Observation of driven-dissipative transitions in a Strontium cavity-QED System

Superradiance refers to the emission phenomenon where the collective emission rate is faster than the simple sum of individual ones’. It provides an interesting playground for exploring equilibrium and out-of-equilibrium phase transitions and holds promise for making an active stable atomic clock. Here we realize a superradiant phase transition in a driven-dissipative setting that is different from the one that is observed in previous work [1]. We trap ⁸⁸Sr atoms in a high-finesse optical cavity with the closest TEM₀₀ mode tuned to resonance with the ¹S₀-³P₁ transition and drive the atoms on resonance with the same optical transition. This is a clean experimental demonstration of cooperative resonance fluorescence that was predicted about 45 years ago [2], in which we are able to probe the dynamics of spin degrees of freedom. We show that at low incident field (i.e. the superradiant phase), the superradiant field cancels the incident drive field, and atoms sustain a non-zero dipole moment with zero intra-cavity field; and at high incident field (the normal phase), atoms start to Rabi flop and have a zero time-averaged dipole moment. We found spontaneous emission can shift the transition point and lead the system to a different steady state. Utilizing the flexible tunability of our cavity-QED system, we also explore how elastic interactions between atoms can change the properties of the phase transition. This opens the door for exploring the rich physics of driven-dissipative systems.

[1] Baumann, K., Guerlin, C., Brennecke, F., & Esslinger, T. (2010). Dicke quantum phase transition with a superfluid gas in an optical cavity. nature, 464(7293), 1301-1306.

[2] Carmichael, H. J. (1980). Analytical and numerical results for the steady state in cooperative resonance fluorescence. Journal of Physics B: Atomic and Molecular Physics, 13(18), 3551.