Quantum superpositions of causal orders as an operational resource

Understand

Causal nonseparability refers to processes where events take place in a coherent superposition of different causal orders.

• These may be the key resource for experimental violations of causal inequalities and have been recently identified as resources for concrete information-theoretic tasks.
• Here, we take a step forward by deriving a complete operational framework for causal nonseparability as a resource.
• Our first contribution is a formal definition of quantum control of causal orders, a stronger form of causal nonseparability (with the celebrated quantum switch as best-known example) where the causal orders of events for a target system are coherently controlled by a control system.