Optical quantum random number generation: applications of single-photon event timing

This dissertation is the result of research which, although electrical and computer engineering in nature, also aims to improve the performance of many systems in the field of quantum information. For example, random number generators are used in almost all areas of science, and the initial portion of this work details the theory, design, and characterization of two photon-arrival-time quantum random number generators (QRNGs). After the QRNGs were completed, it was realized that their performance was severely limited both by the maximum detection rate of the single-photon detectors used, and the precision at which the arrival times could be resolved. The single-photon detectors used for both QRNGs are single-photon avalanche photodiodes (SPADs), devices which when operated below their breakdown voltage can create a macroscopic amount of current (an avalanche) in response to a single incident photon. Some of this charge can become trapped in defects or impurities; if this trapped charge is released when the SPAD is active, a secondary ‘false’ detection event, or ‘afterpulse’ can occur. To lower the afterpulse probability to reasonable levels (< 1%), we attempted to reduce the amount of avalanche charge by halting its growth promptly with high-speed electronics, so that defects have a lower probability of becoming populated in the first place. Initial results show reductions in afterpulse probability by up to a factor of 12, corresponding to a ~20% decrease in dead time, a value that could be improved further. We developed an FPGA-based time-to-digital converter system for use specifically with SPADs, achieving a time-bin resolution of 100 ps, with lower dead time and higher maximum detection rate than all currently available detection systems. This further allowed for the creation of a new higher-order SPAD characterization technique, which was identified previously unknown subtleties to SPAD operation. Finally, we developed an ultra-low-latency QRNG, which was used in one of the recent loophole-free demonstrations of quantum nonlocality. The final latency was below 2.5 ns, to our knowledge the lowest latency QRNG to date. Of special interest, however, is our subsequent exploration into the characterization of its bit-probability drift using atomic clock stability techniques. By employing the Allan deviation and implementing precision feedback, the additional frequency drift caused by environmental fluctuations is reduced such that the resulting bit stream can pass cryptographic random number tests for sample sizes up to 5 Gb. This system is currently intended for the NIST random-number beacon, a world-wide trusted source of random bits