Fiber Loop Interferometer For Quantum Cryptography

Marek Czachor

Chunyuan Zhou, Heping Zeng

We introduce a single-photon Sagnac interferometer for quantum cryptography using time-division phase modulation. The interferometer was arranged to allow coherent laser pulses to travel clockwise and counter-clockwise along the same fiber loop with the corresponding phases modulated in separated periods, interfering at the exit coupler. High visibility of the single-photon interference was observed over a long-distance Sagnac loop. Stable performance was realized by passive compensation of stress and temperature-dependent drifts of the fiber-optic path. Time-division phase encoding and decoding can be realized by controlling the applied electric pulses on integrated phase modulators in the Sagnac loop, which makes this system suitable for a practical quantum cryptography system.

Yu‐Hui Luo

WUGuang, ZHOUChunyuan, ZENGHeping

We introduce a stable, long-distance single- photon Sagnac interferometer, which has a balanced configuration to efficiently compensate phase drift caused by change of the fiber-optic path. By using time-division phase modulation, single-photon interference was realized at 1550 nm in a 5-km-long as well as 27-km-long Sagnac fiber loops, with a fringe visibility higher than 90% and long-term stability. The stable performance of the single-photon interference indicated that the time-division phase-modulated Sag- nac interferometer might readily lead to practical applications in single-photon routing and quantum cryptography.

S. N. Molotkov

Three fundamentally new experimental prototypes of optical fiber quantum-cryptography schemes are described. They are appreciably simpler than the available prototypes, contain fewer optical fiber components, and do not require adjustment during key generation. These cryptosystems can naturally be called quantum time-shift cryptography. They realize the B92 protocol and use a pair of nonorthogonal single-photon states as information states. One such scheme does not use fiber-optic Mach-Zehnder interferometers and, therefore, provides natural realization of the multiplex secure-key distribution regime.

Romain Alléaume