Indefinite Causal Order in a Quantum Switch

TL;DR

This paper demonstrates a photonic quantum switch that exhibits indefinite causal order, verified by a causal witness measurement significantly surpassing the classical bound, advancing understanding of quantum causality.

Contribution

The authors realize a photonic quantum switch with operations indistinguishable by position, providing experimental evidence of indefinite causal order using a causal witness.

Findings

Causal witness value measured at 18 standard deviations beyond the classical bound.

The setup avoids limitations of earlier implementations by ensuring operations are indistinguishable.

Experimental verification of indefinite causal order in a photonic system.

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.