Chinese researchers have demonstrated a handheld quantum magnetometer built for submarine detection by using it on a subway train, tracking the carriages as they moved beneath a city street.
The device was developed by Peng Xinhua and colleagues at the University of Science and Technology of China, and is described in a study published on 14 September in the journal Acta Physica Sinica.
Though it is scarcely larger than a pencil, with a volume of about seven cubic centimetres, and draws only five watts of power, it can isolate the magnetic disturbance caused by a moving train within the Earth's vast background field. It recorded the train braking into the station, its stop, and its departure under traction.
The instrument works by passing laser light through a small cell containing rubidium vapour. Changes in the surrounding magnetic field alter how the rubidium atoms respond, and how much light gets through the cell; from that difference, the system calculates the field strength.
In the low-frequency range from 0.1 to 10 hertz, its sensitivity is around 10 picotesla per square root of hertz. The authors note that this is less sensitive than some superconducting and atomic magnetometers, but that their device is designed to work in a moving environment and under strong magnetic interference.
Its rate of tracking changes in the field reaches 25,200 nanotesla per second, against a figure of about 10,000 nanotesla per second which the paper gives for the Mz magnetometer made by the American company QuSpin. An algorithm controls the atomic resonance and, if a stable signal is lost, reacquires it in under a second.
The device was also run during a G4-level geomagnetic storm, with its readings compared against those of an INTERMAGNET network station in Cheongyang, South Korea. In a separate test it was used to locate a magnetic rod buried about half a metre deep in an agricultural plot measuring 40 metres by 25.
The authors listed navigation, mineral exploration, urban traffic monitoring and submarine detection among the technology's potential applications.
An instrument, in the end, is defined by what it has found. The published record for this one now reads in full: one subway train, braking, stopping and departing; and one metal rod, half a metre down, in a plot forty metres by twenty-five.
The device does not distinguish between these things, and was never asked to. It reports a disturbance in the local magnetic field. A disturbance is a disturbance.
Consider what a subway train is, from the instrument's point of view. It is a long metal object, moving slowly, at depth, beneath a dense medium, generating a field the background does not account for. That is also, line for line, the description of the intended target. The train is not a poor substitute for a submarine. It is a submarine with a timetable, and the buried rod is a submarine at rest.
The Department of Unnecessary Research has reviewed the log. No submarine has yet been detected, and none has been ruled out.