Experimental Study Of Multi-user Quantum Communication Over Fiber Channel

Quantum communication, guaranteed by the law of quantum machnics,offers information-theoretical security for truly secret communication. With the development and maturity of quantum communication system, quantum communication has been experiencing from point-to-point laboratory demonstrations via short distance with low key rate to multi-user network via long distance with high key rate.The thesis foucuses on experimental study of multi-user quantum communication over fiber channel. Detailed topics are as blew:Two experimental quantum secret sharing schemes based on a single photon source have beed presented: 1) a circular quantum secret sharing scheme; 2) a quantum secret sharing scheme based on a single qubit. Without an additional compensating device, the two schemes stably and automatically compensate the birefringence in fiber. Futhermore, compared to previous schemes of a polarization insensitive modulator, our schemes easily implement a polarization insensitive modulator via simply eclectrial clock control,thanks to a structure that a polarization sensitive modulator tails a Faraday mirror. Experiments are performed to test the interfence and the visibility. Experimental results prove that our schemes are stable and practical.A novel quantum network scheme is proposed. The scheme utilizes a sagnac-type interferometer which contains Faraday mirrors and an optical switch. There are four advantages in the scheme: 1) the scheme is a full network in which a quantum key can be distributed between two arbitrary network users. 2) Two expensive single-photon detectors are located on the detecting user Charlie, and other users only need a phase modulator and a Faraday mirror. It dramatically reduces cost of network construction.3) Quantum signals are refected by a faraday mirror after transmitting over fibers so that the birrefrigence is auto-compensating. This structure easily implement a polarization insensitive phase modulator. 4) The scheme defends against all potential attacks using loopholes in a detector via an improved BB84-type protocol.For imperfect source preparation in a practical system, we present experimental realization of the loss-tolerant protocol, which is an effective way,by modifying a commercial plug&play QKD system. we experimentally show that with imperfect source encodings, it is still able to perform secure decoy QKD in exceeding 50km distancesA series of research on a Jones matrix of Faraday mirror in an auto-compensating system are accomplished. Its existing analysis has two drawbacks: only the auto-compensating process is analyzed and existing systems do not fully consider laser phase affected by a Faraday mirror. To improve these trawbacks, we present a detailed Jones-calculus analysis of the output after a light pulse transmitting in a plug& play quantum key distribution system. the same analysis is made to a home-made auto-compensating two-FMs system. More importantly, we show that theoretical and experimental results are different in the plug & play interferometric setup due to the fact that a conventional Jones matrix of FM neglected an additional phase 7r on alternative polarization direction. To resolve the above problem,we propose a new Jones matrix of an FM according to the coordinate rotation.