02. April 2021

Non-Hermitian phase transition at an exceptional point in a Bose-Einstein condensate of light Observation of a non-Hermitian phase transition in an optical quantum gas

Exceptional point photon BEC
Exceptional point photon BEC © Universität Bonn/ Johann Kroha
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Quantum gases of light, such as photon or polariton condensates in optical microcavities, are collective quantum systems enabling a tailoring of dissipation from, for example, cavity loss. This characteristic makes them a tool to study dissipative phases, an emerging subject in quantum many-body physics. We experimentally demonstrate a non-Hermitian phase transition of a photon Bose-Einstein condensate to a dissipative phase characterized by a biexponential decay of the condensate’s second-order coherence. The phase transition occurs because of the emergence of an exceptional point in the quantum gas. Although Bose-Einstein condensation is usually connected to lasing by a smooth crossover, the observed phase transition separates the biexponential phase from both lasing and an intermediate, oscillatory condensate regime. Our approach can be used to study a wide class of dissipative quantum phases in topological or lattice systems.

Read article at ScienceMag:   DOI: https://doi.org/10.1126/science.abe9869

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