Indefinite Causal Orders from Superpositions in Time

Abstract

Treating the time of an event as a quantum variable, we derive a scheme in which superpositions in time are used to perform operations in an indefinite causal order. We use some aspects of a recently developed space-time-symmetric formalism of events. We propose a specific implementation of the scheme and recover the Quantum SWITCH, where quantum operations are performed in an order which is entangled with the state of a control qubit. Our scheme does not rely on any exotic quantum gravitational effect, but instead on phenomena which are naturally fuzzy in time, such as the decay of an excited atom.

arXiv comment5 pages, 2 figures, comments and emails welcome (v2: minor address amendments and corrections)

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• Embedding cyclic information-theoretic structures in acyclic spacetimes: no-go results for indefinite causality
• Fundamental limits for realising quantum processes in spacetime
• Quantum switch instabilities with an open control
• Indefinite causal order and quantum coordinates
• Quantum Supermaps are Characterized by Locality
• Correlations in a quantum switch-based heat engine with measurements: A proof-of-principle demonstration