Strong Interactions in a High Number Density Rydberg Vapour
We use a 3-step excitation process to create Rydberg atoms in a Caesium vapour confined within a glass cell. By heating the cell, we can control the vapour pressure and the number density of excited atoms, and by measuring the fluorescence and transmission properties of the medium we can interrogate the strong interactions between the excited atoms.
The experiments are either pulsed (looking for transient effects) or continuous wave CW (exploring the steady states of the system). In the CW setup we find ‘intrinsic optical bistability’ where a hysteresis effect means that the medium has two distinct responses to the same stimulus. In the pulsed regime we measure ‘critical slowing down’ where the response time of the medium diverges as a first-order phase transition between the two bistable responses is approached. Although the optical bistability is a steady-state phenomenon, the system is far from equilibrium, as it is being driven and dissipating energy. Our experiment therefore provides an opportunity to study an unusual example of a non-equilibrium phase transition.
Nonequilibrium Phase Transition in a Dilute Rydberg Ensemble
C. Carr, R. Ritter, C. G. Wade, C. S. Adams, and K. J. Weatherill
Phys. Rev. Lett. 111, 113901 (2013)
We have demonstrated a nonequilibrium phase transition in a dilute thermal atomic gas. The phase transition, between states of low and high Rydberg occupancy, is induced by resonant dipole-dipole interactions between Rydberg atoms. The gas can be considered as dilute as the atoms are separated by distances much greater than the wavelength of the optical transitions used to excite them. In the frequency domain, we observe a mean-field shift of the Rydberg state which results in intrinsic optical bistability above a critical Rydberg number density. In the time domain, we observe critical slowing down where the recovery time to system perturbations diverges with critical exponent α=-0.53±0.10. The atomic emission spectrum of the phase with high Rydberg occupancy provides evidence for a superradiant cascade.