An advanced version of the famous double-slit experiment has directly measured a single photon in two places at once – or at least, that’s the claim made by a team of physicists who say these results could destroy the concept of a multiverse. This interpretation remains highly contested, however, with other physicists arguing that the experiment can’t really tell us anything new about the nature of reality.
The double-slit experiment, first performed in 1801, has played a key role in the development of quantum mechanics. It shows that when light is shone through two thin slits, it produces a wave-like interference pattern on the other side. Bizarrely, this occurs even when particles of light, called photons, are fired through one by one, with seemingly no chance of interfering with each other.
Many physicists interpret this fact as evidence that even a single photon has a wave-like quality, which can be described by its wave function, a mathematical construct that describes all possible locations for the photon, smeared across space. In some sense, this wave-like nature allows a single photon to travel through both slits at once.
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But mysteriously, placing a detector at each slit in an attempt to pin down which one the photon passes through destroys the interference pattern. The conventional view is that this is the result of the wave function “collapsing” from a measurement and localising in space, restricting the photon’s ability to pass through both slits. But the true nature of the wave function – whether it really exists or is just a mathematical description of reality – is a highly contentious topic.
For instance, some physicists argue for a “many-worlds” interpretation, where a superposition of possible universes exist on top of each other, each of which contains photons moving through different paths, and both of these paths can interfere with each other. A detector set up at one of the slits will cause reality to fork and choose one of these universes from the possible multiverse.
But now, at Hiroshima University in Japan and his colleagues claim that they have direct evidence of this photon travelling through both slits, using a more complex version of the double-slit experiment. This shows that the wave function is less of a mathematical tool and closer to what is really happening, says Hofmann.
“Previously, the assumption was that it’s a speculation. You don’t know what happens to the particle,” he says. “This [experiment] really makes this totally new and even a bit provocative, because what we are saying is that there is evidence for a physical delocalisation, and it’s not a speculation, it’s experimental evidence.”
Hofmann and his team use an interferometer, which splits a photon’s wave function between two paths using a type of mirror, before both paths meet again at an exit, where two detectors measure the photon’s interference pattern. Similar to the double-slit experiment, this interference suggests the photon has travelled down both arms, but again it isn’t possible to measure which path exactly without upsetting the wave function.
To get around this, Hofmann and his team used a technique called weak measurement, which makes very faint recordings of a particle’s properties without causing it to collapse and repeats an experiment many times, building up a statistical picture of the particle. Here, they added a glass plate that slightly twists the photon, changing what is known as its polarisation, to each interferometer arm. The plates work in opposite directions for each path, meaning that if the photon truly did travel down both paths, then these twists would cancel out when measured at the end.
Indeed, by measuring the photon’s polarisation at the two exits and comparing how often the polarisation changed in each one over many runs of the experiment, Hofmann and his team found their results matched a scenario where a single photon delocalised and travelled down both arms.
“What we are claiming here is that the rate at which the photon flips its polarisation is a direct measure of the concept of delocalisation,” says Hofmann. “If the photon delocalises, this flip rate goes down; that’s a direct physical effect of delocalisation.”
The fact that the team could perform this measurement challenges the many-worlds interpretation of quantum mechanics, says Hofmann, because it removes the need for a superposition of different universes. “A superposition should not be confused with simultaneous parallel realities of any kind. In our case, I think we have actual evidence that this is not the case, because we are seeing an effect that corresponds to a distribution of a single photon.”
at Newcastle University, UK, says this could make it slightly more difficult for some physicists to argue that the wave function is a mathematical smokescreen for what is going on. “It makes it harder to believe that quantum mechanics is all just epistemic and probability distributions about real, normal things that behave like we expect them to.”
But at Tel Aviv University in Israel argues that these results can still make sense within a many-worlds interpretation, because we are only seeing the delocalisation of the photon in one possible branch of reality – there could be another branch that sees the photon travel down one path or another, which we don’t see.
“In a parallel world, [the photon] was found in another output port of the interferometer, and when it’s found in another output port, in this other world the photon was in a different arm and had a different presence,” says Vaidman.
More fundamentally, the concept of weak measurements is hotly debated by physicists, with some arguing that you can’t use repeated statistical measurements to infer properties about single particles. “I think you can’t make claims about a single photon with this,” says at Chapman University in California.
“I do expect disagreements, because we are stepping on a lot of people’s feet by actually taking sides in interpretation issues and claiming that interpretation issues could be solved by experiment,” says Hofmann. “The problem has always been that we had peace between the different interpretation camps because there was an agreement that nobody can decide, and we are claiming that experimental tests are possible.”
arXiv
