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Existence of processes violating causal inequalities on time-delocalised subsystems
Julian Wechs1,2, Cyril Branciard3, Ognyan Oreshkov4
1QuIC, Ecole Polytechnique de Bruxelles, C.P. 165, Université Libre de Bruxelles, 1050, Brussels, Belgium. julian.wechs@ulb.be.
Nature Communications
|March 17, 2023
Summary
This study demonstrates that processes violating definite causal order can be realized using time-delocalized quantum subsystems. This provides a practical framework for exploring quantum and classical processes without a fixed causal sequence.
Area of Science:
- Quantum Information Theory
- Foundations of Quantum Mechanics
- Quantum Causality
Background:
- Theoretical possibility of quantum and classical processes lacking well-defined causal order.
- Open question regarding the practical realization of such processes.
- Introduction of time-delocalized quantum subsystems as a framework.
Purpose of the Study:
- To demonstrate the realizability of processes violating causal order using time-delocalized subsystems.
- To analyze unitary extensions of tripartite processes within this framework.
- To investigate the implications for causal inequalities and the concept of definite causal order.
Main Methods:
- Analysis of unitary extensions of tripartite processes.
- Application of the time-delocalized quantum subsystem concept.
- Study of a specific tripartite classical process with a unitary extension.
Main Results:
- Realizations on time-delocalized subsystems exist for all unitary extensions of tripartite processes.
- This includes processes that violate causal inequalities in a device-independent manner.
- A known tripartite classical process with a unitary extension is shown to be realizable.
Conclusions:
- The findings confirm the existence of practical realizations for processes that challenge definite causal order.
- The study validates the meaningfulness of causal inequalities in demonstrating the absence of definite causal order.
- This work bridges theoretical concepts of quantum causality with practical quantum information processing.
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