Chinese-German team builds ultra-sensitive ‘floating compass’ to hunt dark matter

Chinese researchers have developed a table-top magnetic sensor that operates at room temperature and can spot magnetic signals a billion times fainter than the Earth’s own field. The Levitated Magnet Magnetometer (LeMaMa) functions like a compass with a needle that floats in mid-air with virtually no friction. By tracking the needle’s tiny deflections, the team can measure magnetic fields down to the femtotesla level – an advance that could widen the hunt for elusive dark matter particles or monitor brain signals. “The combination of room-temperature operation, tiny components and high sensitivity makes LeMaMa promising for fundamental physics experiments,” Ji Wei, assistant professor at Peking University’s school of physics and corresponding author of the study, told Guangming Daily on August 17. Magnetic fields are everywhere, ranging in strength from about 1.5 tesla for a hospital magnetic resonance imaging machine to the vanishingly weak femtotesla, or a quadrillionth of a tesla, generated by human brain activity. Measuring the tiniest magnetic fields has long been a challenge. Now, scientists from China’s Peking University and Johannes Gutenberg University Mainz in Germany have built the world’s first room-temperature ultraprecise magnetometer based on a levitated magnet. Their work was published in the journal Science on August 6. Until now, femtotesla-level measurements have mainly relied on two approaches. One is the superconducting quantum interference device (Squid) magnetometer, which offers excellent sensitivity but must operate at cryogenic temperatures near absolute zero, requiring bulky and expensive liquid-helium cooling. The other is the spin-exchange relaxation free (Serf) atomic magnetometer, which also reaches femtotesla sensitivity but needs heated atomic vapour and only works in a near-zero magnetic field inside heavy shielding – meaning the sample must be brought into the shielded room, limiting close-range, high-resolution detection. LeMaMa works at room temperature, needs no magnetic shielding and is miniaturised. It requires no liquid helium, no heating and no bulky shielded room. The entire device fits inside a vacuum chamber about the size of a lunchbox. This opens up a new path for taking ultrasensitive magnetic detection out of the lab and into the real world. LeMaMa is a bit like a compass. For a compass to be truly sensitive, its needle needs to turn with no resistance – ideally, floating in the air. At the heart of LeMaMa is a tiny sensing magnet just 0.4 mm (less than 0.02 inch) thick. The team suspended another magnet above this “needle” to provide an upwards force that counteracts gravity. But a single magnet alone cannot hold it steady. “It’s like balancing an egg on a smooth table – it might stay for an instant, but the slightest disturbance knocks it over,” Guangming Daily quoted Ji as saying. “To solve that, we place specially designed diamagnetic materials underneath, which push against the magnet instead of attracting it – like an invisible hand supporting it from below.” With these two forces working together, the tiny sensing magnet floats stably in mid-air, becoming a compass without a pivot or suspension thread. When an external magnetic field is applied, the sensor swings slightly. By measuring that deflection, the team can infer the strength and variation of the outside field. To reach extreme sensitivity, they also had to cut down environmental noise. The sensing magnet is housed in a vacuum chamber, and the whole set-up sits on a vibration-isolation stage. In lab tests, LeMaMa showed extreme sensitivity to ultra-weak magnetic signals, despite operating in a strong background field. This means the device can pick up tiny magnetic fluctuations at the femtotesla level – even against background noise from the Earth’s natural magnetic field that is billions of times stronger. The sensor could also help search for new particles such as dark matter. “The team has already applied LeMaMa to detect axion dark matter, improving sensitivity over previous best results by multiple orders of magnitude in a specific mass range,” Guangming Daily quoted Ji as saying. According to a report on the Peking University website, because LeMaMa’s core sensor is just a few hundred micrometres wide, the device is both highly sensitive and small. This makes it well suited for high-resolution magnetic imaging and close-range magnetic field detection. In biomedicine, it could measure neural magnetic signals for brain research and neurological diagnostics. In geophysical exploration, its small size makes it ideal for high-precision magnetic mapping and mineral resource surveying.

Chinese-German team builds ultra-sensitive ‘floating compass’ to hunt dark matter

.css-1c6uqr6{color:inherit;font-weight:inherit;font-size:inherit;font-family:inherit;line-height:inherit;overflow-wrap:break-word;}The device detects vanishingly weak magnetic signals, such as those produced by the human brain, and can operate at room temperature

The device detects vanishingly weak magnetic signals, such as those produced by the human brain, and can operate at room temperature