Research Of Quantum Cryptographic System And Its Key Techniques

The research of quantum information has shown that if one encodes messages onto quantum states, he will be able to achive a perfect crytographic communication whose security is unconditionally guaranteed by the well-known uncertainty principle of quantum mechanics. This technique is called quantum cryptography or quantum key distribution.The main purpose of this work is to explore novel schemes and techniques for potentially practical quantum cryptography. And the experimental work here is concentrated on building up a long-distance quantum cryptosystem with long-term stability.In order to increase the system stability and the quantum key creation rate, we have proposed an innovative setup on the basis of a Sagnac-based quantum key distribution system which takes the advantage of time-division phase modulation to implement phase coding and decoding. It automatically compensates for phase drifts without any additional optic components.And we have also proposed for the first time a novel scheme of quantum key distribution using the techniques of Faraday mirror and differential phase shift. This scheme skillfully combines a phase shift sevosystem and the back-and-forth optical circuit design which enables it a quantum key creation efficiency as high as 2/3. And such a scheme can be readily extended to reach a higher key creation rate up to (n-l)/n, where n=3,4,5When stepping toward the goal of a practical quantum key distribution system, we have developed many key techniques for it.The single-photon detector is considered the most important part for quantum information discrimination. In this thesis, we also report several novel designs of electronic work for single-photon detection using InGaAs avalanche photodiodes (APDs). With one of these techniques, we have successfully built up a near-infrared single-photon detection module. The essential performance of dark count probability over quantum efficiency (Pd/η) is even one order lessthan that of the commercial product, which may lead to lower quantum-bit error rate and longer key distribution distance.By using capacitor balanced InGaAs/InP avalanche photodiodes, multi-gate detection of single photons at 1550 nm was achieved, with which we have experimentally demonstrated the efficient discrimination of single-photon timing by counting the single-photon clicks and the corresponding after-pulses within the multiple gates. The results show that multi-gated single-photon detection can be used for timing discrimination, which is of practical use in quantum key distribution and other researches based on temporal coding of quantum information.And we also report the first realization of a long-distance Sagnac single-photon interferometer, which showed a stable performance in 50 km optical fiber with a high visibility of 95%. By using the similar technique, we constructed the first long-distance Sagnac single-photon router.In our work, we also developed a high quality quantum random-number generator where we used polarized photons as the random seeds. And the technique of USB2.0 interface was used for data acquisition of qubits, which made it a "plug and play" device. The random number sampling rate can be as high as 5 MHz. As we measured the randomness of the experimental data, we found that its serial correlation coefficient was of 10 3 level, and its byte entropy was no less than 7.99.At last, we have successfully realized a 50.6 km long distance stable quantum key distribution system, in which we introduced high accuration electronic synchronization and a technique of polarization-independent phase modulation for a "plug and play" setup. The mean photon we used was less than 0.1 and the overall error rate of quantum key was only 4%, which guaranteed the security of quantum cryptographic communication. The whole system can stably work over more than 12 hours. And we have constructed the first fiber-optic quantum cryptography testbed in China with which we demonstrated cryptographic transimission of bit-map pictures. Our testbed system has been invited to attend the Shanghai...