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Large Scale Energy Efficient Sensor Network Routing Using a Quantum Processor Unit

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Existence of processes violating causal inequalities on time-delocalised subsystems.

Julian Wechs^1,2, 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.

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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.