11 February 2020

Parametric Instabilities of Interacting Bosons in Periodically Driven 1D Optical Lattices

New states of matter can arise when a system is exposed to a periodic drive, such as by irradiating a solid with light or by mechanically shaking a gas of ultracold atoms. While experiments have validated this method, the target often exhibits a drastically reduced lifetime because it continuously absorbs energy from the drive. Moreover, in systems of bosonic particles the periodic drive can trigger the rapid growth of collective excitations, known as parametric instabilities, which typically decay and result in additional heating. Now, an international team headed by LMU and MPQ physicists report on observations of collective modes in the early-stage dynamics of ultracold bosons in shaken optical lattices.

In this experiment, the international team monitors the momentum distribution of a Bose-Einstein condensate after releasing it from the lattice. The appearance of additional momentum components upon shaking directly signals the presence of collective excitations. The dominant mode is known to appear at very specific momenta, which depend on the parameters of the drive. The team's observations confirm recent theoretical predictions and directly show the important role played by parametric instabilities during early evolution times for a wide class of shaken quantum systems with bosonic elementary excitations.

Understanding the onset of parametric instabilities in driven quantum matter is crucial for determining stable parameter regimes, thereby opening the door to a panoply of exciting new phenomena including the engineering of genuine out-of-equilibrium many-body systems with exotic properties that have no static counterpart.

Parametric Instabilities of Interacting Bosons in Periodically Driven 1D Optical Lattices
K. Wintersperger, M. Bukov, J. Näger, S. Lellouch, E. Demler, U. Schneider, I. Bloch, N. Goldman, and M. Aidelsburger,
Phys. Rev. X 10, 011030, 2020.

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