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.
as written by the authors arXiv:2605.28479
Due to their exceptional isolation from the environment, magnetically levitated particles are explored as extremely sensitive mechanical sensors. For future gravity experiments on quantum superpositions, such systems need to be cooled close to their ground state. To demonstrate the combination of state of the art vibration isolation, milligram levitated high Q mechanical resonators and position detection with low noise, we present linear feedback cooling of a magnetically levitated gravity sensor to below 2 picometer amplitude and below 10 millikelvin mode temperature for two translational modes (the x- and y-mode) simultaneously. The sensor is a levitating permanent magnet in a type I superconducting trap, where its six resonance frequencies are measured with a superconducting coil coupled to a DC SQUID. This signal is measured with a lock-in amplifier and a feedback signal is sent to a piezoelectric actuator, allowing the cooling of resonant modes at 50.6 and 68.0 Hz simultaneously. These two translational modes have Q factors of $$3.8 \cdot 10^6$$ and $$5.5 \cdot 10^6$$ respectively. The experiment is mounted inside a dry dilution refrigerator where it is vibrationally attenuated with 110-130 dB at these frequencies. In this work, we discuss future improvements on the setup which may enable quantum ground state cooling on a magnetically levitated particle, that has previously been shown to be a gravitational sensor.
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.