# Measurements of nonlocal variables and demonstration of the failure of the product rule for a pre- and postselected pair of photons

## TL;DR

This paper demonstrates the first experimental implementation of nonlocal variable measurements in pre- and postselected photons, revealing the failure of the product rule in quantum systems through strong measurements.

## Contribution

It introduces a novel method for measuring nonlocal variables in pre- and postselected photons, showcasing the failure of the product rule with high-precision, strong measurements.

## Key findings

- Successful implementation of von Neumann measurements of nonlocal variables
- Verification of the failure of the product rule in quantum systems
- High-precision, nondemolition measurements suitable for quantum information protocols

## Abstract

We report the first implementation of the von Neumann instantaneous measurements of nonlocal variables which becomes possible due to technological achievements in creating hyperentangled photons. Tests of reliability and of the nondemolition property of the measurements have been performed with high precision showing the suitability of the scheme as a basic ingredient of numerous quantum information protocols. The method allows to demonstrate for the first time with strong measurements a special feature of pre- and postselected quantum systems: the failure of the product rule. It has been verified experimentally that for a particular pre- and postselected pair of particles a single measurement on particle $A$ yields with certainty $\sigma_x^A=-1$, a single measurement on particle $B$ yields with certainty $\sigma_y^B=-1$, and a single nonlocal measurement on particles $A$ and $B$ yields with certainty $\sigma_x^A \sigma_y^B=-1$.