A matter-wave Fabry-Pérot cavity in the ultrastrong driving regime

When the length of an optical cavity is strongly modulated, photons stroboscopically propagate as if in a curved spacetime, with white and black hole event horizons corresponding to stable and unstable fixed point trajectories of a Floquet map. Though this theoretical description underlies both the dynamical Casimir effect and proposed schemes for pulse generation and Floquet cooling, the ultrastrong driving regime has eluded direct experimental verification due to the difficulty of achieving relativistic mirror acceleration. We report results of an experiment which overcomes this limitation by exchanging the roles of light and matter in the cavity, conditioning a matter wave with a quasi-relativistic dispersion relation and reflecting it between two blue-detuned optical barriers acting as mirrors. This approach features both massless mirrors and a much lower effective speed of light; these features enable us to experimentally observe the predicted bright and dark fixed point trajectories in the strongly-driven cavity, and study their emergence and decay.