Fig. 5. Concurrence between the external photons and the polaritons as a function of the power of the classical laser impinging onto the sample.

Each data point (black bullet) is obtained from the 36 measurements of coincidences in all the combinations of polarization (the dotted line serves as a guide). The solid blue line is a theoretical simulation of a model of fluctuating polarized lower polaritons that interact with the single upper polariton injected by the quantum source with an interaction strength of 0.9% of the radiative broadening, which is in quantitative agreement with the observation. The dashed-dotted red line shows the theoretical simulation of a model of a condensate assuming thermal fluctuations, which gives the worst fit for the experimental observation and suggests that, as expected, the condensate has Poissonian fluctuations. The dashed green line shows the loss of concurrence without polariton interactions, due only to the polaritons scattered to the state at which the single photon arrives (cf. Supplementary Material), confirming that the amount of noise in the experiment is small enough to observe the effects of interactions on a single polariton.