Superconducting nanowire single photon detector at 532 nm and demonstration in satellite laser ranging
In this Optics Express article, Li et al. report on a new application for superconducting single photon detectors: precise laser ranging measurements on a satellite. The distance to the European satellite LARES, which is equipped with retroreflectors, was measured with a precision of about 9 mm by measuring the return trip of 532 nm laser pulses. The innovation lies in the detector, which is based on a superconducting nanowire, in contrast to avalanche photodiodes that are commonly used for these measurements. The team fabricated superconducting detectors optimized for detection at 532 nm for coupling to a multimode optical fiber. This was realized with detectors made on a high-reflectance substrate and whose active area was made large enough to match the core of a multimode optical fiber. The results were obtained by directing laser pulses with 1 kHz rep rate at LARES with a 21 cm telescope, the reflected pulses being collected with a 60 cm telescope and coupled to the detector via a multimode optical fiber. In the 420 seconds that the measurement lasted, 123 reflected photons were detected, yielding a distance measurement precision similar to that achieved with state-of-the-art silicon avalanche photodiodes. Several aspects of this experiment are very exciting: the time resolution can be far better than the 140 ps measured on these detectors (see for example the 19 ps time jitter achieved by Single Quantum with their Superconducting Single Photon Detectors), other wavelengths can be used where atmospheric scattering is reduced and operation is totally eye-safe, and the coupling efficiency to a telescope can still undergo large improvements. New exciting possibilities are opening up where high-performance single photon detectors and satellite ranging measurements with sub-mm resolution will enable new applications in geodynamics to monitor tiny movements of the Earth crust but also new fundamental experiments combining quantum optics and relativistic effects.