Experimental observation of Hardy-like quantum contextuality

Breno Marques, Johan Ahrens, Mohamed Nawareg, Adan Cabello, Mohamed Bourennane

Published 2014-08-28Version 1

Contextuality is a fundamental concept for understanding quantum probabilities and the origin of the power of quantum systems for computation and information processing. A natural question is: Which is the simplest and cleanest version of quantum contextuality? It has been very recently found that there is a version that is analog to "the simplest and cleanest" version of quantum non-locality, but only requires single three-level quantum systems (qutrits) rather than composite systems. Here we report the first experimental observation of this "Hardy-like" quantum contextuality. We measured the correlations between the outcomes of five combinations of two compatible measurements performed sequentially, in any possible order, on heralded photonic path-encoded qutrits. The experiment adopts a novel configuration which allows for performing two sequential measurements on the same photon with a high fidelity and allows for observing the independence of the correlations with respect to the order in which measurements are performed. The experimental results match the conditions needed to define a Hardy-like argument and the predictions of quantum theory. In addition, they violate the relevant non-contextuality inequality, even when the non-contextual bound is corrected to take into account experimental imperfections. The experimental observation of this form of quantum contextuality is of fundamental importance, since it connects the Kochen-Specker theorem with the simplest non-contextuality inequality. In addition, the method introduced here opens the door to applications requiring sequential quantum measurements on photonic systems.