Single photon detection for quantum cryptography

Abstract

The nature of light at low-light-level has the peculiar characteristics. Based itself on this principle of physics, Quantum Cryptography suggests the possibility of secure and safe communication that are beyond the limits of classical computer science. Here, one of the most essential problems of quantum cryptography is to control single quanta: single photon. When the ideal single photon source is used in experiments on the quantum mechanics having non-classical properties and the quantum information, it is necessary to detect it. And it is important that such a detector provides high quantum efficiency and low dark counts. Detectors focusing on these factors have been developed at a few research groups and are commercially available. This thesis reports on photon counting experiment with the high sensitive detectors such as a photomultiplier tube and a avalanche photodiode in the near-infrared (NIR, 1~2μm wavelength). Important properties of the detectors were measured: Noise-Equivalent Power (NEP), Signal-to-Noise Ratio (S/N), Quantum Efficiency (QE), Dark Counts. The APD was operated with higher quantum efficiency (~10%) than PMT (~0.1%). In order to decrease dark counts, the APD used in Geiger-mode was operated under a gated-condition, where an AC voltage signal is added to the DC bias voltage upon triggering. The stable photon router based on fiber-optic Sagnac interferometer was experimentally demonstrated. To study the photon statistics, the total counts probability was measured for the two different optical source: below-threshold laser with Bose-Einstein distribution, and above-threshold laser with Poisson distribution.