Albert Einstein is heading out for his daily stroll and has to pass through two doorways. First he walks through the green door, and then through the red one. Or wait — did he go through the red first and then the green? It must have been one or the other. The events had have to happened in a sequence, right?
Not if Einstein were riding on one of the photons ricocheting through Philip Walther's lab at the University of Vienna. Walther's group has shown that it is impossible to say in which order these photons pass through a pair of gates as they zip around the lab. It's not that this information gets lost or jumbled — it simply doesn't exist. In Walther's experiments, there is no well-defined order of events.
This finding1 in 2015 made the quantum world seem even stranger than scientists had thought. Walther's experiments mash up causality: the idea that one thing leads to another. It is as if the physicists have scrambled the concept of time itself, so that it seems to run in two directions at once.
In everyday language, that sounds nonsensical. But within the mathematical formalism of quantum theory, ambiguity about causation emerges in a perfectly logical and consistent way. And by creating systems that lack a clear flow of cause and effect2, researchers now think they can tap into a rich realm of possibilities. Some suggest that they could boost the already phenomenal potential of quantum computing. “A quantum computer free from the constraints of a predefined causal structure might solve some problems faster than conventional quantum computers,” says quantum theorist Giulio Chiribella of the University of Hong Kong. (Emphasis added)