Quantum non-Gaussian states, which cannot be written as mixtures of Gaussian states, are necessary to achieve a quantum advantage in continuous variable systems. They represent an important benchmark for the realization of an advanced quantum light source, as they cannot be made by simple means such as displacement and squeezing. We introduce an attenuation-resistant sufficient criterion for quantum non-Gaussian states based on the second- and third-order correlation functions, $g^{(2)}$ and $g^{(3)}$. The general nonlinear bound for classical mixtures of Gaussian states is $\sqrt{g^{(3)}}+3\sqrt{g^{(2)}}\ge 2$. Any mixture of Gaussian states must fulfill this inequality, thus, the violation of it represents a direct confirmation of quantum non-Gaussianity. We experimentally show the non-Gaussianity of the state produced by a quantum dot single-photon source, where we obtain $\sqrt{g^{(3)}}+3\sqrt{g^{(2)}}=0.174(13)$, which represents a statistical significance of more than 100 standard deviations.

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• Non-gaussian quantum states
• Quantum states of light
• Single photon sources
• Photonic crystal waveguide
• Quantum dots
• Correlation function measurements