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 train in the Earth’s vast background field, clearly recording the train braking into a station, its stop and its departure under traction.

The team said the same technology was intended for anti-submarine warfare (ASW), where detecting the magnetic anomaly of a submerged vessel is the main means of locating it.

According to the paper, its other potential uses also include navigation and positioning, field mineral exploration, mine clearance and urban traffic flow monitoring.

The magnetometer works by shining laser light through a small cell of rubidium vapour. When the magnetic field around it changes, the rubidium atoms respond differently, which changes how much laser light passes through. The system measures that change to calculate the magnetic field strength.

The magnetometer works by shining laser light through a small cell of rubidium vapour. When the magnetic field around it changes, the rubidium atoms respond differently, which changes how much laser light passes through. The system measures that change to calculate the magnetic field strength.

The probe is only about 7 cubic cm (0.4 cubic inches). In the low-frequency window of 0.1 to 10 hertz, its sensitivity is about 10 picotesla per square root hertz. For comparison, devices such as superconducting quantum interference devices and spin-exchange relaxation-free atomic magnetometers are much more sensitive. They can detect fields below one femtotesla, the best performance now possible for weak magnetic fields. But they usually need very low temperatures or complete magnetic shielding, so they are hard to use when moving quickly or when magnetic interference is strong. This small magnetometer takes a different route: it is built to work reliably in real, complex and moving environments.

When measuring, the magnetometer works like a radio locked onto one channel: the resonance frequency of the atoms. If the magnetic field changes too quickly, the probe tilts, or the field is uneven, it can drift off that channel. This is known as losing lock.

The paper describes an algorithm that watches for this. It checks not only the signal strength but also whether the resonance peak still has the right shape. If the lock is lost, the system immediately scans the full frequency range to find the resonance centre again, regaining the lock in under a second.

The algorithm plays a major role in the instrument’s robustness. The hardware provides the signal, while the algorithm keeps the lock stable. Without these algorithms, the magnetometer would easily lose lock in complex external fields.

The paper notes that the Mz magnetometer made by the US company QuSpin has a slew rate of about 10,000 nanotesla per second. This magnetometer reaches 25,200 nanotesla per second, about 2½ times higher. That means it can track sudden magnetic field changes more effectively and is less likely to lose lock.

The team tested the device in other real-world settings. In one test, it monitored Earth’s magnetic field over a long period and recorded a G4-level geomagnetic storm on June 1, last year. These are severe space weather events that can cause satellite drag and navigation system errors. The storm pushed the aurora to latitudes far lower than usual – such as California – and a rare pink aurora even appeared in Heilongjiang, China.

Its data closely matched readings from an International Real-time Magnetic Observatory Network, or INTERMAGNET, station in Cheongyang, South Korea, including wave shapes, spike timing and overall trends. It kept running for a long time without stopping or losing its signal lock.

The team also used it to locate a buried magnetic rod. In a blind test in a 40-metre (131 feet) by 25-metre area of farmland, with the rod buried about half a metre deep, they scanned in a serpentine pattern with the handheld device. It produced a 2D magnetic map and located the centre of the magnetic anomaly, which matched the rod’s actual position well.

The paper also said this device could be used in urban traffic flow sensing, fault diagnosis of underground metal pipeline networks, resource exploration and the search for unexploded ordnance.