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State-of-the-art nano-fabrication process allow to control the spatial confinement of electronic carriers, with close to atomic precision. Structures are thus designed to separate spatially oppositely charged electrons and holes, notably bilayer heterostructures. When electrons and hole are separated in such adjacent layers they experience a strong Coulomb attraction pairing them into spatially indirect excitons, i.e. boson-like particles, which exhibit a giant electric dipole of about 500 Debye. Excitons then experience a repulsive dipolar potential stabilizing cold gases against collapse while providing access to a rich variety of collective quantum phases, possibly ranging from superfluidity to supersolidity [1]. Here, first signatures for the superfluid crossover of such dipolar excitons are reported [2]. When confined in a microscopic trap we show that they realize a four-component superfluid at sub-Kelvin temperatures, distributed between two optically active and two optically inactive spin states. By imaging the condensate bright part, we study in-situ the profiles of the exciton density and the phase coherence in the trap. We thus reveal quantum spatial coherence, in a sub-Kelvin regime bound to very dilute densities and probably limited by the strength of dipolar interactions. Also, we evidence quantized vortices, efficiently trapped in the slight electrostatic disorder of our trapping potential [2]. Analyzing the interplay between quasi long-range order, vortex formation, and density profiles across the range of explored parameters it is finally shown that our experimental findings provide a direct evidence for a Berezinskii-Kosterlitz-Thouless crossover [3]. The work presented here results from contributions of S.Dang, R.Anankine, M.Beian, M.Alloing, E.Cambril, A.Lemaitre and M.Holzmann.

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Francois Dubin
Seminar Room of the Institute of Physics II
Contact: Sebastian Diehl