Beyond-mean-field effects in Rabi-coupled two-component BEC

Alfred Hammond, L. Lavoine, Alessio Recati +2 more

L. Lavoine, Hammond A., Recati A. +2 more

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 ?, the BMF energy density crosses over from the nonanalytic Lee-Huang-Yang scaling ?n5/2 to an expansion in integer powers of density, where, in addition to a two-body BMF term ?n2?, there emerges a repulsive three-body contribution ?n3/?. We experimentally evidence these two contributions, thanks to their different scaling with ?, in the expansion of a Rabi-coupled two-component K39 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.

F. Kh. Abdullaev, R. M. Galimzyanov, Akbar M. Shermakhmatov

L. Lavoine, A. Hammond, Alessio Recati +2 more

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 Ω, the BMF energy density crosses over from the nonanalytic Lee-Huang-Yang scaling ∝n^{5/2} to an expansion in integer powers of density, where, in addition to a two-body BMF term ∝n^{2}sqrt[Ω], there emerges a repulsive three-body contribution ∝n^{3}/sqrt[Ω]. We experimentally evidence these two contributions, thanks to their different scaling with Ω, 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 the vanishing mean field.

Giovanni Francesco Bertacco

In the first part of the thesis we investigate properties of multicomponent quantum Bose gas with a Rabi coupling, using theoretical methods based on functional integration. At very low temperatures beyond-mean-field contribution crucially affect the equation of state of many-body systems. We derive the equation of state of the gas including the quantum fluctuations. The divergent zero-point energy of gapless and gapped elementary excitations of the uniform system is properly regularized obtaining a meaningful analytical expression for the beyond-mean-field equation of state in the weak Rabi coupling approximation. In the case of attractive inter-particle interaction we show that the quantum pressure arising from Gaussian fluctuations can prevent the collapse of the mixture with the creation of a self-bound droplet. In the second part we study the effects of Rabi coupling along the BCS-BEC crossover of a Feshbach resonance for a two-spin-component Fermi gas. Results such as the gap equation, the number equation and the critical temperature in the mean-field approximation have been obtained. In the last part we include quantum fluctuations contributions in the previous calculations in order to investigate the phase diagram of the system.

Martin Dunn, W. Blake Laing, D. K. Watson +1 more