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The phase randomized light is one of the key assumptions in the securityproof of Bennett-Brassard 1984 (BB84) quantum key distribution (QKD) protocolimplemented with an attenuated laser. Though the assumption has been believedto be satisfied for conventional systems, it should be reexamined for currenthigh speed QKD systems. The phase correlation may be induced by the overlap ofthe optical pulses, the interval of which decreases as the clock frequency. Thephase randomness was investigated experimentally by measuring the visibility ofinterference. An asymmetric Mach-Zehnder interferometer was used to observe theinterference between adjacent pulses from a gain-switched distributed feedbacklaser diode driven at 10 GHz. Low visibility was observed when the minimumdrive current was set far below the threshold, while the interference emergedwhen the minimum drive current was close to the threshold. Theoreticalevaluation on the impact of the imperfect phase randomization provides targetvalues for the visibility to guarantee the phase randomness. The experimentaland theoretical results show that secure implementation of decoy BB84 protocolis achievable even for the 10-GHz clock frequency, by using the laser diodeunder proper operating conditions.

Evaluation of the phase randomness of the light source in quantum key distribution systems with an attenuated laser