Violation of Leggett-type inequalities in the spin-orbit degrees of freedom of a single photon

TL;DR

This paper demonstrates experimentally that certain hidden-variable theories, which assume independent degrees of freedom within a single photon and allow contextual influences, are incompatible with observed quantum behavior, evidenced by violating Leggett-type inequalities.

Contribution

The first experimental violation of Leggett-type inequalities using spin and orbital angular momentum degrees of freedom of a single photon, challenging crypto non-local hidden-variable models.

Findings

Achieved maximum violation of seven standard deviations.

Confirmed incompatibility of hidden-variable theories with quantum predictions.

Validated quantum contextuality in single-photon degrees of freedom.

Abstract

We report the experimental violation of Leggett-type inequalities for a hybrid entangled state of spin and orbital angular momentum of a single photon. These inequalities give a physical criterion to verify the possible validity of a class of hidden-variable theories, originally named "crypto non-local", that are not excluded by the violation of Bell-type inequalities. In our case, the tested theories assume the existence of hidden variables associated with independent degrees of freedom of the same particle, while admitting the possibility of an influence between the two measurements, i.e. the so-called contextuality of observables. We observe a violation the Leggett inequalities for a range of experimental inputs, with a maximum violation of seven standard deviations, thus ruling out this class of hidden variable models with a high confidence.