NIST Up-Conversion Detector Achieves Ultra Low Noise Level
The ITL Quantum Communications research group recently published details showing significant performance improvements in frequency up-conversion technology. The group demonstrated an improved frequency up-conversion detector with ultra low dark count (i.e. noise). The dark count rate of this detector is lower than 100 counts/second at 10% detection efficiency.
Due to the poor performance of InGaAs-based single photon detectors in the near infrared (NIR) range, frequency up-conversion detectors have been a better alternative for efficient detection of single photons in the NIR range. The dark count rate of these up-conversion detectors typically ranges from several thousands to several tens of thousands per second. For some applications, such as high speed quantum key distribution (QKD) systems, this level of dark counts is acceptable, and these detectors have been successfully used into fiber-based QKD systems. However, for other applications in quantum optics, where low light signals are only a few thousand photons per second or less, such high dark count rate can limit the effectiveness of an up-conversion detector. A dark count rate at several hundreds and even tens of counts per second is desired.
This up-conversion detector is designed to detect single photons at 1310 nm, one of the standard telecom wavelengths. A 1310-nm photon is up-converted to 710 nm in a PPLN waveguide pumped by 1550-nm laser and then detected by a Si-APD. In this detector, we use a Si-APD with a low dark count rate (Perkin Elmer SPCM-AQR-16), and its intrinsic dark count rate is less than 25 Hz. The noise from the pump was filtered by the WDM couplers and the holographic filter after the waveguide. The Raman scattering caused by the strong pump is the major noise source. Because the anti-Stokes component of the Raman process is much weaker than the Stokes component, we use a pump at a wavelength longer than the signal wavelength. Although the longer wavelength pump can reduce the dark count rate significantly, the dark count rate is still thousands of counts per second. The wavelengths of the noise photons are spectrally broadened and flat and are evenly distributed around the narrow signal spectrum at 710 nm. Therefore, a narrow filter can help to further reduce the dark count rate. We use a holographic band-pass filter after the waveguide. By adjusting the distance between the holographic grating and the iris, the bandwidth of the filter is about 2 nm.
CONTACT: Xiao Tang (ITL), ext. 2503