19 February 2014 Evaluation of the phase correlation between the optical pulses for transmission in quantum key distribution
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One of the key assumptions in the security proof of BB84 quantum key distribution (QKD) protocol with a weak coherent light source is that the phase of each pulse emitted from the source is random. If the phases correlated, an efficient eavesdropping method exists, and more sacrifice bits are necessary to guarantee the security of the protocol. The phase correlation between the adjacent pulses was examined experimentally, for the first time, by measuring the visibility of interference. An asymmetric Mach-Zehnder interferometer was used to observe the interference between adjacent pulses from a distributed feedback (DFB) laser diode driven by 10 GHz clock signal. Clear interference was observed when the DC bias current exceeds the laser threshold. This high visibility implies phase correlation between pulses. In contrast, only a low visibility, i.e. very weak phase correlation, was observed when the DC bias current was set far below the threshold. The phase correlation became stronger as the DC bias current increased. The experimental results imply that a QKD equipment implementing BB84 protocol with a coherent light source is secure as long as the laser is operated in gain-switch even for 10 GHz clock frequency.
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Toshiya Kobayashi, Toshiya Kobayashi, Akihisa Tomita, Akihisa Tomita, Atsushi Okamoto, Atsushi Okamoto, } "Evaluation of the phase correlation between the optical pulses for transmission in quantum key distribution", Proc. SPIE 8997, Advances in Photonics of Quantum Computing, Memory, and Communication VII, 899707 (19 February 2014); doi: 10.1117/12.2037594; https://doi.org/10.1117/12.2037594


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