## Final Report Summary - HANDY-Q (Quantum Degeneracy at Hand)

Quantum fluids differ drastically from our everyday fluids, in that their properties are dominated by the law of quantum mechanics. They are for instance characterized by the notion of phase coherence, leading to interference fringes of the particles density, and by non-local quantum correlations. Such fluids are laboratory objects, which require in general very low temperatures. Let us mention for instance ultra-cold atom gases confined within a magneto-optical trap, liquid Helium below T=2.17K, and Cooper pair liquids in superconductor materials. Owing to their fascinating properties these fluids have been under thorough experimental and theoretical investigation for decades.

Another class of quantum fluid has emerged ten years ago, which is even more intriguing as (i) it lives within solid-state environment (ii) it is made for a large fraction of light, and as such, (iii) it is short lived and not in thermal equilibrium with its environment. This is the exciton-polaritons fluid, which can be created within purposely designed semiconductor microcavities in the so-called “strong coupling regime” between the excitonic transition and the cavity field. The aim of this project has been to achieve a deeper understanding of the properties and potentials of such exotic fluids.

Within this research project we have fabricated and used unconventional semiconductor microcavities (in terms of materials and nanostructuration) to carry out different experimental investigations. We could for instance demonstrate that even in the limit of very weak photonic fraction (i.e. very large excitonic fraction), and a usually detrimental one-dimensional highly disordered environment, a polariton condensate can still be in the mean filed limit, i.e. dominated by a single wave-function with negligible thermal or quantum fluctuations. We have also shown that polaritons can exchange heat with the thermal bath of solid lattice vibrations (phonons). Thus, we have found that for an even “stronger” coupling regime, this exchange can be virtually fully quenched; or, on the contrary, we have also shown that this exchange made quite large and used to cool down the solid itself using a cold polariton fluid. Finally we have extended this investigation to ensembles of semiconductor quantum dots, for which we have demonstrated a large coupling strength with a macroscopic mechanical motion, using the same interaction mechanism as with phonons (strain field). This project has opened new perspectives in the domain of non-equilibrium thermodynamics and opto-mechanics of polariton quantum fluids.

Another class of quantum fluid has emerged ten years ago, which is even more intriguing as (i) it lives within solid-state environment (ii) it is made for a large fraction of light, and as such, (iii) it is short lived and not in thermal equilibrium with its environment. This is the exciton-polaritons fluid, which can be created within purposely designed semiconductor microcavities in the so-called “strong coupling regime” between the excitonic transition and the cavity field. The aim of this project has been to achieve a deeper understanding of the properties and potentials of such exotic fluids.

Within this research project we have fabricated and used unconventional semiconductor microcavities (in terms of materials and nanostructuration) to carry out different experimental investigations. We could for instance demonstrate that even in the limit of very weak photonic fraction (i.e. very large excitonic fraction), and a usually detrimental one-dimensional highly disordered environment, a polariton condensate can still be in the mean filed limit, i.e. dominated by a single wave-function with negligible thermal or quantum fluctuations. We have also shown that polaritons can exchange heat with the thermal bath of solid lattice vibrations (phonons). Thus, we have found that for an even “stronger” coupling regime, this exchange can be virtually fully quenched; or, on the contrary, we have also shown that this exchange made quite large and used to cool down the solid itself using a cold polariton fluid. Finally we have extended this investigation to ensembles of semiconductor quantum dots, for which we have demonstrated a large coupling strength with a macroscopic mechanical motion, using the same interaction mechanism as with phonons (strain field). This project has opened new perspectives in the domain of non-equilibrium thermodynamics and opto-mechanics of polariton quantum fluids.