Persistent currents from the decay of quantum turbulence: signatures of an inverse energy cascade in Bose-Einstein condensates

We report the formation of persistent currents from the decay of turbulence in Bose- Einstein condensates (BECs). In our experiments, a BEC is pierced with a blue-detuned laser beam. By moving the trap center relative to the beam's position, vortices are stirred into the BEC, creating a quantum turbulent state. At finite temperatures, the turbulent state can decay to a persistent current about the blue-detuned laser beam that can last for up to 50 seconds; winding numbers up to 8 have been observed. Our experimental observations correspond well with numerical simulations of the non-equilibrium dynamics and calculations of vortex pinning by a laser beam. We interpret our results as evidence for an inverse energy cascade in dilute-gas BECs.