Low noise frequency up-conversion SPD demonstrated
ITL announces new frequency up-conversion technique:
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.
Outline of the NIST frequency Up-Conversion module
The figure above shows the configuration of the up-conversion detector. An amplified 1557-nm optical pulse train is used to pump two quantum channels. An optical filter, FLT0, is used to suppress noise from the pump. After proper polarization alignment, the pump signals are combined with the 1306-nm quantum signal and sent into periodically-polling LiNbO3 waveguide (PPLN). Through the sum-frequency generation process inside PPLN the quantum signal is up-converted to 710 nm. The output of PPLN is then efficiently detected by Si-APD once its noise is filtered off by the filters FLT1/2.
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.