In an ultracold fluid where atoms move as one, an impurity surrounds itself with a cloud of disturbances — a polaron (term coined by Landau). Like a swimmer in viscous water, it experiences drag. But if you make the cloud alternately attract and repel, at high switching frequencies the drag turns into acceleration. The impurity reaches a steady velocity, gaining self-propulsion — called an acton (from the word 'action').

Computer simulations showed that the acton's motion generates entropy (disorder) and extends quantum thermodynamics to active matter. To test this, researchers plan to use a Bose-Einstein condensate and optical traps, controlling quantum decoherence — the loss of quantum coherence. Surprisingly, the acton moves not smoothly, but in jolts, as if bouncing off invisible bumps.