Quantum computers get a lot of attention, even though they are not ready for prime time, but quantum sensors are already doing useful work. These sensors measure fields, forces and motion so small that ordinary background noise can drown them out. Some sensors are already in daily use, while others are moving from research labs into flight tests, hospitals and field instruments.

For example, a human brain produces magnetic signals in the femtotesla-to-picotesla range – billions of times weaker than a refrigerator magnet – far weaker than the magnetic noise in an ordinary room. That is why brain scanners that measure these signals need ultrasensitive detectors and strong magnetic shielding. In some hospitals, these detectors use quantum technology to help map brain activity before epilepsy surgery, without touching the brain.

Quantum sensors are showing up in other fields as well, including in navigation when GPS signals are jammed or spoofed, mapping gravity to reveal what’s underground, and boosting astronomers’ ability to measure gravitational waves. I am a photonics and quantum technologies researcher. My lab applies physics to develop a range of devices, including quantum sensors.

What is a quantum sensor?

A sensor turns a physical effect – temperature, pressure, light, acceleration or magnetic field – into a number. Most sensors do this with engineered parts: springs, coils or computer chips. But these can drift, or become less accurate, as they age or warm up.

A quantum sensor uses a tiny quantum system as the “active ingredient” that interacts with the world to measure a physical quantity. The most common choices for quantum systems are atoms, electron spins, and superconducting circuits.

An atom has a fixed set of energy levels, like rungs on a ladder. Light or microwaves can move it between those levels only at exact frequencies. A magnetic field, motion or gravity can shift those frequencies or change the phase of the atom’s wave, and the sensor turns that shift into a measurement.

A spin is a built-in property of electrons that makes them act like an infinitesimal cross between a spinning top and a bar magnet. Using spins as a sensor means measuring how a magnetic field causes the spin to “wobble.” The spin is like a spinning top and the magnetic field is like your finger gently touching the top. How much the top........

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