A definitive version of the experiment which demonstrates what Richard Feynman called ‘the central mystery’ of quantum physics was carried out by a Japanese team at the end of the 1980s. This was at the time the ultimate version of the double-slit experiment (see here), using individual electrons to demonstrate both wave-particle duality and the holistic nature of the quantum world.
In the classic version of the double-slit experiment, light is sent through two holes and spreads out on the other side to make an interference pattern, proving that light is a wave. In the version developed by Akira Tonomura and his colleagues at the Hitachi research laboratories, individual electrons were fired one at a time past a very thin conducting wire at right angles to the path of the electrons to give them a choice of two routes past the wire. This setup is known as an electron biprism. On the other side of the wire there was a screen, essentially the same as a TV screen or a computer screen, on which each electron made a spot of light as it arrived. But the spots did not just flash on and off; each spot stayed on the screen as more electrons arrived. And the whole thing was recorded, so that the team had a movie showing the pattern that was building up on the screen as more and more electrons arrived and more spots were produced.
If electrons behaved like particles in the everyday world – like tennis balls, perhaps – you would not expect much of a pattern to build up at all. The balls that went one way round the wire would combine to make a blob on one side of the screen, and the balls that went the other way round the wire would combine to make a blob on the other side of the screen. But that is not what the Hitachi team saw. Each electron did indeed make a single spot on the screen, and at first these spots seemed to be distributed randomly across the screen. But as more spots arrived, they made a pattern. The pattern that built up was the typical stripy pattern produced by interference. Although the electrons started out as particles and arrived as particles, on their way through the experiment they seem to have behaved like waves, apparently interfering with each other even though only one electron was passing through the biprism at any time. They seemed to be ‘aware’ of the whole experimental setup, in both time and space.
The Hitachi team, who published their results in 1989, were not the only people to observe this phenomenon. Pier Giorgio Merli, Giulio Pozzi, and Gianfranco Missiroli carried out interference experiments with single electrons in Bologna in the 1970s. There has been some debate about who deserves priority, but the Japanese team seems to have dotted the ‘i’s and crossed the ‘t’s. Pozzi and his colleagues also carried out the first such experiment using an actual double slit and announced their results in 2008. As expected, they showed interference.
This work helped to encourage others to go a step further and build an experiment which not only literally uses two slits, instead of an electron biprism, to make electrons interfere, but in which the opening and closing of the slits can be controlled at will. After three years of work, a team headed by Herman Batelaan, of the University of Nebraska-Lincoln, announced their results in 2013. In their version of the experiment, electrons were fired one at a time at a wall made of a gold-coated silicon membrane. There were two slits in the wall, each 62 nanometres wide, with the centres of the slits 272 nanometres apart. One or both slits could be closed by a tiny sliding shutter whenever the experimenters chose. As usual, the electrons were captured on a screen after they had passed through the slits. Only about one electron was detected each second.
With just one slit open, they observed the build up of the same kind of blob that you would expect if electrons behaved like tennis balls. But with both slits open they saw an interference pattern, just as the Italian and Japanese teams had seen in their experiments. The electrons ‘knew’ how many slits were open.
Don’t worry if you cannot understand how this is possible. As Feynman said in his book, The Character of Physical Law, ‘I think I can safely say that nobody understands quantum mechanics.’ And he advised, ‘Do not keep saying to yourself, if you can possibly avoid it, “But how can it be like that?” because you will get “down the drain”, into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that.’
And yet, in spite of not understanding how quantum physics can be like that, scientists are able to put it to practical use, not least in the realm of quantum computing (see here).