Chunyan Li

Quantum communication (QC) is one of the most important branches of quantum information. It takes advantage of some basic principles in quantum mechanics to accomplish the task of transmitting secret message securely. This novel accomplishment of communication has great power for the development of information field. Since Bennett and Brassard published their first QKD scheme in 1984 (BB84), a great number of theoretic models have been proposed. Experiments have demonstrated the practical of QC and have a significant development. However, the photons taking the information are incident to be influenced by the noise of environment, such as thermal fluctuation and the imperfection of the fiber. It affects the efficiency and security of communication, and plenty of methods have been proposed to solve the noise problem in QC. The decoherence-free subspace (DFS) is one of the most available methods based on special noise conditions. Collective-rotation noise and collective-dephasing noise are two basic effect of environment and form the general noise. We present quantum key distribution (QKD) schemes over the two kinds of collective-noise channel based on the DFSs. In the QKD schemes, each logical qubit, composed of two physical qubits which are in states of decoherence-free subspaces, is immune to a collective noise and can carry one bit of information in theory. Although the receiver should prepare entangled two-photon quantum systems, he can read out the information encoded by the sender with two unitary operations on two photons, resorting to only two single-photon measurements, not Bell-state measurements, which makes these protocols simpler than others in experiment. These QKD protocols are deterministic, not random, which makes the classical information exchanged be reduced largely. Also, they have a high intrinsic efficiency.