Single photon detection

Can be used to detect single photons.

Richard Feynman likes them, he describes the tube at Richard Feynman Quantum Electrodynamics Lecture at University of Auckland (1979) at one point.

It uses the photoelectric effect multiple times to produce a chain reaction. In particular, as mentioned at youtu.be/5V8VCFkAd0A?t=74 from Video 1. "Using a Photomultiplier to Detect single photons by Huygens Optics" this means that the device has a lowest sensitive light frequency, beyond which photons don't have enough energy to eject any electrons.

Here is a vendor showcasing their device. They claim in that video that a single photon is produced and detected:

Concrete device described at: Video "How to use an SiPM - Experiment Video by SensLTech (2018)".