Observing dynamical currents in a non-Hermitian momentum lattice

Dynamic transients are a natural ingredient of non-equilibrium quantum systems. A paradigmatic example is Dicke superradiance, describing the collectively enhanced population inversion of an ensemble of two-level atoms coupled to a single mode of light.

In our experiment, we leverage superradiance in a quantum degenerate gas to engineer dynamical currents in a synthetic lattice geometry. Our experimental implementation is based on a spinor Bose-Einstein condensate coupled to a single mode of an ultrahigh finesse optical cavity. Two transverse laser fields induce cavity-assisted Raman transitions between discrete momentum states of two spin levels, which we interpret as tunneling in a momentum space lattice. As the cavity field depends on the local density and spin configuration, the tunneling rate evolves dynamically with the atomic and photonic states. By monitoring the cavity leakage, we gain real-time access to the emerging currents and benchmark their collective nature. Moreover, frequency-resolved measurements of the leaking photon field allow us to locally resolve individual tunneling events as well as cascaded dynamics.

Our results provide prospects to explore dynamical gauge fields and transport phenomena in driven-dissipative quantum systems.