Determining the exact number of photons in a very weak light pulse is important for performing reliable experiments in quantum optics and computing. A paper published online this week in Nature Photonics reveals a design for ‘photon-number-resolving detectors’ that is simple, efficient and compatible with cost-effective mass production.
So far, photon-number-resolving detectors have been complex and suffered from issues such as a poor efficiency, the need for cooling to cryogenic temperatures, or a slow response time. Andrew Shields and co-workers report a specially adapted avalanche-photodiode detector, showing that it is able to discriminate between weak laser pulses containing different numbers of photons, such as 0, 1, 2 or 3, and fractional averages within this range.
Avalanche photodiodes are routinely used to detect weak light signals. They rely on an avalanche multiplication effect in which a single photon generates a large number of electrons, and therefore an electrical current that is easy to detect. Usually this avalanche effect masks the actual number of photons received and makes determining the exact number received impossible. The researchers solved this problem by implementing an electronic circuit that measures the avalanche currents at a much earlier stage in their development, allowing the number of photons striking the detector to be determined.