Three-body entanglement induced by spontaneous emission in a three two-level atoms system

We study three-body entanglement induced by spontaneous emission in a three two-level atoms system by using the entanglement tensor approach. The results show that the amount of entanglement is strongly dependent on the initial state of the system and the species of atoms. The three-body entanglement is the result of the coherent superposition of the two-body entanglements. The larger the two-body entanglement is, the stronger the three-body entanglement is. On the other hand, if there exists a great difference in three two-body entanglement measures, the three-body entanglement is very weak. We also find that the maximum of the two-body entanglement obtained with nonidentical atoms is greater than that obtained with identical atoms via adjusting the difference in atomic frequency.     

Quantum teleportation by entanglement swapping with trapped ions

An effective teleportation scheme for an unknown ionic internal state via trapped ions is proposed without joint Bell-state measurement (BSM). In the constructed quantum channel process, we make use of entanglement swapping to avoid decrease in entanglement during the distributing of particles. Thus our scheme provides new prospects for quantum teleportation in a longer distance. The distinct advantage of our scheme is insensitive to the heating of vibrational mode. Furthermore, our scheme has no any individual optical access, and the successful probability also can reach 1.     

Teleportation for an Ionic Entangled Internal State by Entanglement Swapping

We present an effective scheme to teleport an unknown ionic entangled internal state via trapped ions without joint Bell-state measurement. In the constructed quantum channel process, we adopt entanglement swapping to avoid decrease of entanglement during the distribution of particles. Thus our scheme provides new prospects for quantum teleportation over longer distance. The distinct advantages of our scheme are that our scheme is insensitive to heating of vibrational mode and can be generalized to teleport an N-ion electronic entangled GHZ class state. Furthermore, in our scheme the success probability can reach 1.     

The entanglement between two isolated atoms in double mode--mode competition model

Extending the double Jaynes--Cummings model to a more complicated case where the mode--mode competition is considered, we investigate the entanglement character of two isolated atoms by means of concurrence, and discuss the dependence of atom--atom entanglement on the different initial state and the relative coupling strength between the atom and the corresponding cavity field. The results show that the amplitude and the period of the atom--atom entanglement evolution can be controlled by the choice of initial state and relative coupling strength, respectively. We find that the phenomenon of entanglement sudden death (ESD) is sensitive to the initial conditions. The length of the time interval for zero entanglement depends not only on the initial degree of entanglement between two atoms but also on the relative coupling strength of atom--field interaction. The ESD effect can be weakened by enhancing the mode--mode competition between the three- and single-photon processes.     

Quantum logic gates with two-level trapped ions beyond Lamb--Dicke limit

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.     

Fast scheme for generating quantum-interference states and GHZ state of N trapped ions

We propose a fast scheme to generate the quantum-interference states of N trapped ions. In the scheme the ions are driven by a standing-wave laser beam whose carrier frequency is tuned such that the ion transition can take place. We also propose a simple and fast scheme to produce the GHZ state of N hot trapped ions and this scheme is insensitive to the heating of vibrational motion, which is important from the viewpoint of decoherence.     

Effect of mode mode-competition on atom-atom entanglement

A system consisting of two atoms interacting with a two-mode vacuum is considered, where each atom is resonant with the two cavity modes through two different competing transitions. The effect of mode--mode competition on the atom--atom entanglement is investigated. We find that the entanglement between the two atoms can be induced by the mode--mode competition. For the initial atomic state |\varPsi(0)\rangle, whether the atoms are initially separated or entangled, a large or even maximal entanglement between them can be obtained periodically by introducing the mode--mode competition. For the initial atomic state |\varPhi(0)\rangle, the strong mode--mode competition can prevent the two atoms entangled initially from suffering entanglement sudden death; besides, it makes them in a more stable and longer-lived entanglement than in the non-competition case.     

Quantum Entanglement in a System of Two Spatially Separated Atoms Coupled to the Thermal Reservoir

We study quantum entanglement between two spatially separated atoms coupled to the thermal reservoir. The influences of the initial state of the system, the atomic frequency difference and the mean number of the thermal field on the entanglement are examined. The results show that the maximum of the entanglement obtained with nonidentical atoms is greater than that obtained with identical atoms. The degree of entanglement is progressively decreased with the increase of the thermal noise. Interestingly, the two atoms can be easily entangled even when the two atoms are initially prepared in the most mixed states.     

The entanglement of two dipole--dipole coupled atoms interacting with a thermal field via a two-photon process

This paper studies entanglement between two dipole-dipole coupled atoms interacting with a thermal field via a two-photon process. It shows that the entanglement is dependent on the mean photon number of the thermal field and the dipole-dipole interaction. The results also show that the atom-atom entanglement through the two-photon process is larger than that through the one-photon process and a remarkable amount of entanglement between the atoms still remains at certain times even for a very highly noisy thermal field.     

A scheme for conditional quantum phase gate viabimodal cavity and a Λ-type three-level atom

We propose a scheme to implement a two-qubit conditional quantum phase gatefor the intracavity field viaa single three-level $\Lambda$-type atom driven by two modes in a high-Q cavity. The quantum information is encoded on the Fock states of the bimodal cavity. The gate's averaged fidelity is expected to reach $99.8\%$.