Beyond-mean-field effects in Rabi-coupled two-component Bose-Einstein condensate

TL;DR

This paper combines theoretical calculations and experimental measurements to explore beyond-mean-field effects in a Rabi-coupled two-component Bose-Einstein condensate, revealing a crossover in energy scaling and the emergence of three-body interactions.

Contribution

It provides the first experimental evidence of the transition from Lee-Huang-Yang to integer power density scaling in BECs with Rabi coupling, highlighting the role of three-body effects.

Findings

BMF energy density crosses over from LHY scaling to integer powers of density with increasing Rabi frequency

Two distinct BMF contributions are experimentally observed, scaling differently with Rabi frequency

Rabi coupling stabilizes spin composition, aiding the observation of BMF effects in mixtures

Abstract

We theoretically calculate and experimentally measure the beyond-mean-field (BMF) equation of state in a coherently-coupled two-component Bose-Einstein condensate (BEC) in the regime where averaging of the interspecies and intraspecies coupling constants over the hyperfine composition of the single-particle dressed state predicts the exact cancellation of the two-body interaction. We show that with increasing the Rabi-coupling frequency $\Omega$, the BMF energy density crosses over from the nonanalytic Lee-Huang-Yang (LHY) scaling $\propto n^{5/2}$ to an expansion in integer powers of density, where, in addition to a two-body BMF term $\propto n^2 \sqrt{\Omega}$, there emerges a repulsive three-body contribution $\propto n^3/\sqrt{\Omega}$. We experimentally evidence this two contributions, thanks to their different scaling with $\Omega$, in the expansion of a Rabi-coupled two-component $^{39}$K condensate in a waveguide. By studying the expansion with and without Rabi coupling, we reveal an important feature relevant for observing BMF effects and associated phenomena in mixtures with spin-asymmetric losses: Rabi coupling helps preserve the spin composition and thus prevents the system from drifting away from the point of vanishing mean field.