Experimental test of the robustness of the non-classicality of single photons

Quantum non-classicality is not only a fundamental feature which shows the solely quantum character of an emitted photon, but is also necessary for modern applications based on the quantum character of light like quantum information processing [1]. The usual way to show that a light source is non-classical is to measure its auto-correlation [2]. In our work instead we used a different criterion developed in [3] which puts a bound on states that cannot be expressed as a mixture of coherent states. With the same measurement technique we have already proven the non-Gaussianity of the photon [4] using a similar criterion [5,6].