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.
The quantum switch could have applications in budding technologies that, for example, manipulate and transmit information encoded in the quantum states of individual photons and other quantum particles. Such devices must pass particles through quantum channels, such as optical fibers, that invariably suffer from noise. But even if two such channels are too noisy to transmit quantum information, they could in principle be fashioned into a quantum switch to enable the information to flow, Jacquiline Romero, a quantum physicist and member of the Queensland team, says. “You introduce indefinite order and suddenly you can communicate,” she says. “That’s pretty cool!”
Quantum mechanics defies causal order, experiment confirms
An experiment has confirmed that quantum mechanics allows events to occur with no definite causal order. The work has been carried out by Jacqui Romero, Fabio Costa and colleagues at the University of Queensland in Australia, who say that gaining a better understanding of this indefinite causal order could offer a route towards a theory that combines Einstein’s general theory of relativity with quantum mechanics
In classical physics – and everyday life – there is a strict causal relationship between consecutive events. If a second event (B) happens after a first event (A), for example, then B cannot affect the outcome of A. This relationship, however, breaks down in quantum mechanics because the temporal spread of a particles’s wave function can be greater than the separation in time between A and B. This means that the causal order of A and B cannot be always be distinguished by a quantum particle such as a photon.
As well as making an experimental connection between relativity and quantum mechanics, the researchers point out that their quantum switch could find use in quantum technologies. “This is just a first proof of principle, but on a larger scale indefinite causal order can have real practical applications, like making computers more efficient or improving communication,” says Costa.