Measuring faint magnetic fields is important for everything from mapping brain activity to monitoring hearts to probing fundamental physics. But the most sensitive instruments typically need bulky and expensive support: liquid-helium cooling tanks or specially shielded rooms that block out Earth's magnetic field. Now researchers report a miniature sensor that achieves remarkable sensitivity at room temperature, with none of that infrastructure.
The team suspended a tiny permanent magnetic disk — smaller than a grain of rice — inside a glass vacuum chamber, where it levitates in midair. An overhead stack of magnets pulls the disk upward against gravity, while a graphite plate beneath it provides a repulsive force that keeps it stable. Because the magnet is the sensor — it reacts to any external magnetic field — its motion can be read out to detect the faintest of signals.
In tests, the levitated magnetometer reached a sensitivity of about 32 femtoteslas per square root of hertz: fields roughly a hundred million times weaker than Earth's magnetic field. That performance is comparable to much larger and costlier systems.
The approach, described in a new paper, belongs to the fast-moving field of levitated optomechanics, where micron-scale objects are trapped and used as ultrasensitive probes. A magnetometer that works at room temperature and can be shrunk further could bring high-precision magnetic sensing out of specialized laboratories.
Potential applications range from non-invasive brain and heart monitoring to detecting subtle magnetic anomalies for geophysics and even tests of fundamental physics. For now, the floating grain of rice is a proof of principle — but it shows that the most delicate measurements do not always need the most elaborate machinery.
