Entanglement Evolution of Two-Qubit States in the Presence of Local Decoherence

Using the factorization Law for entanglement decay derived by Konrad, T., et al.: Nature Phys. 4, 99 (2008), we investigate the one-side and two-side local decoherence effect on the dynamics of the two-qubit entangled states. Under the depolarizing channel and GAD channel with the nonzero temperature, we have found that any two-qubit states will experience ESD. Furthermore, we also investigate the entanglement dynamics under the non-Markovian case.     

Quantum discord dynamics of two-qubit system in a quantum spin environment

We study the dynamics of quantum discord of two-qubit system in a quantum spin environment at finite temperature in the thermodynamics limit. Special attention is paid to the difference between the entanglement and quantum discord when considering the influences of the environment temperature and the initial system states. We show that in the same range of the physical parameters, when the system states behave no entanglement or entanglement sudden death, the quantum discord keeps nonzero. So the quantum discord is more robust than entanglement under this decoherence environment. Furthermore, we also illustrate that we can tune the parameters related to the system and the environment to suppress the decay of quantum discord.     

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.     

Classical Correlation and Quantum Discord in a Two Qubit System Under Dissipation Environments

We study the dynamics of classical correlation and quantum discord of two-qubit system interacted with the thermal reservoir. Special attention is paid to the difference between the entanglement and quantum discord when considering the influences of the nonzero mean photon number and quantum fluctuation of the vacuum. It is shown that in the same range of the physical parameters, the factors leading to the entanglement sudden death only accelerates the decay of quantum discord, and the quantum discord can last for an infinite period when the entanglement disappears. So the quantum discord is more robust than entanglement under this decoherence environment, and quantum discord is a more general measure of quantum correlation than entanglement.     

Enlarged omnidirectional photonic gap in one dimensional ternary plasma photonic crystals that contain metamaterials

We study the dynamics of multipartite quantum correlations measured by the lower bound of concurrence and quantum discord in a three-qubit system coupled to an XY spin chain. For the initial pure GHZ and W state, we find the lower bound of entanglement is more robust than the quantum discord against the decoherence induced by the spin environment. But for the Werner state, the sudden death of discord is not observed even in the presence of entanglement sudden death. By comparing the evolutions for the GHZ and W states, we show that the W state preserves more quantum correlations than the GHZ state. In addition, we put research emphasis on the relation between the dynamics of multipartite quantum correlations and the quantum phase transition of the spin environment.     

Quantum Correlations of Two Relativistic Spin-12 Particles Under Noisy Channels

We study the quantum correlation dynamics of bipartite spin-\(\frac {1}{2}\) density matrices for two particles under Wigner rotations induced by Lorentz transformations which is transmitted through noisy channels. We compare quantum entanglement, geometric discord(GD), and quantum discord (QD) for bipartite relativistic spin-\(\frac {1}{2}\) states under noisy channels. We find out QD and GD tend to death asymptotically but a sudden change in the decay rate of the entanglement occurs under noisy channels. Also, bipartite relativistic spin density matrices are considered as a quantum channel for teleportation one-qubit state under the influence of depolarizing noise and compare fidelity for various velocities of observers.     

Dynamics of super-quantum discord and direct control with weak measurement in open quantum system

Super-quantum discord(SQD) with weak measurement is regarded as a kind of quantum correlation in quantum information processing. We compare and analyze the dynamical evolutions of SQD, quantum discord(QD), and quantum entanglement(QE) between two qubits in the correlated dephasing environmental model. The results indicate that(i) owing to the much smaller influence of weak measurement on the coherence of the system than that of von Neumann projection measurement, SQD with weak measurement is larger than QD, and(ii) dynamical evolution of QD or QE monotonically goes to zero with time, while SQD monotonically tends to a stable value and a freezing phenomenon occurs. The stable value after freezing mainly depends on the measurement strength and the purity of the initial quantum state.     

Quantum Correlation in Circuit QED Under Various Dissipative Modes

Dynamical evolutions of quantum correlations in circuit quantum electrodynamics (circuit-QED) are investigated under various dissipative modes. The influences of photon number, coupling strength, detuning and relative phase angle on quantum entanglement and quantum discord are compared as well. The results show that quantum discord may be less robust to decoherence than quantum entanglement since the death and revival also appears. Under certain dissipative mode, the decoherence subspace can be formed in circuit-QED due to the cooperative action of vacuum field. Whether a decoherence subspace can be formed not only depends on the form of quantum system but also relates closely to the dissipative mode of environment. One can manipulate decoherence through manipulating the correlation between environments, but the effect depends on the choice of initial quantum states and dissipative modes. Furthermore, we find that proper relative phase of initial quantum state provides one means of suppressing decoherence.     

Dynamics of a Moving Two-Level Atom Under the Influence of Intrinsic Decoherence

We present a general and fascinating problem of quantum entanglement (QE) that is calculated with the help of quantum Fisher information (QFI) and von Neumann entropy (VNE) for moving two-level atomic systems. We calculate numerically the temporal evolution of the state vector of the entire system under the influence of intrinsic decoherence for a moving two-level atom. We demonstrate that the phase shifts of an estimator parameter, intrinsic decoherence, and the atomic motion play an important and prominent role during the time evolution of the atomic system. We observe that there is a monotonic relation between the atomic quantum Fisher information (QFI) and quantum entanglement (QE) in the absence of atomic motion. We also show that at the revival time the local maximum values of QFI decreases gradually. A periodic behavior of QFI is observed in the presence of atomic motion, which becomes more important and remarkable for two-level atomic systems. Moreover, the atomic quantum Fisher information and entanglement demonstrate an opposite response during the time evolution in the presence of atomic motion. We show that the evolution of entanglement is more susceptible to the intrinsic decoherence; a considerable change occurs in the degree of entanglement when the intrinsic decoherence parameter increases. Intrinsic decoherence in the atom–field interaction represses the nonclassical effects of the atomic systems. Both the entanglement and the quantum Fisher information saturate to their lower levels for longer time scales in the presence of intrinsic decoherence. For larger values of intrinsic decoherence, the sudden death of entanglement is observed.     

Sudden Death, Birth and Stable Entanglement in a Two-Qubit Heisenberg XY Spin Chain

Taking the decoherence effect due to population relaxation into account, we investigate the entanglement properties for two qubits in the Heisenberg XY interaction and subject to an external magnetic field. It is found that the phenomenon of entanglement sudden death (ESD) as well as sudden birth (ESB) appear during the evolution process for particular initial states. The influence of the external magnetic field and the spin environment on ESD and ESB are addressed in detail. It is shown that the concurrence, a measure of entanglement, can be controlled by tuning the parameters of the spin chain, such as the anisotropic parameter, external magnetic field, and the coupling strength with their environment. In particular, we find that a critical anisotropy constant exists, above which ESB vanishes while ESD appears. It is also notable that stable entanglement, which is independent of different initial states of the qubits, occurs even in the presence of decoherence.     

A study on quantum discord sudden changes

For open quantum systems, quantum discord sudden changes (QDSCs) may appear in the dynamics instead of sudden death appears in quantum entanglement. In this paper, we study the dynamics of quantum discord and explore the relation between the entanglement sudden death and QDSC. We find that the QDSC is sensitive to the initial states and the excitation number in the total system (the qubits  +  environment). By using the geometric measurement of the quantum correlation, the geometric interpretation for the QDSC is presented and the condition for the emergence of the QDSC is given. Furthermore, we observe that the entanglement sudden death and QDSC do not have one to one correspondence for open quantum systems.     

Dynamics of Quantum Correlations for Atoms in Independent Single-Mode Cavity

We investigate the dynamics of two identical atoms resonantly coupled to independent single-mode cavity in zero detuning without rotating wave approximation (RWA). It is shown that for two atoms initially in the ground state, the entanglement (concurrence) and the normalized geometric measure of quantum discord (NGMQD) display similar behavior. There is no sudden death and sudden birth. And the entanglement is always larger than NGMQD in this case. For two atoms initially in excited state, one can see the novel entanglement sudden death (ESD) and sudden birth (ESB) phenomena. The entanglement is not always greater than the NGMQD in this case. Consequently, there is no simple dominance relation between the entanglement and the NGMQD.     

Steady-State Discord Between Two Qubits Coupled Collectively to a Thermal Reservoir

We study the dynamics of quantum discord between two qubits coupled collectively to a thermal reservoir. For comparison, we also consider the dynamics of quantum entanglement. It is shown that we can obtain a stable quantum discord induced by the thermal environment when the discord of the initial state is zero. The thermal environment can also induce a stable amplification of the initially prepared quantum discord for certain X-type states. It is very valuable that the quantum discord is more resistant against the thermal environment than quantum entanglement. And, we have demonstrated that the sudden death of discord in a Markovian regime is impossible even at high temperature. It provides us a feasible way to create and protect quantum correlation in the case of a high-temperature thermal environment for various physical system such as trapped ions, quantum dots or Josephson junctions.     

Quantum Discord Behaviors in Two Qubits Spin Squeezing Model with Intrinsic Decoherence

By taking into account the intrinsic decoherence and the external magnetic field, quantum discord(QD) behaviors in two-qubit spin squeezing model are investigated in detail. It is found that the magnitude of quantum discord is strongly dependent on the initial states, the squeezing interaction μ, the magnetic field Ω and the purity r of initial states. With t → ∞, one can obtain the steady quantum discord (SQD) value, the environmental decoherence cannot entirely destroy the quantum correlation. Based on the analysis of the SQD, the conditions about the existence of SQD are obtained with different initial states. Varying the parameters μ, Ω and r not only can weaken the effects of decoherence but also can improve the magnitude of QD and SQD. The effects of the parameters μ and Ω on the QD and SQD display so different and complicated features that one cannot get an uniform law about them, while the values of QD and SQD are improved with increasing r. Properly tuning the parameters μ, Ω and r, one can obtain a larger value of QD or SQD.     

Dynamics of Quantum Discord of Two-Qubit Coupled with a Vacuum Cavity

The dynamical behaviors of quantum discord between two atoms coupled with a vacuum cavity are investigated. If the two qubits are initially prepared in two extended Werner-like states, the quantum discord and entanglement can be numerically calculated. There are remarkable differences between the time evolutions of the quantum discord and entanglement under the same conditions. These results imply that quantum discord is not zero for some unentangled states and in some regions entanglement can disappear completely. A large amount of quantum discord exists between the two-qubit. Thus, the quantum discord is more robust than entanglement for the quantum system exposed to the environment. The quantum discord shows sudden change and its existence depends on the initial state of the system. This property of quantum discord may have important implications for experimental characterization of quantum phase transitions.     

Quantum Correlation Properties Between Two Spatially Separated Atoms in Free Space Reinforced by Incoherent Pumping

Quantum correlation dynamics between two identical and spatially separated atoms in free space is investigated by the use of concurrence C and quantum discord (QD). The behaviors of QD differs in many unexpected ways from the entanglement in this system. Firstly, it shows the situations which the concurrence and QD can behave very differently with a “sudden birth” phenomenon of the former but not of the latter, and QD is only oscillating decays with time and the interqubit distance. We also verify the cases which QD is always greater than the concurrence and the region where the concurrence is vanished but with nonzero values for QD. Meanwhile an unexpected situation which the concurrence is greater than QD under the initial state |eg〉 is analyzed. It is revealed that the quantum correlation based only on QD is expected to be more robust than entanglement which is not suitable for all the initial states under the decoherence environment. Then, by introducing the incoherent pumping, we also study the different properties of the steady-state entanglement and QD about this atomic subsystem. It is shown that the incoherent pumping can overcome the decay of the atoms and the influences about the interqubit distance r ₁₂/λ on the steady-state correlation can make the decay of the concurrence obviously quicker than QD, the life of the steady-state QD is evidently larger than the steady-state entanglement.     

Sudden Death,Sudden Birth,and Nonclassical Effects of Photon-Added Power-Law Potential Within the Framework of Subsystem-Environment Correlations

Recently photon-added states that could be detected through the emission rather than the absorption of electromagnetic radiation have been actively explored and investigated. In this paper, we construct the photon-added power-law-potential coherent states (PA-PLPCSs) using generalized Heisenberg algebra. The Klauder minimal set of conditions required to obtain coherent states are satisfied. We study nonclassical effects associated with PA-PLCSs using the Mandel parameter and discuss some of their intriguing nonclassical behavior. These states have interesting significance and can be realized experimentally, exhibiting highly nonclassical behavior that depends on the degree of excitation and other parameters. Finally, we study the dynamics of entanglement and quantum discord for two-mode state within the framework of PLPCSs and show that the sudden death and sudden birth of correlations are due to the change and transfer of the correlation between one mode and its environment, using the monogamic relation between the entanglement and quantum discord.     

Sudden death and robustness of quantum correlations in the weak- or strong-coupling regime

We study the dynamics of quantum discord and entanglement of two entangled two-level atoms within two isolated and dissipative cavities in the weak- or strong-coupling regime. The quantum entanglement are measured by concurrence and relative entropy. The quantum discord of two atoms based on quantum mutual information and relative entropy are also calculated. In the weak-coupling regime, the sudden death of quantum discord and entanglement of two atoms can occur simultaneously within a short interaction time. When the interaction time is long, quantum discord and entanglement of two atoms could be partially preserved due to the long-lived nature of quantum discord and entanglement. However, in the strong-coupling regime, there is no sudden death of quantum discord though the entanglement sudden death phenomenon occurs. In addition, we observe that entanglement and discord will be destroyed eventually when the atom-field interactions are strong. We also address the issue of experimental realization briefly.     

Protecting Qutrit-Qutrit Entanglement Under the Generalized Amplitude Decoherence of the Finite Temperature

We investigate the dynamics and protection of quantum entanglement of a qutrit-qutrit system under local amplitude damping channels with finite temperature. We consider two different initial states. We find that the qutrit-qutrit entanglement decays monotonically as the decoherence strength increases, and may go through entanglement sudden death at higher temperature. Special attention is paid to how to protect the quantum entanglement from decoherence by weak measurement and quantum measurement reversal. Our results show that the entanglement increases with the increase of weak measurement strength when the temperature is lower. However, the protections of entanglement by weak measurement and quantum measurement reversal are almost failed and the decays of entanglement goes up with the increase of weak measurement strength for different decoherence strength when the temperature is higher, even entanglement suffers sudden death.

     

量子关联与原子间相互作用距离关系的研究

本文将两个二能级原子注入一个腔中,用共生纠缠的方法来度量两原子之间的纠缠并推出它们之间的量子失协,还给出了原子与环境之间的共生纠缠与量子失协的计算公式,讨论了原子自发衰变率的变化对量子纠缠及失协的影响,及不同的初始状态情况下,纠缠及失协随原子距离的演化情况。结果表明：原子自发衰变率减小,原子与环境之间的量子纠缠及失协增加;选择不同的初态,可以控制原子间出现纠缠死亡的现象或量子失协为零的状态。