Quantum key distribution over 1 0 1 km of telecom fibre
C. Gobby, Zhiliang L. Yuan, Andrew J. Shields · 2003
Our system is based upon a Mach-Zender interferometer using phase modulation. Photons are generated by a 1.55 pm DFB pulsed laser diode operating with a repetition rate of 2 MHz. The pulses are strongly attenuated so that on average 0. I photons/clock cycle leaves the senders apparatus. Phase modulators. controlled by custom electronics, in the two interfering arms were used to encode the bit information. The signal is multiplexed with pulses from a 1.3 pm clock laser which serves as a timing reference. Cooled InGaAs avalanche photodiodes operating in Geiger mode are used to detect the single photons. It is imperative for operation over long fibres that the dark count rate in the single photon detector is as low as possible. Our detectors typically have a dark count probability of 3.2~10.~ er gate, along with a detection efficiency of 12% at 1.55 pm. This yields a Noise Equivalent Power of I .OXIO-'~ WHz- , whcch IS one of the lowest reported to date. R ' . The interferometer displayed a single photon interference fringe visibility between 99.1% and 99.7% for fibre lengths up to 65km. For longer fibre lengths the visibility falls, but remains around 96% at IOlkm, see Figure 1. The QBER, shown in Figure 2, remains virtually constant at around 3.6% for fibre length up to 50km, suggesting that the dominant contribution derives from inaccuracy in the phase modulation. Beyond 50km the QBER increases with fibre length, due to dark count noise. At 101 km, the QBER averaged over a 2-minute key transfer is typically 7. I %. This error rate is below the 1 I .5% limit, thus allowing us to perform error correction and privacy amplification to form a secure key. The sifted bit rate (shown in Figure 2) decreases with increasing fibre length at a rate of -0.20 dB/km, close to the specified value. At IOOkm the average bit rate was typically 15 Hz, sufficient to allow a cryptographic key to be transferred in several tens of seconds. 100