single-photon interference
Single-photon interference is the demonstration that interference happens even when light is sent through an apparatus one photon at a time, so weakly that there is never more than a single photon inside it. Each photon registers as one discrete click on a detector — fully particle-like. Yet as click after click accumulates over minutes or hours, the dots arrange themselves into the bright-and-dark fringes of an interference pattern.
This rules out the comforting but wrong idea that interference is just photons jostling or pushing on one another. With only one photon present at a time, there is nothing else for it to interfere with except itself. The honest reading is that each photon's wavefunction passes through both arms of the interferometer, and the two contributions recombine; the probability of the photon being detected at a given spot is high where the contributions reinforce and low where they cancel.
Dirac captured this by saying that each photon interferes only with itself, and interference between two different photons never occurs in the simplest sense. The experiment is a clean, modern reprise of the double slit, and it makes the central quantum lesson unavoidable: a single quantum carries wave-like possibilities through the apparatus, but always arrives as one indivisible particle.
Each photon clicks once like a particle, yet the accumulated clicks trace out wave fringes.
Each photon interferes only with itself; the pattern is not built by photons interacting. And the photon is never split into pieces — it is detected as one whole quantum at a single place, with only the probabilities shaped like a wave.