On 21 June, the Chinese Academy of Sciences hailed a breakthrough – a major upgrade to a kind of quantum device that measures magnetic fields. Magnetometers have been used to detect submarines since the second world war. They are able to do this because they can measure an anomaly in Earth’s magnetic field – like one caused by a massive hunk of metal.
The new magnetometer, built by Xiaoming Xie and colleagues at the Shanghai Institute of Microsystem and Information Technology, uses not one SQUID but an array of them. The idea is that by comparing their readings, researchers can cancel out some of the extra artefacts generated by motion. This “would be relevant to an anti-submarine warfare device”, says David Caplin at Imperial College London, who works on magnetic sensors.
Researchers estimate that a SQUID magnetometer of this type could detect a sub from 6 kilometres away, and Caplin says that with better noise suppression the range could be much greater.
Researchers report a low-temperature-superconducting (LTS) SQUID based full tensor gradient system. A symmetrical configuration is used with six planar-type gradiometers mounted on the different faces of a hexagonal-pyramid. A tri-axial SQUID magnetometer was used to compensate the imbalance outputs of each planar gradiometer. Direct readout electronics are used to further increase the system robustness. The SQUID outputs are synchronized with a GPS + INS unit for coordinate projection. During indoor tests, a noise level of 100fT/m/√Hz with corner frequency at 10Hz and a static RMS resolution of 10pT/m(0.01-10Hz) were achieved. Principle demonstration was carried out by a ground test over a 10×10 m2 area using buried iron balls with different weights. The system successfully resolved the abnormalities of all the gradient components at the corresponding locations. The field test was also carried out using a helicopter.
Here is a paper (Magnetometer Underwater Detection Range, A Mechanical Expansion Amplifier Application) that discusses magnetic detection of submarines using a different device but with similar sensitivity. The company Intrel has a 2014 patent for a proposed device for detecting submarines. China may have a device that has similar capabilities to the Intrel proposal.
Magnetic Anomaly Detection (MAD) employs magnetometers to detect very small changes in the earth’s magnetic field. They are used for geophysical mineral and oil exploration, archeology, environmental surveys, ordnance and weapons detection (UXO), maritime intrusion detection, Anti-Submarine Warfare (ASW), and earth science experiments.
Compact, low power, temperature tolerant magnetometers such as the flux-gate design lack sensitivity, while the sensitive instruments based on molten potassium or cryogenic superconducting quantum interference devices (SQUIDs) require bulky insulation and significant resources to maintain their operating temperature.
The Mechanical Expansion Amplifier (MEA) configured as a magnetometer provides a low-powered, ultra-sensitive magnetometer that can operate at any temperature in the -40C to +85C range with little change in sensitivity. The inherent noise limit is lower than that of the SQUIDs.
The Gotland-class submarine has a hastalloy stainless steel hull. It has a magnetic susceptibility similar of +2000e-6/cubic meter.
The submarine is equipped with electromagnets to reduce its magnetic signature. Assume the compensation is 99% effective in masking its effect on the earth’s magnetic field. The MEA magnetometer is assumed to be equipped with a complete suite of noise suppression systems.