Universal device for two-qubit entangled measurements via photonic quantum walks

TL;DR

This paper demonstrates a photonic quantum walk-based device that performs optimal two-qubit entangled measurements, significantly improving direction guessing accuracy even with imperfect states, showcasing versatility in quantum measurement implementation.

Contribution

It introduces a universal photonic device for two-qubit entangled measurements using quantum walks, enabling high-fidelity, optimal collective measurements with abstention to enhance quantum information tasks.

Findings

Achieved fidelities above 0.9850 for two-qubit measurements.

More than 10-fold improvement in direction guessing scores with abstention.

Implemented optimal five-output measurements via nine-step photonic quantum walks.

Abstract

Sophisticated quantum measurements are fundamental to obtain a quantum advantage in many informational problems. Here, we consider the task of guessing a direction encoded in a two-qubit pure state. We experimentally demonstrate that abstention can be used to recover optimal direction guessing (measured in terms of the fidelity and maximum likelihood scores) even from non ideal states. Our protocol uses nine-step photonic quantum walks to implement the optimal five-output two-qubit collective measurements with fidelities above 0.9850. Thanks to abstention, we obtain more than a 10-fold improvement of the direction guessing scores (in terms of deviation to the optimal guessing scores). Our work demonstrates the versatility of photonic quantum walks for implementing many-qubit sophisticated measurements.