Photo-detectors are devices which detect individual photons, or particles of light. They play a central role in any sort of quantum optics experiment, where ultimately we rely on photo-detection to perform measurements. They also play a fundamental role in linear optics quantum computing, my area of research.

Theoreticians typically treat photo-detectors in a highly idealized manner, which excludes many important effects. In this paper we present more advanced models for photo-detectors which include such effects, allowing for more realistic modeling of photo-detection in quantum optics. Specifically, the effects we discuss are:

• Detector efficiency: When a photon hits a photo-detector the detector doesn’t necessarily trigger. Instead, the detector might miss the photon and think that there are fewer photons than there actually are.
• Dark-counts: The opposite effect, where a photo-detector accidentally triggers even if there is no photon.
• Dead-time: When a photo-detector triggers, it typically becomes inactive for a short period of time. During this dead-time the detector effectively has zero efficiency.
• Bandwidth: Light comes in many different frequencies. Any detector will only respond to a certain range of frequencies, referred to as the spectral bandwidth.
• Resolution: When a photo-detector detects a photon, we would ideally like to have as much information about the photon as possible. For example, we would like to know exactly what the frequency of the photon was. In practice, detectors have finite resolution, which limits how much information about a photon’s frequency the detector can extract.
• Imperfect information: Aside from the restrictions due to resolution, a detector may not faithfully convey all the information it has about the detected photon to the user. For example, even if the detector obtains very precise information about the frequency of a photon, it may not be accessible to us.

All of these phenomena have very significant effects in quantum optics experiments and can completely change the behaviour of quantum optical systems, which is why having accurate models is important.