Observation of quantum depletion in a nonequilibrium exciton-polariton condensate

TL;DR

This study provides direct experimental evidence of quantum depletion in a nonequilibrium exciton-polariton condensate, revealing complex interactions beyond mean-field theory and highlighting differences from equilibrium Bose-Einstein condensates.

Contribution

First direct detection of quantum depletion in a nonequilibrium exciton-polariton condensate, advancing understanding of quantum fluctuations in driven-dissipative systems.

Findings

Quantum depletion observed via spectral excitation branch detection

Depletion behavior can match or deviate from Bogoliubov theory

Interactions in exciton-polariton condensates extend beyond mean-field models

Abstract

The property of superfluidity, first discovered in liquid 4He, is closely related to Bose-Einstein condensation (BEC) of interacting bosons. However, even at zero temperature, when one would expect the whole bosonic quantum liquid to become condensed, a fraction of it is excited into higher momentum states via interparticle interactions and quantum fluctuations -- the phenomenon of quantum depletion. Quantum depletion of weakly interacting atomic BECs in thermal equilibrium is well understood theoretically but is difficult to measure. This is even more challenging in driven-dissipative systems such as exciton-polariton condensates(photons coupled to electron-hole pairs in a semiconductor), since their nonequilibrium nature is predicted to suppress quantum depletion. Here, we observe quantum depletion of an optically trapped high-density exciton-polariton condensate by directly detecting the spectral branch of elementary excitations populated by this process. Analysis of the population of this branch in momentum space shows that quantum depletion of an exciton-polariton condensate can closely follow or strongly deviate from the equilibrium Bogoliubov theory, depending on the fraction of matter (exciton) in an exciton-polariton. Our results reveal the effects of exciton-polariton interactions beyond the mean-field description and call for a deeper understanding of the relationship between equilibrium and nonequilibrium BECs.