Xiaofan Mo

Faraday–Michelson system for quantum cryptography

Quantum key distribution provides unconditional security for communication. Unfortunately, current experiment schemes are not suitable for long-distance fiber transmission because of instability or backscattering. We present a uni-directional intrinsic-stabilization scheme that is based on Michelson-Faraday interferometers, in which reflectors are replaced with 90 degree Faraday mirrors. With the scheme, key exchange from Beijing to Tianjin over 125 kilometers with an average error rate is below 6% has been achieved and its limited distance exceeds 150 kilometers. Experimental result shows the system is insensitive to environment and can run over day and night without any break even in the noise workshop.Quantum key distribution provides unconditional security for communication. Unfortunately, current experimental schemes are not suitable for long-distance fiber transmission because of phase drift or Rayleigh backscattering. In this Letter we present a unidirectional intrinsically stable scheme that is based on Michelson-Faraday interferometers, in which ordinary mirrors are replaced with 90 degree Faraday mirrors. With the scheme, a demonstration setup was built and excellent stability of interference fringe visibility was achieved over a fiber length of 175 km. Through a 125 km long commercial communication fiber cable between Beijing and Tianjin, the key exchange was performed with a quantum bit-error rate of less than 6%, which is to our knowledge the longest reported quantum key distribution experiment under field conditions.

Field Experiment on a “Star Type” Metropolitan Quantum Key Distribution Network

Quantum key distribution (QKD) networks have recently attracted growing attention. The topology of the local QKD network is the basis of the next-generation global secure communication network. In this letter, we report a realization of a wavelength-routing star type QKD network which can span a metropolis using a commercial backbone optical fiber network without trusted relays. The longest and the shortest fiber lengths between two geographically separated nodes are 42.6 and 32 km, respectively, and the maximum average quantum bit-error rate is below 8%. A novel analysis model with experimental validation is also proposed to evaluate the users performance in this network under the condition of maximum multiuser crosstalk.

Stability of phase-modulated quantum key distribution systems

The stability of double Mach–Zehnder implementation has been investigated in experiment, which is important for practical quantum key distribution systems. The results of experiment show that the random fluctuation of interference visibility comes from polarization disturbance, especially from that of transmission fiber. A theoretical model is built and a necessary condition is given for intrinsic-stabilization unidirectional quantum key distribution system.

EXPERIMENTAL DEMONSTRATION OF THE PERFORMANCE OF QUANTUM KEY DISTRIBUTION SYSTEM AT 1550 nm

We analyze some of the critical factors that confine the transmission length for a quantum key distribution system under a certain Quantum Bit Error Rate (QBER). Emphasis was placed on the relationship between transmission length and QBER. Relationships between the minimum QBER and transmission length in theory for ideal M–Z interferometers and for the real case in theory according to the parameters of our system are given. The relationship between them as obtained by experiment was also given. The results showed that our system is very close to the performance of an ideal experimental system.

Experimental quantum cryptography distribution system

An experimental setup for quantum key distribution (QKD) in special optical fiber at the wavelength of 850nm was presented. The system employs the B92 protocol to establish a secret key between Alice and Bob. The key is encoded in the phase of very weak laser of average photon number 0.1 per pulse. The measured error rate is lower than 8%, the effective transmission rate is about 17bit/s.