Entanglement transfer between atoms in two distant cavities via an optical fibre

This paper presents a treatment of the entanglement transfer between atoms in two distant cavities coupled by an optical fibre. If the atoms resonantly and collectively interact with the local single-mode cavity fields and the dipole--dipole interaction between the atoms is neglected, then it shows that a complete transfer of entanglement from one pair of atoms to another can be deterministically realized. Furthermore, it also investigates the effects of dipole--dipole interaction on entanglement transfer on the condition that the interaction between the atoms and the cavity is much weaker than the coupling between the cavity and the fibre.     

Preserving entanglement and the fidelity of three-qubit quantum states undergoing decoherence using weak measurement

We demonstrate a method to preserve entanglement and improve fidelity of three-qubit quantum states undergoing amplitude-damping decoherence using weak measurement and quantum measurement reversal. It is shown that we are able to enhance entanglement to the greatest extent, and to circumvent entanglement sudden death by increasing the weak measurement strength both for the GHZ state and the W state. The weak measurement technique can also enhance the fidelity to the quantum region and even close to 1 for the whole range of the decoherence parameter in both of the two cases. In addition, the W state can maintain more fidelity than the GHZ state in the protection protocol. However, the GHZ state has a higher success probability than the W state.     

Entanglement and disentanglement from cascade beam-splitter for squeezed vacuum state inputs

Properties of the entanglement at the outputs of a cascade beam-splitter are investigated for two single-mode squeezed vacuum state inputs. It is shown that the entanglement depends on the squeezing amplitudes of the input states and the reflection coefficients of the cascade beam-splitter, and the composite phase shift Δ between the cascade beam-splitter and the input fields has a great effect on the entanglement. In particular, the properties of the entanglement of a cascade beam-splitter differ from those of only one beam-splitter. A further method for manipulating entanglement by adjusting the parameters of the cascade beam-splitter and the input fields is proposed.     

Discord and entanglement in non-Markovian environments at finite temperatures

The dynamic evolutions of the discord and entanglement of two atoms immersed in two independent Lorentzian reservoirs at zero and finite temperatures have been investigated by using the time-convolutionless master-equation method.Our results show that,nonzero temperature can induce the entanglement sudden death and accelerate the decays of discord and entanglement.The discord and the entanglement have different robustness for different initial states and their robustness may change under certain conditions.When both the non-Markovian effect and detuning are present simultaneously,due to the memory and feedback effect of non-Markovian reservoirs,the discord and entanglement can be effectively protected even at nonzero temperature by increasing the non-Markovian effect and the detuning.     

Protecting entanglement by detuning: in Markovian environments vs in non-Markovian environments

The models of two qubits separately trapped in two independent Markovian or non-Markovian environments have been investigated. The distinction of the two-qubit entanglement dynamics in different environments has also been discussed in detail. The results show that, in non-Markovian environments, the possible usage time of entanglement can be extended due to its memory effect. On the other hand, we note that, compared to Markovian environments, the two-qubit entanglement could be protected better in non-Markovian environments by modulating the detuning between qubits and cavities. Finally, an intuitive physical interpretation for these results is given.     

Generation of motional cat states for a trapped ion in a cavity

Beyond the Lamb－Dicke limit, we propose a simple scheme for generating Schr?dinger cat states of the motion of a trapped two-level ion interacting with a quantized light field in a single-mode cavity.     

Position—dependent dynamics of a trapped ion in a standing wave laser

We have investigated the position-dependent dynamics of a trapped ion in a standing wave laser by transforming it to the Jaynes－Cummings-type system under the Lamb－Dicke limit. A variety of novel phenomena are exhibited, e.g. periodic collapse and revival features and long-time scaled revivals of the ionic inversion, depending on its position in the standing wave. Our result provides a way of producing a system equivalent to the two-photon Jaynes－Cummings model in the trapped ion system, with its exact periodicities.     

Entropy as a measure of the noise extent in a two-level quantum feedback controlled system

By introducing the von Neumann entropy as a measure of the extent of noise, this paper discusses the entropy evolution in a two-level quantum feedback controlled system. The results show that the feedback control can induce the reduction of the degree of noise, and different control schemes exhibit different noise controlling ability, the extent of the reduction also related with the position of the target state on the Bloch sphere. It is shown that the evolution of entropy can provide a real time noise observation and a systematic guideline to make reasonable choice of control strategy.     

Entanglement concentration and teleportation of multipartite entangled states in an ion trap

We propose an effective scheme for the entanglement concentration of a four-particle state via entanglement swapping in an ion trap. Taking the maximally entangled state after concentration as a quantum channel, we can faithfully and determinatively teleport quantum entangled states from Alice to Bob without the joint Bell-state measurement. In the process of constructing the quantum channel, we adopt entanglement swapping to avoid the decrease of entanglement during the distribution of particles. Thus our scheme provides a new prospect for quantum teleportation over a longer distance. Furthermore, the success probability of our scheme is 1.0.     

The Holevo capacity of a generalized amplitude-damping channel

The Holevo capacity of a generalized amplitude-damping channel is investigated by using a numerical method. It is shown that the Holevo capacity depends on the channel parameters representing the ambient temperature and fidelity. In particular, under a special condition, the Holevo capacity of the generalized amplitude-damping channel can be written as an analytical expression.