Online optimal exact identification of a quantum change point

TL;DR

This paper demonstrates that in quantum change point detection, optimal identification can be achieved with simple local measurements and minimal memory, matching the performance of more complex global strategies.

Contribution

It shows that local measurement protocols can attain the quantum optimal performance for change point detection, simplifying implementation.

Findings

Local measurements achieve optimal change point detection performance.

Optimal strategy requires only one bit of memory.

Method is feasible with current quantum technology.

Abstract

We consider online detection strategies for identifying a change point in a stream of quantum particles allegedly prepared in identical states. We show that the identification of the change point can be done without error via sequential local measurements while attaining the optimal performance bound set by quantum mechanics. In this way, we establish the task of exactly identifying a quantum change point as an instance where local protocols are as powerful as global ones. The optimal online detection strategy requires only one bit of memory between subsequent measurements, and it is amenable to experimental realization with current technology.