Practical Quantum Communication System And Its Key Techniques

Quantum Communication (QC), as a new cross-field of quantum physics and information science, provides an unconditional security for communication. The development of QC opens up a vast range of applications, including national defense, business and politics.The dissertation was focused on developing a practical quantum communication system. Besides novel experimental demonstration schemes, thanks to the recent improvement on the high speed single-photon detectors, polarization modulators and so forth, an entire quantum communication system was demonstrated.Stable quantum key distribution (QKD) in fiber with polarization encoding is always difficult to achieve due to the random fluctuation of polarization state caused by the birefringence of the optical fiber. Based on the classical polarization control, stable polarization feedback control at single-photon-level by analyzing photon count and pressing fiber was realized, and a polarization-encoding QKD with long-term stability was demonstrated. Furthermore, a QKD system with real-time polarization control was developed based on time division multiplexing. The polarization fluctuation was monitored by the reference pulses which arrived at the single photon detectors in the different temporal detection gate of the signal photons. By stabilizing the polarization of the reference photons, the polarization of the signal photons could maintain stable for a long time. The time division multiplexing technique implemented here contributed to non-interruption and high efficiency key generation.A novel scheme of an experimentally feasible fiber-based quantum direct communication was also presented. The protocol overcame the security problems in practical operation condition with the help of classical encryption and random redundancy. And a secret message was directly transmitted from Alice to Bob in a phase-encoding BB84 system for demonstration. In the process of developing a practical quantum communication system, ultrashort pulsed diode laser and high speed single photon detectors were developed. With these technological improvements, a practical "Plug-Play" QKD prototype and quantum telephone system was demonstrated. With a high speed processor based on FPGA, the quantum telephone system provided high efficiency and integration voice communication with unconditional security, which paves a way to the promising implementation of quantum cryptography in the current communication system.