Quantum computers get all the hype, but quantum sensors could be equally transformative, enabling autonomous vehicles that can “see” around corners, underwater navigation systems, early-warning systems for volcanic activity and earthquakes, and portable scanners that monitor a person's brain activity during daily life.

Quantum sensors reach extreme levels of precision by exploiting the quantum nature of matter—using the difference between, for example, electrons in different energy states as a base unit. Atomic clocks illustrate this principle. The world time standard is based on the fact that electrons in cesium 133 atoms complete a specific transition 9,192,631,770 times a second; this is the oscillation that other clocks are tuned against. Other quantum sensors use atomic transitions to detect minuscule changes in motion and tiny differences in gravitational, electric and magnetic fields.

There are other ways to build a quantum sensor, however. For example, researchers at the University of Birmingham in England are working to develop free-falling, supercooled atoms to detect tiny changes in local gravity. This kind of quantum gravimeter would be capable of detecting buried pipes, cables and other objects that today can be reliably found only by digging. Seafaring ships could use similar technology to detect underwater objects.

Most quantum-sensing systems remain expensive, oversized and complex, but a new generation of smaller, more affordable sensors should open up new applications. Last year researchers at the Massachusetts Institute of Technology used conventional fabrication methods to put a diamond-based quantum sensor on a silicon chip, squeezing multiple, traditionally bulky components onto a square a few tenths of a millimeter wide. The prototype is a step toward low-cost, mass-produced quantum sensors that work at room temperature and that could be used for any application that involves taking fine measurements of weak magnetic fields.

Quantum systems remain extremely susceptible to disturbances, which could limit their application to controlled environments. But governments and private investors are throwing money at this and other challenges, including those of cost, scale and complexity; the U.K., for example, has put £315 million into the second phase of its National Quantum Computing Program (2019–2024). Industry analysts expect quantum sensors to reach the market in the next three to five years, with an initial emphasis on medical and defense applications.