Single photon detectors which respond equally to one or more incident photons are relatively common. However, for many applications in quantum information technology we require a detector that can distinguish between different numbers of photons. We have realised a practical semiconductor device that can resolve the photon number in each incident light pulse.
A photon number resolving detector can be used to signal the successful operation of photonic gates used in quantum computers, as well as in quantum teleportation. Our detectors could also be used for quantum imaging and tomography, as well as the generation and characterisation of quantum light states. More generally, these detectors can make measurements limited only by the fundamental level of quantum noise, in low-light applications such as biomedical imaging, astronomy and optical range-finding.
Our detectors exploit small, unsaturated signals from avalanche photodiodes. Avalanche photodiodes are semiconductor devices which allow a single photon to generate a large photocurrent via avalanche multiplication, much like a single snowflake triggering an avalanche of snow. In a single photon detector this charge will grow until it saturates the device, giving a fixed output regardless of the number of incident photons. In our photon number resolving detectors we prevent this from happening by gating the detector, which limits the time for avalanche growth to less than 1 nanosecond. The output signal is proportional to the number of avalanches, which can be clearly discriminated, allowing the photon number to be determined. We have demonstrated this principle in uniform detectors [1,2] as well as using spatially-multiplexed devices, in which avalanches generated in separate zones within a single small-area diode are summed to give the photon number .
Because these detectors operate close to room temperature, are compact, scalable and simple to fabricate, our approach is ideal for a wide range of applications in quantum photonics.
 B. E. Kardynal, Z. L. Yuan and A. J. Shields, Nature Photonics 2, 425-428 (2008)
 O. Thomas, Z. L. Yuan, J. F. Dynes, A. W. Sharpe and A. J. Shields, Appl. Phys. Lett. 97, 031102 (2010)
 O. Thomas, Z. L. Yuan and A. J. Shields, Nature Communications 3, 644 (2012)