Phonon excitations in a one dimensional Bose gas*

Cold atomic gases provide a powerful tool to investigate quantum many-body systems [1]. Phonons, which are low-energy collective excitations for one dimensional superfluids, play a major role in the relaxation dynamics of such systems. It has been shown that mechanisms of phonon dephasing and rephasing generate, respectively, losses and recurrences of coherence in a quantum isolated system, and that the long time behavior of the system itself is determined by the spectrum of the phononic modes [2]. In our experiment we are able to excite and detect phonon modes individually and to monitor them over time.
Our system consists in a 1D BEC of ⁸⁷Rb atoms confined in a box trap, which provides a dispersion relation where the phonon energies are commensurate. We implement a shaking process that modulates the box walls symmetrically at a fixed frequency. The density profile of the cloud can be obtained via absorption imaging at different times. Doing so, we are able to investigate, within a reasonably broad range of frequencies, the growth of the lower symmetric modes when the system is excited, and their subsequent evolution and damping after the end of the shaking.

[1] Schweigler et al., Nature 545, 323 (2017)
[2] Rauer et al., Science 360, 307 (2018)