Fluctuations of the order parameter of a mesoscopic Floquet condensate

TL;DR

This paper explores nonequilibrium Bose-Einstein condensates in periodically driven systems, revealing that coherence persists in mesoscopic Floquet states but breaks down at larger particle numbers, challenging mean field descriptions.

Contribution

It introduces the concept of Floquet condensates in driven Bose gases and analyzes their coherence properties and limitations in the large particle number limit.

Findings

Floquet states exhibit near-perfect coherence in mesoscopic regimes

Order parameter fluctuations increase with particle number

Mean field limit does not exist for large particle numbers in these systems

Abstract

We suggest that nonequilibrium Bose-Einstein condensates may occur in time-periodically driven interacting Bose gases. Employing the model of a periodically forced bosonic Josephson junction, we demonstrate that resonance-induced ground state-like many-particle Floquet states possess an almost perfect degree of coherence, as corresponding to a mesoscopically occupied, explicitly time-dependent single-particle orbital. In marked contrast to the customary time-independent Bose-Einstein condensates, the order parameter of such systems is destroyed by violent fluctuations when the particle number becomes too large, signaling the non-existence of a proper mean field limit.