Multimode feedback cooling of the collective modes of a Bose-Einstein condensate

TL;DR

This paper demonstrates cavity-free feedback cooling of the three lowest collective modes of a Bose-Einstein condensate, achieving near ground-state occupancy through in situ measurements and active damping.

Contribution

It introduces a novel method of cavity-free feedback cooling for multiple collective modes of a BEC using shadowgraph imaging.

Findings

Successfully cooled dipole modes to <1 phonon occupancy

Measured and damped collective oscillations in real-time

Demonstrated effective multi-mode feedback cooling

Abstract

We experimentally demonstrate cavity-free feedback cooling of the three lowest-lying collective modes of a Bose-Einstein condensate in a prolate harmonic trap. Using shadowgraph imaging as an in situ probe of the atomic density, we measure the time-dependent centers of mass and widths of the condensate in two dimensions and use these measurements to damp oscillations in the two visible dipole modes and the low-frequency quadrupole mode. By inducing oscillations in the condensate, we show that we can efficiently damp the dipole modes to a final mean phonon occupancy per atom of $<1$.