Below a critical temperature, a sufficiently high density of bosons undergoes Bose-Einstein condensation (BEC). Under this condition, the particles collapse into a macroscopic condensate with a common phase, showing collective quantum behaviour like superfluidity, quantised vortices, interferences, etc. Up to recently, BEC was only observed for diluted atomic gases at μK temperatures. Following the recent observations of non-equilibrium BEC in semiconductor microcavities at temperatures of ~10 K, using momentum-1 and real-space 2 trapping, the quest is now towards the observation of the superfluid motion of a polariton BEC. For the same reasons that polaritons benefit from unusually favourable features for condensation, such as very high critical temperatures, it is expected that their superfluid properties would likewise manifest with altogether different magnitudes, such as very high critical velocities. Since they have shown many deviations in their Bose-condensed phase from the cold atoms paradigm, it is not clear a priori to which extent their superfluid properties would coincide or depart from those observed with atoms, among which quantised vortices 6 , frictionless motion 7 , linear dispersion for the elementary excitations 8 , or more recently Čerenkov emission of a condensate flowing at supersonic velocities 9 , are among the clearest signatures of quantum fluid propagation.Microcavity polaritons are two-dimensional bosons of mixed electronic and photonic nature, formed by the strong coupling of excitons-confined in semiconductor quantum wells-with photons trapped in a micron scale resonant cavity. First observed in 1992 10 , these particles have been profusely studied in the last fifteen years due to their unique features. Thanks to their photon fraction, polaritons can easily be excited by an external laser source and detected by light emission in the direction perpendicular to the cavity plane. However, as opposed to photons, they experience strong interparticle interactions owing to their partially electronic fraction. Due to the deep polariton dispersion, the effective mass of these particles is 10 4 -10 5 smaller than the free electron mass, resulting in a very low density of states. This allows for a high state 3 occupancy even at relatively low excitation intensities. However, polaritons live only a few 10 -12 s in a cavity before escaping and therefore thermal equilibrium is never achieved. In this respect, a macroscopically degenerate state of polaritons departs strongly from an atomic Bose-condensed phase. The experimental observations of spectral and momentum narrowing, spatial coherence and long range order-which have been used as evidence for polariton Bose-Einstein condensation-are also present in a pure photonic laser 11 . The recent observation of long range spatial coherence 12 , vortices 4 and the loss of coherence with increasing density in the condensed phase 13,14 , are in accordance with macroscopic phenomena proper of interacting, coherent bosons 15 . But a direct manifestation ...

Although optical technology provides the best solution for the transmission of information, all-optical devices must satisfy several qualitative criteria to be used as logic elements. In particular, cascadability is difficult to obtain in optical systems, and it is assured only if the output of one stage is in the correct form to drive the input of the next stage. Excitonpolaritons, which are composite particles resulting from the strong coupling between excitons and photons, have recently demonstrated huge non-linearities and unique propagation properties. Here we show that polariton fluids moving in the plane of the microcavity can operate as input and output of an all-optical transistor, obtaining up to 19 times amplification and demonstrating the cascadability of the system. Moreover, the operation as an AND/OR gate is shown, validating the connectivity of multiple transistors in the microcavity plane and opening the way to the implementation of polariton integrated circuits.

Superfluidity-the suppression of scattering in a quantum fluid at velocities below a critical value-is one of the most striking manifestations of the collective behaviour typical of Bose-Einstein condensates. This phenomenon, akin to superconductivity in metals, has until now only been observed at prohibitively low cryogenic temperatures.For atoms, this limit is imposed by the small thermal de Broglie wavelength, which is inversely related to the particle mass. Even in the case of ultralight quasiparticles such as exciton-polaritons, superfluidity has only been demonstrated at liquid helium temperatures. In this case, the limit is not imposed by the mass, but instead by the small exciton binding energy of Wannier-Mott excitons, which places the upper temperature limit. Here we demonstrate a transition from normal to superfluid flow in an organic microcavity supporting stable Frenkel exciton-polaritons at room temperature. This result paves the way not only to table-top studies of quantum hydrodynamics, but also to room-temperature polariton devices that can be robustly protected from scattering.

Exciton-polaritons are bosonic quasiparticles that arise from the normal mode splitting of photons in a microcavity and excitons in a semiconductor material. One of the most intriguing extensions of such a light−matter interaction is the socalled ultrastrong coupling regime. It is achieved when the Rabi frequency (Ω R , the energy exchange rate between the emitter and the resonant photonic mode) reaches a considerable fraction of the emitter transition frequency, ω 0 . Here, we report a Rabi energy splitting (2ℏΩ R ) of 1.12 eV and record values of the coupling ratio (2Ω R /ω 0 ) up to 0.6-fold the material band gap in organic semiconductor microcavities and up to 0.5-fold in monolithic heterostructure organic light-emitting diodes working at room temperature. Furthermore, we show that with such a large coupling strength it is possible to undress the exciton homogeneous linewidth from its inhomogeneous broadening, which allows for an unprecedented narrow emission line (below the cavity finesse) for such organic LEDs. The latter can be exploited for the realization of novel monochromatic sources and near-IR organic emitting devices.