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One thing leads to another. It sounds obvious, but in the quantum realm, the saying doesn’t always ring true. A new quantum device can jumble up a sequence of two events so that they take place in both orders simultaneously, researchers report in a paper in press in Physical Review Letters.

“In everyday life, we are used to thinking of events having a definite order,” says physicist Jacqui Romero of the University of Queensland in Australia. For example, in the morning, you might brush your teeth before washing your face, or vice versa. But in the quantum realm, both can be true simultaneously.

The device, known as a quantum switch, works by putting particles of light through a series of two operations — labeled A and B — that alter the shape of the light. These photons can travel along two separate paths to A and B. Along one path, A happens before B, and on the other, B happens before A.

Which path the photon takes is determined by its polarization, the direction in which its electromagnetic waves wiggle — up and down or side to side. Photons that have horizontal polarization experience operation A first, and those with vertical polarization experience B first.

But, thanks to the counterintuitive quantum property of superposition, the photon can be both horizontally and vertically polarized at once. In that case, the light experiences both A before B, and B before A, Romero and colleagues report.

The results mark “the first steps toward controlling a new regime of quantum physics,” says physicist Giulio Chiribella of the University of Oxford and the University of Hong Kong, whose team came up with the quantum switch idea but who was not involved with the experiment.

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Category: Science