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

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 ${\sigma }_x^A{\sigma }_y^B=-1$.

arXiv comment6 pages, 5 figures, to be published in PRL

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• Knowledge of Quantum Hidden Variables Enables Backwards-In-Time Signaling
• Quantum observable's reality erasure with spacelike-separated operations
• Nonlocal Variable-Strength Measurements of N Qubits Using GHZ-like Entanglement
• Interplay between strong and weak measurement: Comparison of three experimental approaches to weak value estimation