Experiments on quantum causality

TL;DR

This paper reviews experiments on quantum causality, focusing on the quantum switch, which demonstrates indefinite causal order with potential implications for quantum foundations and practical quantum technologies.

Contribution

It provides a comprehensive overview of the theory and experimental realization of indefinite causal order, especially through the quantum switch, highlighting recent advances.

Findings

Experimental realization of the quantum switch

Demonstration of indefinite causal order in practice

Potential applications in quantum communication and computation

Abstract

Quantum causality extends the conventional notion of fixed causal structure by allowing channels and operations to act in an indefinite causal order. The importance of such an indefinite causal order ranges from the foundational---e.g. towards a theory of quantum gravity---to the applied---e.g. for advantages in communication and computation. In this review, we will walk through the basic theory of indefinite causal order and focus on experiments that rely on a physically realisable indefinite causal ordered process---the quantum switch.