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arXiv · 1803.04302

Indefinite Causal Order in a Quantum Switch

K. Goswami·C. Giarmatzi·M. Kewming·F. Costa·C. Branciard·J. Romero·A. G. White
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Abstract

In quantum mechanics events can happen in no definite causal order: in practice this can be verified by measuring a causal witness, in the same way that an entanglement witness verifies entanglement. Indefinite causal order can be observed in a quantum switch, where two operations act in a quantum superposition of the two possible orders. Here we realise a photonic quantum switch, where polarisation coherently controls the order of two operations, $\hat{A}$ and $\hat{B}$, on the transverse spatial mode of the photons. Our setup avoids the limitations of earlier implementations: the operations cannot be distinguished by spatial or temporal position. We show that our quantum switch has no definite causal order, by constructing a causal witness and measuring its value to be 18 standard deviations beyond the definite-order bound.

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K. Goswami, C. Giarmatzi, M. Kewming, F. Costa, C. Branciard, J. Romero, A. G. White. 2018-03-12. Indefinite Causal Order in a Quantum Switch. https://doi.org/10.1103/physrevlett.121.090503

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