit state and provide an alternative way to obtain similar monogamy relations for other nonclassical correlation measures, including squared negativity, quantum discord, and geometric quantum discord. Finally, we find that the tight monogamy relation of MIN is violated by some mixed three-qubit states, however, a weaker monogamy relation of MIN for mixed states and even multi-qubit states is still obtained.     

Sufficient and necessary condition for zero quantum entropy rates under any coupling to the environment

We find the necessary and sufficient conditions for the entropy rate of the system to be zero under any system-environment Hamiltonian interaction. We call the class of system-environment states that satisfy this condition lazy states. They are a generalization of classically correlated states defined by quantum discord, but based on projective measurements of any rank. The concept of lazy states permits the construction of a protocol for detecting global quantum correlations using only local dynamical information. We show how quantum correlations to the environment provide bounds to the entropy rate, and how to estimate dissipation rates for general non-Markovian open quantum systems.     

Quantifying Quantumness with Sudden Birth or Death of Entanglement for Two-Qubits System

Quantumness of the correlations between two qubits, coupled to a cavity field whenever the system is open or closed, is investigated when sudden death (or birth) of the entanglement occurs. It is found that quantum correlation not exists only in the entangled state. It’s found that the dephasing parameters and the purity of the initial states play an important role in the dynamics behaviors of the quantum correlations, including entanglement. Quantumness of the correlations and entanglement, due to dephasing of the cavity, are damped until the same their values which are contained in their initial states.     

Creation of quantum correlations via the initial classical-mixed states

Using the pseudomode method, we theoretically analyze the creation of quantum correlations between two two-level dipole-dipole interacting atoms coupled with a common structured reservoir with different coupling strengths. Considering certain classes of initial separable-mixed states, we demonstrate that the sudden birth of atomic entanglement as well as the generation of stationary quantum correlations occur. Our results also suggest a possible way to control the occurrence time of entanglement sudden birth and the stationary value of quantum correlations by modifying the initial conditions of states, the dipole-dipole interaction, and the relative coupling strength. These results are helpful for the experimental engineering of entanglement and quantum correlations.     

Two-Qubit Local Fisher Information Correlation beyond Entanglement in a Nonlinear Generalized Cavity with an Intrinsic Decoherence

In this paper, we study a Hamiltonian system constituted by two coupled two-level atoms (qubits) interacting with a nonlinear generalized cavity field. The nonclassical two-qubit correlation dynamics are investigated using Bures distance entanglement and local quantum Fisher information under the influences of intrinsic decoherence and qubit–qubit interaction. The effects of the superposition of two identical generalized coherent states and the initial coherent field intensity on the generated two-qubit correlations are investigated. Entanglement of sudden death and sudden birth of the Bures distance entanglement as well as the sudden changes in local Fisher information are observed. We show that the robustness, against decoherence, of the generated two-qubit correlations can be controlled by qubit–qubit coupling and the initial coherent cavity states.     

Controlling Sudden Birth and Sudden Death of Entanglement at Finite Temperature

The decoherence and the decay of quantum entanglement due to both population relaxation and thermal effects are investigated for the two qubits initially prepared in the extended Werner-like state by solving the Lindblad form of the master equation, where each qubit is interacting with an independent reservoir at finite temperature T. Entanglement sudden death (ESD) and entanglement sudden birth (ESB) are observed during the evolution process. We analyze in detail the effects of the mixedness, the degree of entanglement of the initial states and finite temperature on the time of entanglement sudden death and entanglement sudden birth. We also obtain an analytic formula for the steady state concurrence that shows its dependence on both the system parameters, the decoherence rate and finite temperature. These results arising from the combination of extended Werner-like initial state and independent thermal reservoirs suggest an approach to control the maximum possible concurrence even after the purity and finite temperature induce sudden birth, death and revival.     

Influences of Initial States on Entanglement Dynamics of Two Central Spins in a Spin Environment

We investigate the entanglement dynamics of two electronic spins coupled to a bath of nuclear spins for two special cases, one is that two central spins both interact with a common bath, and the other is that one of two spins interacts with a bath. We consider three types of initial states with different correlations between the system and the bath, i.e., quantum correlation, classical correlation, and no-correlation. We show that the initial correlations (no matter quantum correlations or classical correlations) can effectively avoid the occurrence of entanglement sudden death. Irrespective of whether both two spins or only one of the two spins interacts with the bath, the system can gain more entanglement in the process of the time evolution for initial quantum correlations. In addition, we find that the effects of the distribution of coupling constants on entanglement dynamics crucially depend on the initial state of the spin bath.     

Quantum correlations dynamics of three-qubit states coupled to an XY spin chain: Role of coupling strengths

We investigate the prominent impacts of coupling strengths on the evolution of entanglement and quantum discord for a three-qubit system coupled to an XY spin-chain environment. In the case of a pure W state, more robust, even larger nonzero quantum correlations can be obtained by tailoring the coupling strengths between the qubits and the environment.For a mixed state consisting of the GHZ and W states, the dynamics of entanglement and quantum discord can characterize the critical point of quantum phase transition. Remarkably, a large nonzero quantum discord is generally retained, while the nonzero entanglement can only be obtained as the system-environment coupling satisfies certain conditions. We also find that the impact of each qubit's coupling strength on the quantum correlation dynamics strongly depends on the variation schemes of the system-environment couplings.     

Induced entanglement revival and entanglement protection in a two qubit system

A system of two initially entangled qubits interacting with a bosonic environment is considered. The interaction induces a loss of the initial entanglement of the two qubits, and for specific initial states it causes entanglement sudden death. An investigation of the modifications on the entanglement dynamics by a single pulse control field, performed in the two qubit system, shows that the control field can not only protect entangled states against sudden death but also induce a revival of entanglement in the two qubit system.     

Disentanglement Dynamics in Nonequilibrium Environments

We theoretically study the non-Markovian disentanglement dynamics of a two-qubit system coupled to nonequilibrium environments with nonstationary and non-Markovian random telegraph noise statistical properties. The reduced density matrix of the two-qubit system can be expressed as the Kraus representation in terms of the tensor products of the single qubit Kraus operators. We derive the relation between the entanglement and nonlocality of the two-qubit system which are both closely associated with the decoherence function. We identify the threshold values of the decoherence function to ensure the existences of the concurrence and nonlocal quantum correlations for an arbitrary evolution time when the two-qubit system is initially prepared in the composite Bell states and the Werner states, respectively. It is shown that the environmental nonequilibrium feature can suppress the disentanglement dynamics and reduce the entanglement revivals in non-Markovian dynamics regime. In addition, the environmental nonequilibrium feature can enhance the nonlocality of the two-qubit system. Moreover, the entanglement sudden death and rebirth phenomena and the transition between quantum and classical nonlocalities closely depend on the parameters of the initial states and the environmental parameters in nonequilibrium environments.     

A Comparison of Quantum Discord and Entanglement in a Micromaser-Type System

The correlations dynamics of a pair of two-level atoms going though a cavity one after another are investigated. It is shown that initial entanglement of two atoms has interesting subsequent time evolution, including the so-called entanglement sudden death as expected, and initial uncorrelated states become entanglement and nonzero quantum discord in the same time. In particular, it is found that the curves of time evolution of entanglement and quantum discord are nearly same profile. It may lead to a better understanding of relations between entanglement and quantum discord.     

Sudden birth of entanglement between two atoms in a double JC model

Sudden birth of entanglement between two initially separate atoms interacting with two entangled photons in a double JC model is investigated, and the influences of different atomic initial states on entanglement among atoms are discussed. The results show that sudden birth of entanglement can occur when the two atoms are initially in excited states.     

Tripartite states Bell-nonlocality sudden death in a quantum-critical environment

We demonstrate that multipartite Bell-inequalities violations can be fully destroyed in finite time in three-qubit states under a quantum-critical environment, which is an Ising model in a transverse field. We use the Mermin--Ardehali--Belinksii--Klyshko (MABK) inequality to detect the degree of nonlocality as measured by the extent of their violations. The effects of system-environment couplings, the size of degrees of freedom of environment and the strength of transverse field on the Bell-inequality violations are given for two different initial states, namely, the W class and GHZ class states. The results indicate that the Bell-inequality violations of the tripartite states will be completely destroyed by the decoherence under certain conditions.     

Quantum Entanglement in a Structured Reservoir

We study the entanglement dynamics of two-level atoms interacting in independent structured non-Markovian reservoirs. The atoms are initially prepared in a pure W state and the reservoir is in the vacuum. We have shown that the degree of entanglement of the initial states, Markovian environments can control the time of the two qubit entanglement sudden death and the reservoirs’ entanglement sudden birth. We show the dependence of entanglement dynamics on the non-Markovianity. When the initial state of atoms is in a pure W state, the entanglement will decay asymptotically.     

Dissipative Dynamics of Quantum Discord of Two Strongly Driven Qubits

The exact dynamics of quantum discord (QD) of two strongly driven qubits, which are initially prepared in the X-type quantum states and inserted in two independent dissipative cavities or in a common dissipative cavity, are studied. The results indicate that both in the two cases, the evolution of QD is independent of the initial cavity state. For the two independent dissipative cavities, it is found that the phenomenon of sudden transition between classical and quantum decoherence exists and the transition time can be greatly delayed by suitably choosing the initial state parameter of the two qubits, the cavity mode-driving field detunning and the decay rate of the cavity. For the common dissipative cavity, it is shown that for some initial states of the two qubits, the QD can increase for a finite time at first, and then it decreases to a steady value, while for some other initial states, the QD can increase monotonously or with oscillation till a stable value is reached. Moreover, the creation of QD for the two qubits in a common cavity is discussed.