Quantum computers get the headlines, but some of the most practical quantum devices of the coming decades may be measuring instruments. The same delicacy that makes qubits so hard to protect turns out to be a gift when the goal is to notice tiny changes.
Fragility as a strength
Quantum sensors turn the fragility of quantum states into a strength: because those states react to tiny disturbances, they can measure very small changes in time, gravity, magnetism and motion. A qubit that decoheres when a magnetic field brushes past it is, seen from another angle, an extremely good magnetic field detector. Many experts expect quantum sensing to reach wide practical use sooner than quantum computing, because it needs only a few well-controlled quantum systems rather than millions of error-corrected qubits.
Clocks that feel gravity
The cesium clocks of level 1 are being overtaken by optical clocks, which use atoms such as strontium or ytterbium whose electrons switch energy levels hundreds of trillions of times per second, tens of thousands of times faster than cesium's rhythm. The best of these clocks would drift by less than a second over billions of years. Einstein's general relativity says time runs slightly slower deeper in a gravitational field, and these clocks can now see it on a laboratory bench. The best optical clocks can detect the slowing of time from gravity across a height difference of about a millimeter, as a team at JILA in Colorado reported in 2022. Clocks this sensitive could map Earth's gravity field and may one day redefine the second, perhaps around 2030.
Weighing the ground
Atoms cooled close to absolute zero behave like waves. Atom interferometers measure gravity and acceleration by splitting clouds of cold atoms into waves that travel different paths and then recombining them; the way the waves interfere reveals tiny differences in the pull each path felt. Instruments built this way, called quantum gravimeters and gradiometers, can sense hidden voids, pipes, tunnels and buried structures from the surface. In 2022 a team at the University of Birmingham in England reported in the journal Nature that its quantum gravity gradiometer had detected a buried tunnel outdoors. Applications include finding sinkholes before they open, surveying archaeological sites without digging and tracking groundwater.
Navigation without satellites
GPS signals are weak, and jamming and spoofing have become common near conflict zones, confusing ships and airliners. Quantum inertial sensors could let ships and planes navigate when satellite signals are jammed or spoofed, because they track motion without any outside signals. Ordinary inertial sensors drift over time; quantum versions, which measure acceleration and rotation with atoms, promise to drift far less. Trials on ships, aircraft and trains, including British tests in 2024, have begun, but the devices are still large and expensive.
Magnetic fields of the brain and beyond
The brain's electrical activity produces magnetic fields a billion times weaker than Earth's. Since the 1970s these have been recorded with SQUIDs, superconducting sensors that need bulky liquid helium cooling and a rigid helmet. A newer device works near room temperature. Optically pumped magnetometers allow wearable brain scanners that can record the brain's magnetic fields while a person moves, which helps in studying children and in locating the source of epileptic seizures. Another sensor uses tiny flaws in diamond, called nitrogen-vacancy centers, whose spins respond to magnetic fields at the scale of single cells.
Listening to the cosmos
LIGO, the detector that first recorded gravitational waves in 2015, measures changes in the length of its arms far smaller than a proton. At that level, the quantum jitter of light itself becomes a limit. LIGO uses "squeezed" light, a quantum trick that reduces noise in one measured property, to cut quantum noise and hear more distant events.
Limits and dual use
Moving sensors from quiet laboratories to bumpy roads and ships is hard; vibration, temperature swings, size and cost all bite. And sensors raise security questions. Some analysts warn that better sensors might one day make submarines easier to find, which could unsettle nuclear deterrence, though many experts doubt this is near, since seawater and ocean noise limit detection range. Sensing also belongs in debates about surveillance.
Worked example: the leaking water main
A city water utility knows a main is leaking somewhere under a busy street but not where. The usual approach is to dig test holes, each one closing a lane of traffic for days. Suppose the utility hires a team with a quantum gravity gradiometer. The team walks the street, taking readings every meter or so. Where water has washed soil away, or where the pipe runs, the slight change in mass beneath the surface changes the local pull of gravity, and the atom interferometer picks it up. The map narrows the search to a few meters, and one hole does the work of five. The example shows both the promise and the limits. The technique is quantum mechanics put to plain civic use, saving money, water and disruption. But the instrument is expensive, needs skilled operators and works best on quiet days with little vibration. Whether such devices become as routine as ground-penetrating radar depends on making them smaller, tougher and cheaper, an engineering problem rather than a physics one.