Visualising Berry phase and diabolical points in a quantum exciton-polariton billiard

TL;DR

This paper demonstrates the experimental visualization of Berry phase and diabolical points in a reconfigurable exciton-polariton quantum billiard, revealing fundamental spectral degeneracies in a controllable optical microcavity system.

Contribution

It introduces a method to create and visualize diabolical points and Berry phase in a quantum billiard of exciton-polaritons using structured optical pumping.

Findings

Observation of diabolical points in exciton-polariton spectra

Direct imaging of Berry phase in a quantum billiard

Reconfigurable potential walls controlled by optical pump

Abstract

Diabolical points (degeneracies) can naturally occur in spectra of two-dimensional quantum systems and classical wave resonators due to simple symmetries. Geometric Berry phase is associated with these spectral degeneracies. Here, we demonstrate a diabolical point and the corresponding Berry phase in the spectrum of hybrid light-matter quasiparticles -- exciton-polaritons in semiconductor microcavities. It is well known that sufficiently strong optical pumping can drive exciton-polaritons to quantum degeneracy, whereby they form a macroscopically populated quantum coherent state similar to a Bose-Einstein condensate. By pumping a microcavity with a spatially structured light, we create a two-dimensional quantum billiard for the exciton-polariton condensate and demonstrate a diabolical point in the spectrum of the billiard eigenstates. The fully reconfigurable geometry of the potential walls controlled by the optical pump enables a striking experimental visualisation of the Berry phase associated with the diabolical point. The Berry phase is observed and measured by direct imaging of the macroscopic exciton-polariton wavefunctions.