Chinese 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...