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Optimal temporal filtering for COW-QKD

We present a procedure to optimize the quantum bit error rate in coherent one-way quantum key distribution (COW-QKD) system. We built the testbed for COW-QKD, which supported a clock rate of 1 GHz. Temporal filtering was realized by varying gate delays applied to the single-photon detector and optimal selection of time window to receive logic bits. We observed that with adjustable temporal-filtering, we can improve on the quantum bit error rate, reducing it to 11.6%. The sifted key rate drops to less than 1 kbps, when we extend this QKD protocol over a distance of 150 km.