Non-Equilibrium Dynamics of a Quenched BEC: Complex Pathways to thermalization

We study the quantum dynamics of an isolated two-dimensional (2D) bosonic system involving a Bose-Einstein condensate (BEC) which is quenched to a highly non-equilibrium state. This issue is relevant to the general state preparation of BECs in higher bands. The dynamical pathways involve an initial formation of a well-organized ring-shape condensate which is consistent with the recently proposed theory of inflationary dynamics[Nature Physics 14 269]. We show how and why this ring-phase subsequently disorders into a cloud state characterized by a large number of bosonic pairs which proliferate and occupy an extended, but a confined region in the Brillouin zone. The associated wavefunction has an intrinsic many-body nature and at long time scales, thermalization and recondensation are shown to follow. We analyze the mechanism leading to this recondensation establishing its relation to a nonlinear kinetic Boltzman equation and building on the analogy with weak turbulence and resonant four-wave processes. Numerical simulations from the Gross Pitaevskii Equation yield these same dynamical pathways and support the theoretical approach described above.