Toshiba researchers develop photon counter

Dr Andrew Shields, leader of the Quantum Information Group, said: “A simple semiconductor device that can count the photons in a light signal is important for several quantum applications. We plan to apply the new device, in conjunction with our semiconductor photon source technology, in practical quantum communication gates that can extend the distance of quantum key distribution and in quantum computers based on photons.” Determining the number of photons in a light signal is of crucial importance for many of these applications. Quantum computers based on photons, for example, need to distinguish between one and two photons on each output. Another application is quantum teleportation, which may be used to send secret digital keys over longer distances than currently possible. According to the researchers, the lack of a suitable photon number resolving detector has been a major obstacle to the deployment of quantum technologies. Until now, the most common semiconductor detector – the avalanche photodiode – could only register the presence or absence of one or more photons. Toshiba’s detector, however, can count the number of individual photons in a pulse. The breakthrough has been enabled by a technique developed by Toshiba to detect weak photon induced avalanches. The electrical current caused by a single photon in a semiconductor is too weak to be detected quickly. Whilst avalanche photodiodes work amplify this tiny current, the strength of the final current does not depend on the number of photons that initiated it. The Toshiba device, by contrast, can detect photon induced avalanches that are 20 times weaker than conventionally and the strength of which scales linearly with the incident number of photons.