How can a photomultiplier tube detect a single photon?

A photomultiplier tube detects a single photon by leveraging the photoelectric effect to convert light into an electrical signal, which is then massively amplified through secondary emission.12

The process functions as follows:

• Initial Emission: When a photon strikes the tube's photosensitive cathode, it causes the emission of an electron.12
• Electron Multiplication: This initial electron is accelerated toward a series of electrodes known as dynodes, each held at a successively higher positive potential.1
• Cascading Amplification: Upon striking the first dynode, the electron triggers the release of several additional electrons. These electrons are then attracted to the next dynode, where the process repeats. This cascade continues through multiple stages, resulting in an enormous increase in the number of electrons.12
• Detection: By the time the electron stream reaches the final stage, the original signal has been amplified by up to 1,000,000 times, making it possible to detect the arrival of even a single photon.12