Emerging dissipative phases and spin currents in a superradiant quantum gas

The interplay between coherent and dissipative processes is at the core of the evolution of open many-body quantum systems. Their competition can lead to new phases of matter, instabilities, and non-equilibrium dynamics that have no closed system counterparts. However, probing these phenomena at a microscopic level in a setting of controlled coherent and dissipative couplings often proves challenging.

In our experiment, we achieve such control by coupling a ⁸⁷Rb spinor Bose-Einstein condensate (BEC) to a single mode of a lossy optical cavity. Two transverse laser fields incident on the BEC drive cavity-assisted Raman transitions between discrete motional states of two atomic spin levels. In a first set of experiments, we adjust the imbalance between the drives to tune the competition between coherent dynamics and dissipation. This results in the emergence of a dissipation-stabilized phase and a region of bistability. We relate the observed phases to the underlying microscopic processes by characterizing the properties of the polariton modes. Moreover, we report on recent experimental observations of spin currents in a momentum space lattice, which are mediated by a density-dependent photon field. We identify the superradiant nature of these currents and employ real-time, frequency-resolved measurements of the leaking cavity to locally resolve individual tunneling events and cascaded dynamics.

Together, our results provide prospects for the exploration of spin-orbit coupling, artificial magnetic fields and transport phenomena in non-Hermitian light-matter systems.