A new technique for phase tracking in quantum cryptography systems is proposed that adjusts phase in an optimal way, using only as many photon counts as necessary. We derive an upper bound on the number of photons that need to be registered during phase adjustment to achieve a given phase accuracy. It turns out that most quantum cryptosystems can successfully track phase on a single-photon level, entirely with software, without any additional hardware components or extensive phase-stabilization measures. The technique is tested experimentally on a quantum cryptosystem.
© 2004 Optical Society of America
(030.5260) Coherence and statistical optics : Photon counting
(040.5570) Detectors : Quantum detectors
(060.5060) Fiber optics and optical communications : Phase modulation
(120.5050) Instrumentation, measurement, and metrology : Phase measurement
(270.1670) Quantum optics : Coherent optical effects
Vadim Makarov, Alexei Brylevski, and Dag R. Hjelme, "Real-Time Phase Tracking in Single-Photon Interferometers," Appl. Opt. 43, 4385-4392 (2004)