Indefinite Causal Order in a Quantum Switch
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, and , 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.
Cite
@article{arxiv.1803.04302,
title = {Indefinite Causal Order in a Quantum Switch},
author = {K. Goswami and C. Giarmatzi and M. Kewming and F. Costa and C. Branciard and J. Romero and A. G. White},
journal= {arXiv preprint arXiv:1803.04302},
year = {2018}
}