While solid-state detectors, such as avalanche photodiodes, are starting to put the squeeze on photomultiplier tubes and other older single-photon detectors, still newer technology is making progress. Physicists at Toshiba Research Europe and the University of Cambridge have developed a novel single-photon detector based on a quantum dot - a tiny silicon disc measuring only 30 x 8 nm2.
The dot's size makes it like an artificial atom (albeit with a very non-Coulomb potential) with the usual quantized electron energy states. The quantum dot is encased within a resonant tunnelling diode designed so that under normal circumstances no current can flow through it. If, however, the dot absorbs a photon, tunnelling becomes allowed and a current flows. The measured quantum efficiency is around 12% but is expected to reach 65%.
Further reading
J C Blakesley et al. 2005 Phys. Rev. Lett. 94 067401.