A billiard ball hangs in midair. Tiny peas shoot at it from a blowgun. Each hit is barely perceptible, but if the ball is very light and barely trembles from heat, you can make out individual nudges. That's essentially the setup: a glass sphere the size of a bacterium levitates in a vacuum while a narrow beam of xenon atoms flies at it.

The sphere is cooled to its quantum ground state: its own jitter nearly comes to a halt. The xenon atoms are accelerated to supersonic speeds and released in short bursts, like a paintball marker on full auto. When an atom strikes the sphere, the sphere receives a momentum kick of less than 50 keV/c, as if a mosquito pushed a soccer ball, but billions of times weaker. The instrument catches such kicks with 96% confidence.

The main achievement is distinguishing kicks from the atomic beam against random hits by residual gas molecules in the vacuum chamber. It's like hearing a soft knock on the door over street noise. Now researchers can study how the force between an atom and a surface changes as they nearly touch. Such levitating spheres could also become ultrasensitive scales for single impacts — from cosmic particles or molecules, for instance.

So the billiard ball turns into a microphone that hears a single dust mote fall.