The Research Of Some Problems Of Quantum Information

Quantum information science is a new field. It includes such two parts as quantum computation and quantum communication. Quantum communication comprises quantum teleportation, quantum dense coding, quantum secret sharing, quantum cloning, etc. After the development of two decade years, people have acquired a series of important breakthrough. It makes this subject become one of the hotspot in international leading edge.In recent years, quantum computers have attracted much more attention because they can show stronger computational capacity than classical ones. As we know, the quantum logical gates are one of the essential building blocks of a quantum computer. They play an important role in quantum computation and quantum communnication. For quantum computer, because of the no-cloning theorem and the linearity of the Hilbert space, an arbitrary unknown quantum state is not copied exactly. Therefore much more attention has been turned to approximate quantum cloning. In this paper, we will study some problems of quantum information as quantum teleportation, quantum dense coding, quantum logic gate and quantum cloning. The results of our study are mainly given as follows,1. The study of realizing quantum teleportation and dense coding in quantum cavity QEDWe propose a scheme to realize quantum teleportation and dense coding, in our scheme, the atoms interact simultaneously with a highly detuned cavity mode with the assistance of a classical field. In the whole process, our scheme does not require the transfer of quantum information between the atoms and cavity and the cavity decay is negligible. In addition, cavity field is only virtually excited and prolong the effective decoherence time, so our scheme can be realized.2. The study of realizing quantum SWAP gate in ion-trapped systemWe consider two three-level ions which are confined in a linear trap interact with two different frequency laser pulses in the case of resonant. In this scheme, decoherence is negligible. Thus this scheme may be realized with presently available ion-trap techniques.3. The study of schemes for realizing quantum cloning(1) Optical realization of optimal symmetric real state quantum cloning machineWe present an experimentally uniform linear optical scheme to implement the optimal 1→2 symmetric and optimal 1→3 symmetric economical real state quantum cloning machine. Decoherence is very low because photons have relatively less interaction with the environment. This scheme is feasible by current experimental technology.(2) Implementation of two optimal symmetric economical state-dependent cloners in ion-trap systemAs we well know, at present, a series of progress has been made in quantum information processing with trapped ions, especially, the advancement of information exchange between trapped ion and photon and scalable ion-trapped system. Multiqubit quantum cloning has important significance in quantum information theory and application. It is possible to implement multiqubit quantum cloning with trapped ions, owing to the experimental progress in controlling multiqubit in ion trap system. Based on the current advancement, we propose an experimentally unified scheme to realize two optimal symmetric economical state-dependent cloners. Our scheme has some outstanding features:First, the two clones work without the need of an ancilla—this is called economical cloning—thus it is likely to be much less sensitive to noise and decoherence. Second, the two clones are optimal. This property can help the information encoded in the initial system be optimally distributed to the final system. The two cloners that have high fidelity can provide a more risky eavesdropping strategy for quantum cryptographic protocol. In addition, by adjusting the tunable time parameters, several quantum cloning machines such as optimal symmetric (asymmetric) universal quantum cloning and optimal symmetric (asymmetric) phase-covariant quantum cloning can also be performed.