The tiny device is the size of a pencil and uses just 5 watts of power but can record a train braking into a station under a city street
3-MIN READ3-MINZhang Tongin BeijingPublished: 10:00am, 25 Sep 2026Chinese researchers have showed a handheld quantum magnetometer built for hunting submarines in a striking real-world test: it tracked a subway train rumbling beneath a city street.The device, developed by Peng Xinhua and colleagues at the University of Science and Technology of China, is described in a study published on September 14 in Acta Physica Sinica.
Though it is scarcely larger than a pencil and draws only 5 watts of power, it can isolate the magnetic disturbance caused by a moving train in the Earth’s vast background field, clearly recording the train braking into a station, its stop and its departure under traction.
