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For fiber-based QKD over transmission distances longer than 10 km, the wavelength of the quantum signal must be in the low-loss band of telecommunications fiber, usually around 1310 nm or 1550 nm. The available single-photon detectors that are directly sensitive to these wavelengths are InGaAs avalanche photodiodes (APDs) and superconducting single-photon detectors. Due to strong after-pulsing effects, InGaAs APDs are usually operated in a gated mode, typically limiting the clock rate of the system to several MHz. As a result, the key rate is also limited. Superconducting single-photon detectors can operate in the free-running mode and their only limitation to the sifted-key rate is the dead time, usually below 10 ns. Moreover, the time response of superconducting single-photon detectors can be less than 100 ps. However superconducting single-photon detectors are not generally available and need to be operated at low temperatures (typically 4 K). In contrast, silicon-based APDs (Si-APDs) are readily available and easy to operate. Their dead time is approximately 50 ns and their timing resolution is 300 ps or less [10, 11]. Unfortunately, while the peak detection efficiency of Si-APD can be as high as 70% around 650 nm their detection efficiency decreases rapidly at wavelengths longer than 1000 nm.
To resolve this complication, we have applied sum frequency generation to up-convert the transmitted photons from the low-loss fiber wavelengths to wavelengths where they can be efficiently detected by Si-APDs. Using periodically poled LiNbO3 (PPLN) the internal sum frequency conversion can be achieved with nearly 100% efficiency. The overall detection efficiency is 20%. Such an up-conversion single-photon detector is used in our 1310-nm QKD system and good performance is achieved.
When compared with other up-conversion detectors, ours has the advantage of low dark count rate. Most of the dark counts are induced by the strong pump via Raman-Stokes effects. When we set the signal wavelength shorter than the pump wavelength, the Raman-Stokes effects are greatly reduced. Moreover, we modulate the pump to a pulse train that is synchronous to the quantum signal. By this, the dark count rate is further reduced.
H. Xu, L. Ma and X. Tang. "Low noise PPLN-based single photon detector" Proceedings of SPIE, Vol.6780, pp. 6780OU.
H. Xu, L. Ma, A. Mink, B. Hershman and X. Tang. "1310-nm quantum key distribution system with up-conversion pump wavelength at 1550 nm". Optics Express, Vol. 15, Issue 12, pp. 7247-7260.