In the everyday world, events occur in a definite order—your alarm clock rings before you wake up, or vice versa. However, a new experiment shows that when fiddling with a photon, it can be impossible to say in which order two events occur, obliterating our common sense notion of before and after and, potentially, muddying the concept of causality. Known as a quantum switch, the setup could provide a useful new tool in budding quantum information technologies.
Quantum mechanics already torpedoes our notion that an object can be in only one place at a time. Thanks to the weirdness of quantum mechanics, a tiny particle like an electron can be in multiple places at once. The quantum switch achieves something similar for two events, A and B, showing that A can occur before B and B can occur before A.
“I’m very excited to see people realizing our idea with an actual experiment,” says Giulio Chiribella of the University of Oxford in the United Kingdom, one of the theorists who in 2009 first proposed the concept.