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.

In their experiment, Romero, Costa and colleagues created a “quantum switch”, in which photons can take two paths. One path involves being subjected to operation A before operation B, while in the other path B occurs before A. The order in which the operations are performed is determined by the initial polarization of the photon as it enters the switch.

The experiment involves using a polarizing beam splitter, which sends photons of different polarizations along different paths. The photon source is diagonally polarized with respect to the beam splitter, which means that there is a 50% chance that a photon will take either route.

Out of order

The two paths are then recombined, and the polarization of the photons are measured. The operations A and B are designed such that the order in which they are applied to the photons affects the polarization of the output photons – if the system has definite causality.

The team did the experiment using several different types of operation for A and B and in all cases they found that the measured polarization of the output photons was consistent with their being no definite causal order between when A and B was applied. Indeed, the measurements backed indefinite causal order to a whopping statistical significance of 18σ – well beyond the 5σ threshold that is considered a discovery in physics.

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.

The research is described in Physical Review Letters.