New SPAD Control System: High Efficiency, Low Error Rate

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By employing a technique analogous to the operation of noise-canceling headphones, PML researchers have created an exquisitely sensitive, semiconductor-based, single-photon detection system that has the highest reported detection efficiency of any device of its type and is capable of operating at gigahertz frequencies with very low noise. Such capabilities are in critical demand as part of the worldwide effort to provide practical and provably secure communication systems that encode information in the quantum-mechanical states of individual photons to generate secret encryption keys. A widely used technology for detecting those photons is the single-photon avalanche diode (SPAD).* But the performance of SPADs has, to date, been a limiting factor in the overall encryption system. One issue with SPADs, particularly those sensitive to telecom wavelengths, is a process called afterpulsing, in which an actual photon detection can sometimes cause a second, false output at a time when no photon was present. To suppress this, electronic signals are applied to the detector, but these signals are many orders of magnitude larger than the tiny signal generated by a single photon and can make it difficult to identify a definitive “hit” amid the electrical interference. The PML team attacked these problems by devising a

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