A new experiment hints at surprising hidden mechanics of quantum superpositions

It is the central question in quantum mechanics, and no one knows the answer: What really happens in a superposition—the peculiar circumstance in which particles seem to be in two or more places or states at once? Now, in a forthcoming paper a team of researchers in Israel and Japan has proposed an experiment that could finally let us say something for sure about the nature of this puzzling phenomenon.

Their experiment, which the researchers say could be carried out within a few months, should enable scientists to sneak a glance at where an object—in this case a particle of light, called a photon—actually resides when it is placed in a superposition. And the researchers predict the answer will be even stranger and more shocking than “two places at once.”

The classic example of a superposition involves firing photons at two parallel slits in a barrier. One fundamental aspect of quantum mechanics is that tiny particles can behave like waves, so that those passing through one slit “interfere” with those going through the other, their wavy ripples either boosting or canceling one another to create a characteristic pattern on a detector screen. The odd thing, though, is this interference occurs even if only one particle is fired at a time. The particle seems somehow to pass through both slits at once, interfering with itself. That’s a superposition. And it gets weirder: Measuring which slit such a particle goes through will invariably indicate it only goes through one—but then the wavelike interference (the “quantumness,” if you will) vanishes. The very act of measurement seems to “collapse” the superposition. “We know something fishy is going on in a superposition,” says physicist Avshalom Elitzur of the Israeli Institute for Advanced Research. “But you’re not allowed to measure it. This is what makes quantum mechanics so diabolical.”

For decades researchers have stalled at this apparent impasse. They cannot say exactly what a superposition is without looking at it; but if they try to look at it, it disappears. One potential solution—developed by Elitzur’s former mentor, Israeli physicist Yakir Aharonov, now at Chapman University, and his collaborators—suggests a way to deduce something about quantum particles before measuring them. Aharonov’s approach is called the two-state-vector formalism (TSVF) of quantum mechanics, and postulates quantum events are in some sense determined by quantum states not just in the past—but also in the future. That is, the TSVF assumes quantum mechanics works the same way both forward and backward in time. From this perspective, causes can seem to propagate backward in time, occurring after their effects.