Theoretical Study Of W-State Fusion Based On Optical System And Quantum-Dot System

Quantum entanglement is a unique concept in quantum mechanics, and it is an important quantum resource in the quantum information field. Therefore the creation of entangled states has been paid much attention in the quantum infor-mation. With the increase of the number of particles forming an entangled state, there are many different kinds of entangled states, such as W states, GHZ states, Dicke states, cluster states and so on. The entanglement would be destroyed, when one particle in the GHZ states is detected. Different from the GHZ state, W s-tate is a special kind of entangled states which is highly robust against the qubits loss. When one particles in the W states is detected, the other particles are still entangled. Hence, W states, as an important quantum resource, are widely used in the field of quantum computing and information science. For example, W states have been shown to be the only pure state to exactly solve the problem of lead-er election in anonymous quantum networks and secure quantum communication. However, large-scale quantum information tasks, the multipartite entangled states are needed. Expansion and fusion operations have been proposed as an efficient way to prepare large-scale entangled states. In this dissertation, we present the w-state fusion schemes in different physical systems. The main research contents are as following:(1) We propose an effective W-state fusion scheme with the help of weak cross-Kerr nonlinearities. Different from the previous schemes, the present scheme can fuse a n-qubit W state and a m-qubit W state to a (n+m-l)-qubit W state without the Fredkin gate and an ancillary photon, that is, the present scheme can be used to not only create large W state with small ones, but also prepare 3-qubit W states with Bell states. In this sense, the present scheme reduces the requirements of the initial resources of the W-state fusion. We analyze the resource cost and the success probability of the scheme, which shows that the present scheme requires less resource cost than the previous ones and can be achieved with high probability under the current experiment technology.(2) We propose effective fusion schemes for stationary electronic W state and flying photonic W state, respectively, by using the quantum-dot-microcavity coupled system. The present schemes can fuse a n-qubit W state and a m-qubit W state to a (m+n-l)-qubit W state. These schemes are based on the optical selection rules and the transmission and reflection rules of the cavity and can be achieved with high probability. We evaluate the effect of experimental imperfections and the feasibility of the schemes, which shows that the present schemes can be realized with high fidelity in both the weak coupling and the strong coupling regimes. The feasibilities have been discussed, which indicates these schemes can be achieved with high suc-cess probability under the current experimental technology. These schemes may be also meaningful for the large-scale solid-state-based quantum computation and the photon-qubit-based quantum communication.