Witnessing causal nonseparability

TL;DR

This paper demonstrates that the quantum switch exemplifies indefinite causal order using a new causal witness, advancing understanding of quantum resources beyond fixed causal structures, although it does not violate causal inequalities.

Contribution

It introduces the causal witness as a tool to detect causal nonseparability and establishes the quantum switch as a genuine example of indefinite causal order.

Findings

Quantum switch is a genuine indefinite causal order resource.

Causal witness can detect causal nonseparability.

Quantum switch does not violate causal inequalities.

Abstract

Our common understanding of the physical world deeply relies on the notion that events are ordered with respect to some time parameter, with past events serving as causes for future ones. Nonetheless, it was recently found that it is possible to formulate quantum mechanics without any reference to a global time or causal structure. The resulting framework includes new kinds of quantum resources that allow performing tasks - in particular, the violation of causal inequalities - which are impossible for events ordered according to a global causal order. However, no physical implementation of such resources is known. Here we show that a recently demonstrated resource for quantum computation - the quantum switch - is a genuine example of "indefinite causal order". We do this by introducing a new tool - the causal witness - which can detect the causal nonseparability of any quantum resource that is incompatible with a definite causal order. We show however that the quantum switch does not violate any causal nequality.