Automatic detection efficiency measurements of Superconducting Nanowire Single Photon Detectors

Abstract

Superconducting nanowire single-photon detectors (SNSPDs) are characterized by their quantum limited ability to accurately detect single photons, with low jitter, high detection efficiency and low dark count rate. To achieve this, the detector is cooled to 2-3K, bringing the device in a superconductive state, and is then biased with a direct current (DC) close to its critical current. When a photon impinges the detector, the depairing of Cooper-pairs by the photon leads to local destruction of the superconductivity. The growth of this non-superconducting area, first across and then along the nanowire, leads to the development of a measurable resistance and hence the production of detection pulses. Increasing system detection efficiency (SDE) of detectors has been a long-term goal in the community. Recently ultrahigh efficiency detectors (SDE>98%) have been demonstrated. It has also been shown that the wavelengths dependence of SDE, typically defined by a quarter wavelength cavity, is modulated by fiber-detector airgap (Fabry–Pérot). Measuring such modulations and finding the optimal operation wavelength manually is a time consuming and tedious process. In this thesis a setup for automatic measurement of SDE versus wavelength was developed and benchmarked. For the tested detector, efficiencies were found ranging between 22% and 95% in the wavelengths range between 1260nm and 1650nm. For the optical circuit using Single Mode (SM) fibers only, the automated SDE measurements were unreliable due to shifts in polarisation during measurements. Using PM fibers led to efficiencies very similar to the values measured manually.