Entanglement Concentration Of Three-Particle And Four-Particle Less-Entangled States

Quantum entanglement plays an important role in the field of quantum infor-mation and has been widely applied to quantum communication as an important quantum resource. It is a crucial quantum resource with respect to various quan-turn information processing, such as quantum teleportation, quantum dense coding, quantum secret sharing, and so on. However, in practical transmission and storage, quantum entangled systems will inevitably interact with the environment, and the noise will degrade the entanglement. Entanglement concentration is an effective and feasible method to solve the problem by using local operations and classical communication. It is significant and meaningful to quantum communication.Recently, more and more attention has been focused on the concentration for multi-particle entangled state, mainly including GHZ state, W state and cluster state. In this thesis, we mainly study the entanglement concentration of the par-tially entangled three-particle GHZ state and W state with unknown coefficients, and the partially entangled four-photon cluster state with known coefficients. In cav-ity QED system, based on the input-output process of ancillary coherent light pulse in the nitrogen-vacancy centers coupled to low-Q microresonators, we propose two entanglement concentration schemes of partially entangled three-particle GHZ state and W state with unknown coefficients. The entanglement concentration scheme can be probabilistically realized via one of the participants using local operations and classical communication. It is found that the nonlocal and maximally entan-gled state can be obtained among three remote participants. Our schemes use a conventional photon detector to discriminate the two coherent states|α) and|-α), which is more convenient than homodyne measurement. We propose a scheme to concentrate an arbitrary partially-entangled four-photon cluster state with known coefficients via cross-Kerr nonlinearity. By using an ancillary single-photon resource, quantum non-demolition detectors and the linear’optical elements, the nonlocal and maximally entangled state can be probabilistically obtained among remote partici-pants. Moreover, the scheme can be iterated to obtain a higher success probability.