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We theoretically address the case of two non‐equilibrium polariton superfluids that are continuously injected into the same region of a microcavity by means of two pumping lasers at different energies and angles. First we characterize the subsonic‐supersonic transition for one superfluid, by studying the drag force exerted by the particles on the defect. Then, for the two fluid case, we find that even if the two fluids interact it is possible to independently tune them to the super‐sonic or sub‐sonic regimes.

We study the effect of pure dephasing on the strong‐coupling of a two‐level system (typically, a quantum dot) with the single mode of a microcavity. Whereas dephasing merely broadens the line in the linear regime, it results in a new spectral shape when transitions between higher manifolds of the Jaynes‐Cummings ladder are involved, namely, a triplet, that we show does not imply weak‐coupling. Higher excitation brings the system into lasing. Description of this intermediate triplet allows to demonstrate quantum nonlinearities even in noisy systems and to measure the magnitude of dephasing.

We study the hydrodynamics of a coherent polariton fluid lasting for hundreds of picoseconds and moving inside of a semiconductor microcavity. The momentum of the polariton droplets can be well controlled by selecting the angle and energy of two excitation laser beams. We observe unique behaviours due to the quantum nature of the polariton states, such as the linearization of the polariton dispersion around the energy of the signal states, and the propagation at constant speed of the polariton fluids. The experiments have been realized making use of a new technique for the detection of images simultaneously resolved in time, energy and real‐ or reciprocal‐ space.