Abstract
Avalanche photodiodes are widely used as practical detectors of single photons1. Although conventional devices respond to one or more photons, they cannot resolve the number of photons in the incident pulse or in a short time interval. However, photon-number-resolving detectors are urgently needed for applications in quantum computing2,3,4, communications5 and interferometry6, as well as for extending the applicability of quantum detection generally. Here we show that, contrary to current belief3,4, avalanche photodiodes are capable of detecting photon number, using a technique to measure very weak avalanches at the early stage of their development. Under such conditions the output signal from the avalanche photodiode is proportional to the number of photons in the incident pulse. As a compact, mass-manufactured device, operating without cryogens and at telecom wavelengths, it offers a practical solution for photon number detection. Determining the exact number of photons in a weak light pulse is an important requirement for many applications in quantum optics. Now, contrary to popular belief, Andrew Shields and colleagues have demonstrated that an avalanche-photodiode detector can perform the task.
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