We propose an entanglement swapping scheme in cavity QED. In the scheme, the previously used joint measurement is not needed. The entanglement swapping in our proposal is a non-post-selection one, i.e., after the swapping is done, the swapped entanglement is still there.

Abstract.We survey all the coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronagraph Experiment (LASCO) aboard the Solar & Heliospheric Observatory (SOHO) in the interval from 1997 to 2001, and select 197 frontside halo CMEs whose associated near-surface activity could be clearly identified from SOHO EUV Imaging Telescope (EIT) and other space-borne and ground-based observations. A statistical analysis has been made with the emphasis on the correlation between CMEs and solar surface activity. We have found in our sample that all the CMEs were accompanied by local brightening in the CME source regions in EIT and/or Hα images. However, if we only classify the events with a brightening increase more than 50% above the quiet background as flares, as measured from EIT data, approximately 88% of the earth-directed CMEs are associated with flares and more than 94% are associated with eruptive filaments. With regard to the locations of CME source regions, there are about 79% CMEs initiating from active regions, while only 21% originate outside active regions. We evaluate the symmetry between CMEs and associated solar surface activity in spatial distributions. We find that in about half the events the surface activity is asymmetric with respect to the CME. For the flares having GOES X-ray records in this study, allowing for the uncertainties specified by the data cadence, 59% of the CME initiations seem to precede the flare onset in the CME source regions, while 41% are preceded by flare onsets. The statistical correlation seems to suggest that CMEs are intrinsically related to surface activity.

An entanglement concentration scheme for unknown atomic entanglement states is proposed via entanglement swapping in cavity QED. Because the interaction used here is a large-detuned one between two driven atoms and a quantized cavity mode, the effects of the cavity decay and thermal field have been eliminated. These advantages can warrant the experimental feasibility of the current scheme.PACS numbers: 03.67. Hk, 03.67.Mn, 03.67.Pp In quantum teleportation, an unknown quantum state will be sent from sender to receiver via a quantum channel with the help of classical communication. In the process, the carrier of the unknown quantum information (state) is not transmitted. From the original discussion of quantum teleportation [1], a perfect quantum channel is needed if we want to teleport the quantum state faithfully, i.e. the quantum channel must be a maximally entangled state. But in real experiment, the actually available quantum channels are usually nonmaximally entangled states, such as pure non-maximally entangled states and mixed entangled states etc. To realize the faithful quantum teleportation, we must convert the non-maximally entangled states into the pure maximally entangled states. This process are usually named as entanglement concentration (pure non-maximally entangled states case) [2] and entanglement purification or distillation (mixed entangled states case) [3]. The original contribution on entanglement purification has discussed the general purification and concentration processes. The main idea of the original contribution is that, from the local operations on the ensemble of nonmaximally entangled pairs (raw entangled pairs), we will sacrifice some pairs of the raw entangled pairs and keep the remaining ones. Then entanglement of the remaining pairs will be enhanced. To realize the entanglement purification process experimentally, many physical schemes have been proposed. For mixed entangled states of polarization photons, Pan et al have presented their physical scheme [4] and experimental scheme [5], where polarization beam splitter is the most important element. J. L. Romero et al proposed a physical scheme for the purification of the mixed entangled cavity fields, where the entanglement degradation is caused by the cavity decay. To realize the purification process, two auxiliary atoms have been introduced [6]. For pure non-maximally entangled states of polarization photons, S. Bose et al have proposed a concentration scheme via entanglement swap- * Electronic address: mingyang@ahu.edu.cn † Electronic address: zlcao@ahu.edu.cn ping [7].In the previous contributions to entanglement purification and concentration, there is fewer schemes discussing the purification and concentration of entangled atomic states [8,9,10,11]. Hitherto, the purification and concentration of entangled photon states have been demonstrated in experiment [5,12,13], but the realization of purification and concentration of entangled atomic states are still under way. So it is necessary to explore the experimentally fea...

We prove that the squared Rényi-α entanglement (SRαE), which is the generalization of entanglement of formation (EOF), obeys a general monogamy inequality in an arbitrary N -qubit mixed state. Furthermore, for a class of Rényi-α entanglement, we prove that the monogamy relations of the SRαE have a hierarchical structure when the N -qubit system is divided into k parties. As a byproduct, the analytical relation between the Rényi-α entanglement and the squared concurrence is derived for bipartite 2 ⊗ d systems. Based on the monogamy properties of SRαE, we can construct the corresponding multipartite entanglement indicators which still work well even when the indicators based on the squared concurrence and EOF lose their efficacy. In addition, the monogamy property of the µ-th power of Rényi-α entanglement is analyzed.

We propose two schemes for the generation of the cluster states. One is based on cavity quantum electrodynamics (QED) techniques. The scheme only requires resonant interactions between two atoms and a singlemode cavity. The interaction time is very short, which is important in view of decoherence. Furthermore, we also discuss the cavity decay and atomic spontaneous emission case. The other is based on atomic ensembles. The scheme has inherent fault tolerance function and is robust to realistic noise and imperfections. All the facilities used in our schemes are well within the current technology.

The entanglement is investigated in a two-qubit Heisenberg XXZ system with Dzyaloshinskii-Moriya(DM) interaction. It is shown that the entanglement can be efficiently controlled by the DM interaction parameter and coupling coefficient Jz. Dx(the x-component parameter of the DM interaction) has a more remarkable influence on the entanglement and the critical temperature than Dz(the z-component parameter of the DM interaction). Thus, by changing the direction of DM interaction, we can get a more efficient control parameter to increase the entanglement and the critical temperature.

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In this paper, we propose two general entanglement distillation protocols which can concentrate the state (non-maximally entangled pure W state). The general protocols are mainly based on the unitary transformation on the auxiliary particle and one of the three entangled particles, and in the second protocol, the entanglement distillation includes two meanings, namely, extracting the concentrated tripartite entangled W state and obtaining the maximally entangled bipartite state from the ‘garbage’ state, which gives no contribution to the distillation of the state. We can make use of the ‘garbage’ in the distillation process, and make the entanglement waste in the quantum communication as small as possible. A feasible physical scheme is suggested based on the cavity QED.