Quantum Entanglement Swapping in a Non-Markovian Environment

We propose an entanglement swapping scheme for mixed states in a classical non-Markovian noises, which is modelled as the so-called Ornstein-Uhlenbeck processes. The two mixed states before entanglement swapping are two X-type mixed states, which are caused by the environment-induced decoherence on the initially Bell states. This is more practical than the pure state case in quantum information processing. The fidelity and concurrence of the post-swapping states are discussed.     

Concentration of Unknown Atomic Entangled States via Entanglement Swapping through Raman Interaction

We show that entanglement concentration of unknown atomic entangled states is achieved via the implementation of entanglement swapping based on Raman interaction in cavity QED. A maximally entangled state is obtained from a pair of partially entangled states probabilistically. Due to Raman interaction of two atoms with a cavity mode and an external driving field, the influence of atomic spontaneous emission has been eliminated. Because of the virtual excitation of the cavity mode, the decoherence of cavity decay and thermal field is neglected.     

Multiparty secret sharing of quantum information via cavity QED

The scheme on multiparty secret sharing of an atomic quantum state information via entanglement swapping in cavity QED [Y.Q. Zhang, X.R. Jin, S. Zhang, Phys. Lett. A 341 (2005) 380] is revisited and accordingly an improved version is proposed. The possible decoherence effect in the original version can be avoided after revision of assuming the prior distribution of entanglement. Moreover, the success probability of teleportation has been raised from 6.25% in the original version to 100% in the present version.     

Multipartite entanglement dynamics and decoherence from a quantum-critical environment

We investigate the entanglement dynamics and decoherence of a multipartite system under an environment which can exhibit a quantum phase transition. Our result implies that the entanglement evolution depends not only on the size of the system and the quantum states of concern but also on the environment. In the sense of the linear entropy to measure decoherence induced by the environment, the decoherence-free subspaces have been identified for our model.     

Demonstration of Quantum Entanglement Control Using Nuclear Magnetic Resonance

With the two forms of the quantum entanglement control, the quantum entanglement swapping and preservation are demonstrated in a three-qubit nuclear magnetic resonance quantum computer. The pseudopure state is prepared to represent the quantum entangled states through macroscopic signals. Entanglement swapping is directly realized by a swap operation. By controlling the interactions between the system and its environment,we can preserve an initial entangled state for a longer time. The experimental results are in agreement with the experiment.     

Entanglement Dynamics and Bell Violations of Two Atoms in Tavis-Cummings Model with Phase Decoherence

Considering the dipole-dipole coupling intensity between two atoms and the field in the Fock state, the entanglement dynamics between two atoms that are initially entangled in the system of two two-level atoms coupled to a single mode cavity in the presence of phase decoherence has been investigated. The two-atom entanglement appears with periodicity without considering phase decoherence, however, the phase decoherence causes the decay of entanglement between two atoms, with the increasing of the phase decoherence coefficient, the entanglement will quickly become a constant value, which is affected by the two-atom initial state. Meanwhile the two-atom quantum state will forever stay in the maximal entangled state when the initial state is proper even in the presence of phase decoherence. On the other hand, the Bell violation and the entanglement do not satisfy the monotonous relation, a large Bell violation implies the presence of a large amount of entanglement under certain conditions, while a large Bell violation corresponds to a little amount of entanglement in certain situations. However, the violation of Bell-CHSH inequality can reach the maximal value if two atoms are in the maximal entangled state, or vice versa.     

存在相位退相干时原子与光场相互作用的纠缠演化和保持

应用全量子理论研究了存在相位退相干时单模相干光场与一个二能级原子相互作用系统纠缠的时间演化规律;分别讨论了原子—光场耦合常数、光场的平均光子数以及失谐量的大小对场与原子纠缠的影响.结果表明:随着原子—光场耦合常数的增大和光场平均光子数的增加,系统纠缠的振荡频率都会明显增大.不存在相位退相干时,纠缠的时间演化明显受到失谐量的影响,若选取适当的失谐量,系统的纠缠可长时间保持在最大纠缠态.若考虑相位退相干的影响,则在共振情况下系统纠缠的时间演化是一个逐渐衰减的过程,且最终衰减到零;但若存在适当的失谐量,则在初始一段时间内系统的纠缠也是一个波动幅度逐渐衰减的过程,但随着时间的演化,失谐量抵消了相位退相干的影响,使系统的纠缠不再衰减到零.如果增大失谐量,纠缠在初始一段时间内波动的幅度会相应的减小,并且纠缠趋于稳定的时间也随着失谐量的增大而缩短;当失谐量适当时,系统可保持在纠缠相对较大的状态而无消纠缠态.     

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.     

Generation of remote W-type entangled state via tripartite entanglement swapping of continuous variables

We propose a scheme for generating remote W-type entangled state via tripartite entanglement swapping of continuous variables, where two EPR pairs and a local W-type entangled state are required. Because of the co-existence of both bipartite and tripartite entanglement in a W-type entangled state, the three involved remote regions, without direct interaction, will become entangled after the prescribed entanglement swapping.     

Stability investigation on quantum correlations of coupled qubits in non-Markovian process

Utilizing the concurrence and the quantum discord as the measure method, in this paper we compare and investigate the dynamic evolution features of quantum correlations of coupled qubits in non-Markovian process. We focus attention on decoherence effect influences the stability of quantum correlations. The investigation results show that because of the decoherence influence between the system and environment, the concurrence always evolves with time in oscillation form in the way of deaths and survivals, however, the quantum discord time evolution does not appear the deaths and survivals. The quantum discord survives obviously longer than concurrence, which indicates that quantum discord has a stronger ability to resist decoherence than entanglement. Through regulating and controlling the purity and entanglement of the initial quantum state, we can effectively suppress the decay of the quantum correlations, which is advantaged to complete the quantum information processing.     

Heisenberg Limit Phase Sensitivity in the Presence of Decoherence Channels

In the present study, time evolution of quantum Cramer–Rao bound of entangled N00N state, as phase sensitivity, is determined by the aid of quantum estimation theory in the presence decoherence channels. Also, the dynamic quantum process as decoherence approach is characterized by quantum fisher information flow and entanglement amount in order to distinguish between Markovian and Non-Markovian process. The comparison between quantum fisher information and quantum fisher information flow assists to comprehend the phase sensitivity evolution corresponding to Non-Markovian and Markovian process. Furthermore, as result of backflow of information from the environment to system, the phase sensitivity corresponding memory effect of environment are revived after complete decay and increase in the few times.     

Quantum teleportation

Quantum Teleportation of one qubit of information using entangled state of two qubit is explained. It is shown that if quantum state of N qubits is to be teleported, the requirement is entangled state of at least 2N qubits. A scheme of teleportation of a superposition of even and odd coherent states was suggested by Van Enk and Hirota for teleportation of superposed coherent state, success of which is ? according to the authors. It is shown how this scheme can be modified so as to make the success nearly 1. It is also explained how decoherence can be taken into account and how such schemas can be applied to similar phenomena of entanglement diversion and entanglement swapping.     

State transfer in intrinsic decoherence spin channels

By analytically solving the master equation, we investigate quantum state transfer, creation and distribution of entanglement in the model of Milburn’s intrinsic decoherence. Our results reveal that the ideal spin channels will be destroyed by the intrinsic decoherence environment, and the detrimental effects become severe as the decoherence rate γ and the spin chain length N increase. For infinite evolution time, both the state transfer fidelity and the concurrence of the created and distributed entanglement approach steady state values, which are independent of the decoherence rate γ and decrease as the spin chain length N increases. Finally, we present two modified spin chains which may serve as near perfect spin channels for long distance state transfer even in the presence of intrinsic decoherence environments.     

Dynamics of entanglement under decoherence in noninertial frames

In this paper,we investigate the entanglement dynamics of a two-qubit entangled state coupled with its noisy environment,and plan to utilize weak measurement and quantum reversal measurement to study the entanglement dynamics under different decoherence channels in noninertial frames.Through the calculations and analyses,it is shown that the weak measurement can prevent entanglement from coupling to the amplitude damping channel,while the system is under the phase damping and flip channels.This protection protocol cannot prevent entanglement but will accelerate the death of entanglement.In addition,if the system is in the noninertial reference frame,then the effect of weak measurement will be weakened for the amplitude damping channel.Nevertheless,for other decoherence channels,the Unruh effect does not affect the quantum weak measurement,the only exception is that the maximum value of entanglement is reduced to√2/2of the original value in the inertial frames.     

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.     

Quantum key distribution by comparing Bell states

We propose a quantum key distribution (QKD) scheme based on entanglement swapping. In this scheme, the methods to form secret keys are so interesting. By comparing initial Bell state and outcome of entanglement swapping, the secret keys between Alice and Bob are generated involuntarily.     

Effects of Intrinsic Decoherence on Information Transport in a Spin Chain

Considering Milburn＇s intrinsic decoherence effect on quantum communication through a spin chain, we show that the transfer quality for quantum state and entanglement will obviously decrease with the increasing intrinsic decoherence rate. Some odd chains are much higher than even ones for the state transfer efficiency. The state transfer of a long chain is very sensitive to the intrinsic decoherence, which turns out to be an obstacle for information transport.     

Protecting Entanglement in a Common Phase Decoherence Environment using Weak Measurement and Quantum Measurement Reversal

Protection of entanglement from disturbance of the environment is an essential task in quantum information processing. We investigate the entanglement protection of a qubit-qubit system interacting with a phase decoherence reservoir by employing the weak measurement (WM) and quantum measurement reversal (QMR). We show explicitly that the quantum entanglement can be obviously protected by means of the proper WM and QMR. In particular, we found that there is a specific initial state parameter region, the entanglement protection ratio, which is determined only by the initial state parameter and independent of the form of the spectral density of the reservoir.     

A Continuous Variable Quantum Key Distribution Protocol Based on Entanglement Swapping of Quasi-Bell Entangled Coherent States

A continuous variable quantum key distribution protocol with entanglement swapping of quasi-Bell entangled coherent states is proposed. As the preliminary step, a sender shares quasi-Bell entangled coherent states with a receiver. After their measurements to distinguish the cases of a zero response, a nonzero even-photon response and an odd-photon response, two legitimate participants fulfill the task of key distribution. The correlations resulting from entanglement swapping of quasi-Bell entangled coherent states and the order rearrangement of transmitted states provide the possibility to protect secret key distribution. In the ideal channel, the success probability increases with the amplitude of the coherent state, and approaches unity when the amplitude of the coherent state is larger than two. However, in the loss channel, the decoherence will introduce error in the generated key, and the error rate increases with the amplitude of the coherent state. When the amplitude of the coherent state is smaller than 0.2 (or so), the error rate approaches zero, although the success probability is less than 0.5 in this case.