Quantum Information Processing without Strong Optical Nonlinearity

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It has been said that gigantic optical nonlinearity is necessary for realization of quantum information processing. Since 2001, however, it became clear, by theoretical research contribution including our results, that some of quantum information processings are possible using linear optics and practically reasonable magnitude of nonlinear optics.This research deals with the fundamental properties of "linear optics quantum gates" and "light-exciton interaction" as the element of such quantum information processing, and also seek the possibility of "entanglement manipulation" and "quantum game" as the application of such quantum information processing.We obtained a rigorous solution for exciton dynamics under linear and Coulomb interactions between quantum dots. As a result, it became clear that a strong entanglement can be generated between the dots using such interactions. Also, the robustness of even/odd-coherent-state qubits against decoherence and its dot-size dependence have been … More investigated.In order to treat individual proposals of linear optics quantum information processing(LOQIP) in an organized way, We built up a systematic theory for LOQIP. Particularly, the influence of detector imperfections has been taken into account numerically. As a result, it became clear that several information processing can be accomplished with high fidelity using exsisting photon counters.In quantum game theory, we theoretically compared the effects of entanglement and classical correlations on non-cooperative games, and obtained a result that in general entanglement is more influential than classical correlations in changing the phase of games. In comparing this kind of effects, it is necessary that a quantum version of a game should include the original classical game as a special case. According to our research, it became clear that GHZ entanglement can be used for this purpose but W entanglement cannot.An application example of LOQIP is B92 quantum cryptography, whose security under practical imperfection has not been proven in a satisfied way. We theoretically proven the security of B92 protocols and obtained the security conditions versus the magnitude of the imperfections. Less