A magnet the size of a grain of rice floats in a vacuum without touching the walls. It is held by the magnetic field of a superconducting trap — a ring cooled to a state where current flows without resistance. Levitation removes friction: without it, even a weak push makes the magnet oscillate for a very long time, like a tuning fork after being struck.
But for delicate measurements, even such residual jitter is a nuisance. It is eliminated by feedback: a superconducting coil senses the magnet's motion, and a computer sends a signal through an amplifier to a piezoelectric pusher. The pusher nudges the trap in antiphase, as if you were sitting in a boat and moving against the wave to cancel it. This reduced the oscillation amplitude to two picometers — a hundred times smaller than the diameter of an atom.
The motional temperature dropped below ten millikelvins. For comparison, interstellar space is warmer — about three kelvins. The entire experiment is housed in a dilution cryostat that suppresses external vibration by a factor of one hundred billion. In the future, such a system could be cooled to the quantum limit — then the magnet would oscillate only due to the uncertainty principle, not from external disturbances. This would pave the way for gravity detectors that could sense the attraction of objects as small as a grain of sand.
An oscillation amplitude of two picometers is so small that if the magnet were the size of Earth, its motion would be less than the thickness of a human hair.