Hypersonic Bose–Einstein condensate In this nature paper, a BEC is accelerated around a smooth ring-shaped waveguide within a vacuum chamber at UHV. The BEC reaches speeds up to 16 times the condensate's speed of sound. What is the significance of quoting the speed as a multiple of the speed of sound?
Considering the BEC is trapped and moving within a vacuum, isn't this a bit like quoting the speed of a box of trapped air moving through space as a multiple of the speed of sound in air? Or has this something to do with the BEC traveling around a ring path such that it is not in an inertial frame?
 A: The point they are making is lossless hypersonic transport, which is allowed by the smoothness of the waveguide/trapping potential:

this is the first demonstration of lossless hypersonic transport of
condensates in a matter-wave guide. Such smooth guides form an ideal
base for guided matter-wave interferometry.

The reason for the relation between lossness, smoothness, and hypersonic motion is explained here:

The BEC lifetime of 5.3 s is particularly impressive because the atoms travel in the waveguide at a hypersonic speed of 16 times the Landau critical velocity in the BEC. At such high velocities, any roughness or corrugation of the guide would couple the longitudinal velocity to transverse excitations and thus rapidly destroy the coherence of the condensate. Therefore, the absence of any measurable heating is only possible because the TAAP matter-wave guides are perfectly smooth.

So they have demonstrated that they can make very fast BECs and that they live long enough to be investigated. Hence they can use them to study high angular momentum states & effects like Quantum Hall or Laughlin states.
