One at a Time

In 1801 Thomas Young let sunlight through two narrow openings and saw the light on the far wall fall into stripes, bright and dark, where a stream of particles should have made two patches. Light was a wave, and the stripes were where crests from the two openings met crests, and where they met troughs. For a century that settled it.

Then light turned out to arrive in lumps, and electrons — undeniably particles, with mass and charge — turned out to make the same stripes. In 1961 Claus Jönsson sent electrons through slits a few micrometres apart and photographed fringes. In 1989 Akira Tonomura's team at Hitachi, using an electron biprism in place of the slits, turned the beam down so far that only one electron was ever in the apparatus at a time, and filmed the screen. Each electron arrives as one dot, in one place, apparently at random. Keep going and the dots pile up into fringes. Readers of Physics World later voted it the most beautiful experiment in physics.

That is what this page reproduces, and the thing to watch is the surprise in it. No single dot shows a wave. There is nothing for one electron to interfere with except itself, and yet the dark fringes are places where it almost never lands, even though either slit on its own would happily send it there. The pattern only makes sense if whatever passed through the apparatus went through both slits. The second screen runs the identical stream with a detector at each slit that records which one every particle used. Nothing else changes, and the fringes do not appear: the dots make two plain bands, one behind each slit, with a visibility the counter shows sliding toward zero.

The same arithmetic runs much of modern technology and research. Electron microscopes and electron holography — Tonomura's own field — depend on electrons interfering with themselves. Neutrons, atoms and molecules of close to two thousand atoms have now made fringes. And the lesson of the watched screen, that gaining which-path information erases interference, is the working principle behind quantum cryptography, where any eavesdropper who learns which way a photon went leaves exactly this kind of mark.