Many-Body Decay of the Gapped Lowest Excitation of a Bose-Einstein Condensate

We study the decay mechanism of the gapped lowest-lying axial excitation of a quasipure atomic Bose-Einstein condensate confined in a cylindrical box trap. Owing to the absence of accessible lower-energy modes, or direct coupling to an external bath, this excitation is protected against one-body (linear) decay, and the damping mechanism is exclusively nonlinear. We develop a universal theoretical model that explains this fundamentally nonlinear damping as a process whereby two quanta of the gapped lowest excitation mode couple to a higher-energy mode, which subsequently decays into a continuum. We find quantitative agreement between our experiments and the predictions of this model. Finally, by strongly driving the system below its (lowest) resonant frequency, we observe third-harmonic generation, a hallmark of nonlinear behavior.

Automated summary

The decay mechanism of the gapped lowest-lying axial excitation of a quasipure atomic Bose-Einstein condensate confined in a cylindrical box trap is studied, revealing that the damping mechanism is exclusively nonlinear. This nonlinear damping is explained as a process where two quanta of the gapped lowest excitation mode couple to a higher-energy mode, which subsequently decays into a continuum. Experimental results show quantitative agreement with the predictions of this model, and third-harmonic generation is observed when the system is strongly driven below its lowest resonant frequency, indicating nonlinear behavior.