Beyond-Mean-Field Effects in Rabi-Coupled Two-Component Bose-Einstein Condensate

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 scaling proportional to n(5/2) to an expansion in integer powers of density, where, in addition to a two-body BMF term proportional to n(3)/root Omega, there emerges a repulsive three-body contribution proportional to n(3)/root Omega. We experimentally evidence these two contributions, thanks to their different scaling with Omega, in the expansion of a Rabi-coupled twocomponent K-39 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 the vanishing mean field.

Automated summary

In this study, the beyond-mean-field (BMF) equation of state in a coherently coupled two-component Bose-Einstein condensate was theoretically calculated and experimentally measured. It was found that with increasing Rabi-coupling frequency Omega, the BMF energy density transitions from nonanalytic scaling to an expansion in integer powers of density, revealing the emergence of a repulsive three-body contribution in addition to the two-body BMF term. The experimental evidence supported these findings, showing the importance of Rabi coupling in preserving the spin composition and preventing the system from drifting away from the vanishing mean field point.