Experimental generalized contextuality with single-photon qubits

TL;DR

This paper experimentally demonstrates quantum contextuality using single-photon qubits by violating a generalized noncontextuality inequality, highlighting the nonclassical nature of quantum measurements.

Contribution

First experimental test of the Liang-Spekkens-Wiseman inequality with single-photon qubits, avoiding assumptions of determinism and extending contextuality tests to two-level systems.

Findings

Violates the LSW inequality by over 16 standard deviations

Shows that noncontextual hidden variable models cannot reproduce the results

Confirms contextuality as a fundamental feature of quantum mechanics with qubits

Abstract

Contextuality is a phenomenon at the heart of the quantum mechanical departure from classical behaviour, and has been recently identified as a resource in quantum computation. Experimental demonstration of contextuality is thus an important goal. The traditional form of contextuality -- as violation of a Kochen-Specker inequality -- requires a quantum system with at least three levels, and the status of the assumption of determinism used in deriving those inequalities has been controversial. By considering `unsharp' observables, Liang, Spekkens and Wiseman (LSW) derived an inequality for generalized noncontextual models that doesn't assume determinism, and applies already to a qubit. We experimentally implement the LSW test using the polarization states of a heralded single photon and three unsharp binary measurements. We violate the LSW inequality by more than 16 standard deviations, thus showing that our results cannot be reproduced by a noncontextual subset of quantum theory.