Quantum mechanics predicts effects that are counter-intuitive

Quantum mechanics predicts effects that are counter-intuitive and impossible to reconcile with our daily experience. One of the most intriguing examples of these predictions is that a pair of particles can be entangled. When two particles are entangled we cannot describe the properties of one of them without referring to the other, they behave as a single entity.

KS18 and its graph of compatibility
KS18 and its graph of compatibility

Impressively, this correlated behavior is displayed regardless of how far apart the pair of particles is. These correlations gave rise to Bell's theorem. This theorem states that correlations obtained by measuring pairs of entangled particles cannot be reproduced by local hidden variable theories, so quantum mechanics is nonlocal. The non-locality of quantum mechanics can be witnessed by the violation of Bell's inequalities. Nonlocality is an important resource that enables tasks such as secure communication and the generation of randomness. However, Bell's theorem is not the only hidden variable impossibility theorem in quantum mechanics.

Another example is the Kochen-Specker contextuality. Analogous to nonlocality, contextuality can be witnessed by the violation of noncontextual hidden variable inequalities. However, the difference is that for contextuality, only one particle and sequential measurements are required, whereas for nonlocality, two particles and spacelike separation are necessary. Furthermore, in certain scenarios it is possible to show contextuality regardless of the quantum state of the measured particle, measurements that achieve this phenomenon are called state-independent contextuality ensembles.

In this work we studied the problem of combining nonlocality and contextuality in the same experiment. We connected both resources by using Bell inequalities that are violated by state-independent contextuality sets. We proved the these Bell inequalities are maximally robust to either noise or detection inefficiency, thus, they are suitable for experimental implementations. Our work paves the way for technologies that harness simultaneously the advantages offered by nonlocality and contextuality.


More information

Optimal and tight Bell inequalities for state-independent contextuality sets - Paper published in Physical Review Research