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China works on quantum gravity sensor to detect US nuclear submarines just by their mass.
China is advancing a superconducting quantum interference device (SQUID)-based gravity sensor through the Chinese Academy of Sciences, aiming to detect submerged nuclear submarines by measuring minute gravitational disturbances generated by their mass.
The system, recently demonstrated outside laboratory conditions, represents a shift toward deployable quantum sensing capabilities that could enable passive detection of stealth naval assets without relying on acoustic or electromagnetic signals. According to a report published on April 3, 2026, by the South China Morning Post, the compact sensor (approximately the size of an office cubicle) achieves sensitivity levels approaching large-scale observatories, marking a critical step toward potential use in anti-submarine warfare.
The development of gravitational sensors means that submarines could be detected in the future just because their mass slightly distorts the spacetime, creating tiny but measurable changes that current sensors can't pick up as they rely on sound or signals. (Picture source: US Navy and China's CAS)
On April 3, 2026, the South China Morning Post announced that a research team from the Chinese Academy of Sciences unveiled a compact gravity-detecting system based on superconducting quantum interference device (SQUID) technology, capable of measuring extremely small variations in gravitational fields with a level of precision approaching large-scale observatories. This gravity sensor now has roughly the volume of an office cubicle and was used outside controlled laboratory environments, marking a transition from fixed experimental instruments toward real-world assets. Initially intended for geophysical survey and subsurface resource detection, this gravity sensor is now said to be considered for military use.
This type of sensor could be particularly relevant for anti-submarine warfare (ASW), where its passive ability to detect a mass through gravitational disturbances introduces a potential application of quantum sensing for detecting stealth assets, including U.S nuclear submarines. The primary technical advance lies in the suppression of gravity gradient measurement noise, which typically originates from seismic motion, structural vibration, and thermal fluctuations, and has been reduced to a level approaching that of large interferometric facilities such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), despite the system’s size being several orders of magnitude smaller.
This indicates that a kilometer-scale length is no longer the sole determinant of sensitivity in gravity measurements, and that noise isolation and signal processing can compensate for reduced scale, which is the key enabling factor for moving from experimental setups to potential field applications across both civilian and military domains. The Chinese gravity sensor system is based on a superconducting quantum interference device (SQUID) architecture that uses superconducting materials cooled to very low temperatures to eliminate electrical resistance and enable quantum interference effects. Please note: I acknowledge that the following content contains a lot of scientific terminology, but it has been made as accessible as possible.
These cryogenic temperatures enable the formation of Josephson junctions that respond to phase differences in quantum wavefunctions, allowing the device to measure extremely small changes in magnetic flux, which are induced by positional shifts of a suspended test mass, typically in the kilogram range, held in a near-frictionless state through magnetic levitation enabled by the Meissner effect. This effect thereby eliminates mechanical disturbances that would otherwise obscure measurements, with displacement sensitivity reaching the nanometer or sub-nanometer scale, which is then converted into electrical signals for analysis, effectively turning the device into a precision instrument that exceeds the sensitivity limits of traditional sensors.
Unlike classical gravimeters that measure absolute gravitational acceleration, this system measures spatial gradients, meaning it compares gravitational differences between two closely spaced points. This allows the detection of localized anomalies caused by nearby mass, with measurable variations on the order of 10⁻⁹ to 10⁻¹⁰ m/s² (9.800000000 to 9.800000002 m/s²), which corresponds to changes in the ninth decimal place of standard gravity. This enables the detection of the movement of nearby massive objects through their gravitational influence rather than relying on electromagnetic or acoustic detectable signals, making the Chinese system inherently passive, without revealing the sensor’s presence, as there is no reliance on external emissions such as emitted energy, reflections, or signal propagation.
This key notion distinguishes it from conventional anti-submarine warfare methods, where sonar depends on acoustic propagation, radar is ineffective underwater, and magnetic anomaly detection (MAD) is limited by range and background noise. In contrast, a gravity-based detection system is theoretically unaffected by acoustic quieting, hull coatings, or electromagnetic shielding, since mass cannot be altered or concealed, introducing a detection parameter that is constant and unavoidable, although the magnitude of the gravitational signal remains extremely small and difficult to isolate, which currently limits operational viability.
In practical terms, a submerged object with a displacement of 18,000 tons, such as a U.S. Ohio-class nuclear ballistic submarine, produces a gravitational anomaly that decreases with distance according to an inverse-square relationship, which could be detected and tracked, particularly if integrated into China's broader surveillance architecture that includes dozens of surface vessels and hundreds of oceanic sensors. However, the detection range of a gravity sensor is highly dependent on sensor sensitivity and environmental noise, and current systems are likely limited to relatively short ranges and do not yet support reliable operational tracking in open ocean conditions, as it would probably require multiple sensors to triangulate a signal.
But still, it introduces, on the paper, the possibility of tracking submarine movement patterns over time, if a sufficient sensor density is achieved across maritime areas, particularly when combined with existing deployments of surface vessels and seabed monitoring infrastructure. As a gravity sensor relies on mass, which cannot be concealed or altered, a new detection parameter that is constant for all physical objects enters the race, shifting the standard submarine detection model from signal-based identification to direct measurement of physical presence, suggesting that such systems could complement existing sensor networks in the future.
Like the LIGO, which features two main facilities, each with vacuum chambers forming L-shaped, 4-kilometer (2.5-mile) long arms, the main limitation of gravitational sensors remains the signal-to-noise ratio, as these systems are highly sensitives to environmental disturbances such as seismic activity, ocean wave motion, atmospheric pressure changes, nearby vessel movement, human activity, and biological sources such as large marine animals, all of which can generate signals larger than those produced by distant targets such as black holes or submarines. This requires more advanced isolation systems, like mechanical vibration damping, magnetic shielding, and computational techniques.
Therefore, measurement precision and signal isolation represent the primary technical bottlenecks, and ongoing work focuses on improving stability, calibration accuracy, and real-time signal processing, including the application of machine learning techniques to distinguish target signatures from background noise. China is already accelerating the development of artificial intelligence across military and scientific applications to mitigate these problems. Furthermore, the claimed ability of the Chinese gravity sensor system to operate outside laboratory conditions indicates that the system can maintain measurement stability in environments with uncontrolled interference, which is a prerequisite for deployment on mobile platforms.
This single fact could support a potential deployment on naval vessels, aircraft, and unmanned systems in the future, enabling its use in ISR missions and the development of distributed sensing networks across maritime environments, where mobility allows sensors to be repositioned based on operational needs and increases coverage. However, like the railguns, physical constraints remain, such as maintaining superconducting conditions in cryogenic temperatures, ensuring continuous calibration under motion, and managing power and cooling requirements alongside other high-end systems such as radars.
From a strategic perspective, the potential deployment of gravity-based detection systems challenges the existing assumptions underlying submarine stealth, particularly for nuclear deterrence forces that rely on concealment for survivability, as it introduces a detection mechanism that focuses solely on the mass/displacement. This could influence second-strike survivability if detection capabilities improve, while also creating incentives for countermeasures and alternative concealment strategies. This could also lead to a race about the integration of quantum technologies in military operations, as such ISR systems theoretically expand a country's detection capabilities beyond electromagnetic and acoustic domains, even though current systems remain at an experimental stage and are not yet capable of consistent operational deployment.
Written by Jérôme Brahy
Jérôme Brahy is a defense analyst and documentalist at Army Recognition. He specializes in naval modernization, aviation, drones, armored vehicles, and artillery, with a focus on strategic developments in the United States, China, Ukraine, Russia, Türkiye, and Belgium. His analyses go beyond the facts, providing context, identifying key actors, and explaining why defense news matters on a global scale.