In the system with two two-level ions confined in a linear trap, this paper presents a simple scheme to realize the quantum phase gate （QPG） and the swap gate beyond the Lamb Dicke （LD） limit. These two-qubit quantum logic gates only involve the internal states of two trapped ions. The scheme does not use the vibrational mode as the data bus and only requires a single resonant interaction of the ions with the lasers. Neither the LD approximation nor the auxiliary atomic level is needed in the proposed scheme. Thus the scheme is simple and the interaction time is very short, which is important in view of decoherence. The experimental feasibility for achieving this scheme is also discussed.
An experimentally feasible scheme for implementing four-atom quantum dense coding of an atom-cavity system is proposed. The cavity is only virtually excited and no quantum information will be transferred from the atoms to the cavity. Thus the scheme is insensitive to cavity decay and the thermal field. In the scheme, Alice can send faithfully 4 bits of classical information to Bob by sending two qubits. Generalized Bell states can be exactly distinguished by detecting the atomic state, and quantum dense coding can be realized in a simple way.
Considering a quantum model consisting of two effective two-level atoms and a single-mode cavity, this paper investigates the entanglement dynamics between the two atoms, and studies the effect of the Stark shift on the entanglement. The results show that, on the one hand the atom-atom entanglement evolves periodically with time and the periods are affected by the Stark shift; on the other hand, the two atoms are not disentangled at any time when the Stark shift is considered, and for large values of the Stark shift parameter, the two atoms can remain in a stationary entangled state. In addition, for the initially partially entangled atomic state, the atom-atom entanglement can be greatly enhanced due to the presence of Stark shift. These properties show that the Stark shift can be used to control entanglement between two atoms.
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.
This paper studies the entanglement properties in a system of two dipole-dipole coupled two-level atoms resonantly interacting with a single-mode thermal field. The results show that, when the temperature of the cavity is high enough （corresponding to the large value of the mean photon number）, the entanglement is greatly enhanced due to the initial atomic coherence. These results are helpful for controlling the atomic entanglement by changing the initial parameters of the system.
In this paper, we accomplish the teleportation of an unknown three-particle maximally entangled W state by using a spin-path entangled quantum channel which may be realized experimentally based on the advanced theory and technique in Bose-Einstein condensate （BEC） of molecule, micro-fabricated wave guide and simple quantum logic gate. Similarly, we can make an arbitrary n-particle entangled Greenberger Horne-Zeilinger （GHZ） state （n ≥ 4） teleported through this kind of quantum channel. It may have important applications due to its resource-economic and practical features.
The effects of distributing entanglement through the amplitude damping channel or the phase damping channel on the teleportation of a single-qubit state via the Greenberger Horne-Zeilinger state and the W state are discussed. It is found that the average fidelity of teleportation depends on the type and rate of the damping in the channel. For the one-qubit affected case, the Greenberger-Horne Zeilinger state is as robust as the W state, i.e., the same quantum information is preserved through teleportation. For the two-qubit affected case, the W state is more robust when the entanglement is distributed via the amplitude damping channel; if the entanglement is distributed via the phase damping channel, the W state is more robust when the noisy parameter is small while the Greenberger-Horne-Zeilinger state becomes more robust when it is large. For the three-qubit affected case, the Greenberger-Horne-Zeilinger state is more robust than the W state.
Recently, a genuine six-qubit entangled state Isix） has been proposed [Chen P X, et al. Phys Rev A, 2006, 74： 032324]. This state does not belong to the well-known three types of multipartite entangled states, i.e., Greenberger-Home-Zeilinger （GHZ） state, W state, and linear cluster state. This state has many potential applications in quantum information processing. We pro- pose a scheme for generating such a genuine six-qubit entangled state for trapped ions in thermal motion. The scheme is insen- sitive to both the initial motional state and heating.
The entanglement dynamics between an isolated atom and a moving atom interacting with a cavity field is investigated. The results show that there appears sudden death of entanglement between the isolated atom and the moving atom and that the time of entanglement sudden death （ESD） is independent of the initial state of the system. It is interesting that the isolated atom can also entangle with a cavity field, though they do not interact with each other originally, which stems from the fact that the entanglement between the isolated atom and the moving atom may turn into the entanglement between the isolated atom and the cavity.
