Fusion Of W States Based On Optical System

With the rapid developing of quantum mechanics and informatics, people combine them together, finally, a new interdiscipline, quantum information, appears. It applies the characteristics of quantum mechanics to informatics. These involve nonlocality, superposition principle, quantum non-cloning theorem and so on. In quantum information, we can realize the storage, transmission and extraction of the information via controlling quantum states. Quantum information contains two important branches, quantum computing and quantum communication. These two branches are based on the generation and control of quantum states. Therefore, more and more researchers try to study the preparation and control of the quantum entangled states in all kinds of physical systems.The common quantum systems involve cavity quantum electrodynamics (cavity QED), superconducting system, ion trap, quantum dot, nuclear magnetic resonance, optical system and so on. Among all kinds of the quantum systems, the fast developing and advantages of the optical system makes it become an ideal quantum information processing system. Its advantages contain:high operability, strong anti-jamming ability, transmitting easy and so on. However, it also has some drawbacks. It is very hard to make photons interact with each other. The application of cross-Kerr nonlinearity can solve this problem well. Cross-Kerr medium can make photons interact with each other. Although nonlinearities of Natural Kerr media are very small, people can solve this problem via using electromagnetically induced transparency (EIT). Therefore, the small-but-not-tiny Kerr nonlinearities can be effectively used in quantum information processing.In this article, we mainly introduce the fusion of W states with the optical system and obtain some achievements.(1) Realizing the fusion of W states with Cross-Kerr nonlinearity and QD-cavity coupled systemIn this section, we proposed a new scheme for fusing two arbitrary-size photonic W (or Bell) states encoded by polarizations. In our scheme, we use cross-Kerr nonlinearity, linear optics, quantum non-demolition measurement (QND measurement) and a quantum dot in an optical cavity (QD-cavity coupled system) to replace quantum logic gates used in previous schemes, so our scheme can solve the problems of the previous schemes. In addition, our scheme needn’t any ancillary photons. Hence, our scheme not only can overcome the drawbacks of the previous schemes but also is more efficient. Trough discussing the loss of the resource, we find two ways, linear growth with recycling and fusing states with similar size with recycling, are very efficient. Comparing with the previous scheme, we find our scheme have lower loss. With the analysis of the feasibility, we find that the scheme can be realized with current experimental technology.(2) Fusing multiple W states with optical systemWe proposed another scheme for fusing W states with the help of cross-Kerr medium, linear optics and QND measurement. This scheme can fuse not only two W states but also three and four W (or Bell) states, and the set-up of our scheme is simple. Comparing with [1], our scheme is not very efficient for fusing two W states. The success probability of our scheme for fusing three W states is same with the scheme [2], but our scheme needn’t any logical gates or ancillary photons. However, comparing with the scheme [3], our scheme is more efficient for fusing four W states without logical gates or ancillary photons. And our scheme can be generalized to realize the fusion of multiple W states.