Researchers at China’s University of Science and Technology have shown that their handheld quantum magnetometer can detect a subway train moving under a city street. In real-world tests, the small device tracked changes in Earth’s magnetic field as the train arrived, stopped, and left the station.

Peng Xinhua and his team developed the magnetometer for places where regular high-sensitivity instruments are hard to use. They believe this technology could one day help detect submarines by picking up their magnetic signatures underwater.

The handheld device is about 7 cubic centimeters (0.4 cubic inches) in size and uses just 5 watts of power. Even though it is small, it can spot magnetic disturbances against Earth’s much larger background magnetic field.

The magnetometer uses a small cell filled with rubidium vapor. Laser light passes through the vapor while changes in the surrounding magnetic field alter the behavior of the rubidium atoms. Those changes affect the amount of transmitted laser light, allowing the system to calculate magnetic field strength.

The device has a sensitivity of about 10 picotesla per square root hertz across the 0.1 to 10-hertz low-frequency range.

More sensitive instruments such as superconducting quantum interference devices and spin-exchange relaxation-free atomic magnetometers can detect fields below one femtotesla. However, they can require extremely low temperatures or extensive magnetic shielding.

The Chinese team’s device is built to work well in changing environments. It keeps track of the resonance frequency of the rubidium atoms all the time, much like how a radio stays tuned to a particular station.

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