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Normally, causal influence is assumed to go only one way-from cause to effect-and never back from the effect to the cause-the ringing of a bell does not cause the pressing of the button that triggered it. Now, researchers from the University of Oxford and the Université libre de Bruxelles have developed a theory of causality in quantum theory, according to which cause-effect relations can sometimes form cycles. This theory offers a novel understanding of exotic processes in which events do not have a definite causal order. The study has been published in Nature Communications.

An international research team from Germany and France has created structures in which light fields interact with electrons so strongly that the quantum vacuum itself is significantly altered. Using extremely short bursts of light, they interrupted this coupling much faster than the timescale of a vacuum fluctuation and observed an intriguing ringing of the emitted electromagnetic field, indicating the collapse of the vacuum state. Their key achievement could improve our understanding of the nature of nothingness-the vacuum of space itself, paving a way toward photonics exploiting vacuum fluctuations. The results are published in the current issue of Nature Photonics.