# Experimental nonlocal measurement of a product observable

## TL;DR

This paper demonstrates a feasible linear optics method for nonlocal measurement of a product observable, avoiding causality violations, and builds on recent theoretical proposals for such measurements.

## Contribution

It provides the first proof-of-principle experimental demonstration of nonlocal product observable measurement using linear optics, based on a recent theoretical framework.

## Key findings

- Successfully measured a nonlocal product observable without causality violation
- Implemented a quantum erasure scheme with linear optics for nonlocal measurement
- Validates the feasibility of nonlocal measurements in quantum systems

## Abstract

Nonlocal measurement, or instantaneous measurement of nonlocal observables, is a considerably difficult task even for a simple form of product observable since relativistic causality prohibits interaction between spacelike separate subsystems. Following a recent proposal for effectively creating the von Neumann measurement Hamiltonian of nonlocal observables [Brodutch and Cohen, Phys. Rev. Lett. 116, 070404 (2016)], here we report a proof-of-principle demonstration of nonlocally measuring a product observable using linear optics without the violation of relativistic causality. Our scheme provides a feasible approach to perform nonlocal measurements via quantum erasure with linear optics.