Bosonic bunching occurs within quantum physics and can be mimicked classically by noncontextual hidden-variable models, which excludes this phenomenon as a means to prove stronger-than-quantum contextuality.
We perform a comprehensive set of experiments that characterize bosonic bunching of up to 3 photons in interferometers of up to 16 modes. Our experiments verify two rules that govern bosonic bunching. The first rule, obtained recently in [1,2], predicts the average behavior of the bunching probability and is known as the bosonic birthday paradox. The second rule is new, and establishes a n!-factor quantum enhancement for the probability that all n bosons bunch in a single output mode, with respect to the case of distinguishable bosons. Besides its fundamental importance in phenomena such as Bose-Einstein condensation, bosonic bunching can be exploited in applications such as linear optical quantum computing and quantum-enhanced metrology.
We argue that the experiment described in the recent Letter by Zu et al. [Phys. Rev. Lett. 109, 150401 (2012); arXiv:1207.0059v1] does not allow to make conclusions about contextuality, since the measurement of the observables as well as the preparation of the state manifestly depend on the chosen context.
Recent research on quantum contextuality has been strongly centered on device-independent frameworks, such as the many graph approaches to contextuality and the celebrated sheaf-theoretical approach. Contextuality is described in these frameworks as a property of data only, making it possible to characterize and quantify the phenomena regardless of the reasons why it occurs. In this paper we look beyond the data and focus on possible explanations for this experimental fact. We show that a classical system generating contextual data can easily be found if the following conditions are satisfied (1) We only have access to a specific collection of epistemic measurements (which, all things considered, is basically Bohrs view on quantum measurements) and (2) There is a limitation on which of these measurements can be jointly performed. The way we see it, this example indicates that contextuality may be a consequence of the type of measurement taken into account, instead of an intrinsic feature of the system upon which these measurements are performed; if this is correct, the widespread idea that quantum contextuality is a non-classical feature can be avoided.
A central result in the foundations of quantum mechanics is the Kochen-Specker theorem. In short, it states that quantum mechanics is in conflict with classical models in which the result of a measurement does not depend on which other compatible measurements are jointly performed. Here, compatible measurements are those that can be performed simultaneously or in any order without disturbance. This conflict is generically called quantum contextuality. In this article, we present an introduction to this subject and its current status. We review several proofs of the Kochen-Specker theorem and different notions of contextuality. We explain how to experimentally test some of these notions and discuss connections between contextuality and nonlocality or graph theory. Finally, we review some applications of contextuality in quantum information processing.