Quantum world defies causal order

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 combining general 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), say, then B cannot affect the outcome of A. This relationship is thought to break down in the quantum world because the temporal spread of a quantum particle’s wave function can exceed the separation in time between A and B. The causal order of A and B cannot therefore always be distinguished by, say, a photon.

In their experiment, Romero, Costa and colleagues created a “quantum switch”, in which a beam splitter sends photons of different polarizations along different paths. The photon source is diagonally polarized with respect to the beam splitter, which means there is a 50% chance that a photon will take either route. One path involves the photons 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 two paths are then recombined, and the polarization of the photons is measured. The operations A and B are designed so that if the system has definite causality, the order in which they are applied to the photons affects the polarization of the output photons.

However, when the team ran the experiment using several different operations for A and B, the measured polarization of the output photons was always consistent with there being no definite causal order of the application of A and B. Indeed, the measurements backed indefinite causal order to a statistical significance of 18σ – well beyond the 5σ considered a discovery in physics.

“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 (Phys. Rev. Lett. 121 090503).