quantachapter 7 · light, one piece at a time7.2

one photon at a time

send light through two narrow slits and a pattern of bright and dark bands appears on the screen behind them. the bands come from waves: where the two paths arrive in step they add, and where they arrive out of step they cancel.

now dim the source until only one photon is in flight at a time. each one lands as a single dot, in a place nobody can predict. yet after thousands of dots the same bands build up, as if every photon had passed through both slits at once.¹

two paths, one pattern

close one slit and the fine bands go. the dots still arrive one at a time, but they settle into a single broad glow behind the open slit. open it again and the bands come back, dark stripes included, in places where light from one slit alone would have landed.

no single dot carries the pattern. it only shows once enough of them have landed, which is why the experiment needs patience more than it needs light.