Speaker: Dávid NAGY (SZFI)

Title: Observation of phase transitions in cavity QED

Date: 28 February 2023, 10:00

Place: Building 1 Meeting room, or online:
Meeting ID: 876 7145 6898
Passcode: 682420



The talk overviews recent developments and presents the latest experimental results achieved in the Quantum Optics Laboratory of Wigner SZFI.

A cloud of laser-cooled rubidium atoms is routinely transported into a high-finesse optical resonator. The strong collective light-matter coupling can be reached even at the limit where a single atom is weakly coupled to the field. As a result, the atomic ensemble significantly alters the resonator's transmission, which can then be used to monitor the dynamics of this nonlinear quantum system. On the other hand, light inside the cavity pumps the multilevel atoms into an uncoupled state via a critical runaway process. With time-resolved measurements of the cavity output, we can track this genuine dynamical transition and measure finite-size scaling relations of the enhanced photon number fluctuations.

By shining an additional repumper laser on the atoms, the two phases of the coupled atom-cavity system can be stabilized. They correspond to different hyperfine ground states of the atoms, and only one of them is coupled to the cavity mode. Atoms being in this latter state block cavity transmission, while the uncoupled state leads to high transmission. We demonstrate experimentally that there is a bistability region where the two phases coexist. We find a hysteresis in the order parameter when either of the two control parameters is swept across the bistability region. We interpret the phenomenon in terms of the recent paradigm of first-order, driven-dissipative phase transitions.