Looking both ways: it’s a question of quantum realities RTimages/Alamy Stock Photo

Alice sent a present to Bob. No wait, Bob sent the present to Alice. Actually… they kind of sent it to each other at the same time. A new experiment shows how gift-giving gets confusing when you’re using quantum mechanics to muck about with causality.

You may have heard of the double-slit experiment, in which a single particle fired at two small gaps appears to interfere with itself, as if it had passed through both slits at once. That happens because, until it is measured by a detector on the other side, the particle is in a quantum superposition of two states. In some sense it is able to take both paths.


It’s weird, and difficult to wrap your head around, but now a team at the University of Vienna in Austria have performed a different kind of experiment that is even more mind-bending: putting the order of events into a superposition.

Normally, it’s easy for us to say that event A happens before event B, or vice versa. But Giulia Rubino and her colleagues have created a situation in which these seemingly contradictory scenarios are in superposition. “If you put together quantum mechanics and causal relations, a situation arises in which there is no pre-defined causal order,” she says. “It’s counter-intuitive.”

Their experiment involves sending a photon through two collections of optical devices, labelled Alice and Bob. These devices transform the quantum state of the photon in different ways, so that going through Alice, then Bob produces a different outcome to Bob, then Alice. “The fact that A is applied before B or B is applied before A actually changes the results,” says Rubino.

To picture how that works, imagine the photon is a present intended for a third party. Alice likes to wrap presents, while Bob prefers a simple ribbon tied into a bow. If Alice gets her hands on the present first, she wraps it and then passes it to Bob, who puts a bow on. If Bob gets it first, Alice’s wrapping covers the bow, resulting in a different outcome. Things are slightly more complicated for the photon, as Alice and Bob can perform different actions with a certain probability, so there are more than two possible outcomes.

Who’s first?

In the team’s experiment, a kind of quantum switch controls which path the photon takes, and thus the order in which Alice and Bob act. To mess with causality, they place this switch itself in a superposition, meaning that in a sense, both act first.

Of course, that’s not quite what’s happening, just as the particle in the double slit experiment doesn’t truly go through both slits at once – it’s just we don’t have the language to describe the truly weird nature of the quantum realm that bubbles beneath our layer of reality.

“Time itself might be undefined in these situations,” says team member Mateus Araújo. “The whole confusion with quantum mechanics is unfamiliarity, something that just doesn’t match our macroscopic, classical experience.”

What’s reality, any way

“We’re really pushing the mysteries and confusion of quantum physics to the absolute limit,” says Matty Hoban at the University of Oxford. “We don’t have a good picture of what reality is.”

But this experiment isn’t just a neat quantum party trick. We already know that causality confusion could theoretically help with some kinds of quantum communications and computation, reducing the number of resources needed to send messages or solve certain problems. In the future, the team want to realise this in an experiment. “We want to recreate the gain proposed in many theoretical papers and demonstrate this advantage,” says Rubino.

Whether that turns out to be useful remains to be seen, says Hoban, as the computational problems in question are fairly esoteric and not directly related to real-world tasks. “It’s not clear if we have a killer application, but it’s very interesting that you can get improvements.”

Journal reference: arxiv.org/abs/1608.01683