We describe the application of high-speed, high-efficiency single-photon detectors using sinusoidally gated InGaAs/InP APDs to the polarization entanglement distribution at 1550 nm. A distribution rate of 0.4 kHz was obtained over 5.3-dB channel loss.
A single-photon detector (SPD) at 1550 nm is essential for quantum key distribution (QKD) based on optical fiber links. A cooled InGaAs/InP avalanche photodiode (APD) is a candidate for the SPD, because it has a high detection efficiency with a low dark count probability. However, the maximum detection speed was very slow (<10 MHz), which is due to the afterpulsing phenomenon. Recently we proposed the sinusoidal gating scheme and demonstrated the high-speed (500-800MHz) single-photon detection with a low afterpulsing probability. In the scheme, the overall afterpulsing probability is somewhat high when the gating frequency is lower than 100MHz. In this paper, we report on the afterpulsing-free (<1%) 80MHz single-photon detection using the sinusoidal gating. The 80MHz mode-locked titanium sapphire laser emits ~90 fs optical pulses at 810 nm. The wavelength of the optical pulses is converted into 1550 nm by the optical parametric oscillator (OPO). The 1550 nm optical pulses (~120 fs) are attenuated to the single-photon level and led to the APD (EPITAXX EPM239BA) cooled to -35 degrees Celsius. The cooled APD was operated with sinusoidal gating.
We have demonstrated quantum key distribution (QKD) over 100 km using single-photon detectors based on InGaAs/InP avalanche photodiodes (APDs). We implemented the differential phase shift QKD (DPS-QKD) protocol with electrically cooled and 2-GHz sinusoidally gated APDs. The single-photon detector has a dark count probability of 2.8 × 10(-8) (55 counts per second) with a detection efficiency of 6 %, which enabled us to achieve 24 kbit/s secure key rate over 100 km of optical fiber. The DPS-QKD system offers better performances in a practical way than those achieved using superconducting single-photon detectors. Moreover, the distance that secure keys against the general individual attacks can be distributed has been extended to 160 km.