Quantum Essentials · Module 3 · Chapter 12 of 17
How does quantum sensing work, and where is it used today?
Your phone knows where you are because a satellite carries an atomic clock. That's quantum sensing, and it's been working for decades.
The short answer (tap to open)
Quantum sensors use atoms, light or tiny defects in diamond whose energy steps are identical everywhere and shift very slightly when gravity, magnetism, motion or time change. Interference turns those shifts into readable signals. Atomic clocks already run GPS and the internet. Quantum gravimeters, brain scanners and navigation systems are in trials now, years ahead of large quantum computers.
Predict
Which quantum technology is already used by billions of people every day?
After this page you can
- explain why quantum sensors are so precise;
- name four kinds of quantum sensor and what each measures;
- give real, dated examples of quantum sensing in use today;
- tell a trial from a product, and a company claim from a checked result.
Part A · Why quantum makes the best rulers
Why are atoms such good measuring tools?
Every caesium atom is identical to every other caesium atom. Its energy steps (chapter 1) sit at exactly the same spacing in London, Tokyo or on a satellite. No factory makes two rulers that alike. So an atom makes a perfect reference: measure its steps, and you're measuring against nature's own standard.
Then how does an atom sense anything?
Those steps shift very slightly when something nudges the atom. A magnetic field moves them. Gravity and acceleration change how an atom's wave travels. A sensor prepares atoms in a known state, lets the world nudge them, then reads how far they moved.
Here's the trick that makes it quantum. The sensor splits each atom into a superposition (chapter 2) of two paths or two states, lets them travel, then recombines them. The tiny difference between the two shows up as interference (chapter 4): bright or dark, more counts or fewer. Interference turns an impossibly small shift into a clear signal.
Compare the push with a ruler's millimetre marks.
Interference reads shifts far smaller than anything you could see.
The fringes, in words
Bright and dark stripes from the recombined atom waves. A tiny push, far smaller than an atom's width, slides the stripes along, so a detector at one spot sees bright turn to dark.
Interference reads shifts far smaller than anything you could see.
Why does fragility help here?
In chapter 8 fragility was the enemy: the world disturbs qubits and ruins the computation. In a sensor, that sensitivity is the whole point. A qubit that reacts to the faintest magnetic field is a terrible memory and a superb magnetometer. Same physics, opposite job.
Part B · The four main kinds of quantum sensor
What do quantum sensors measure?
- Clocks measure time. Atoms switch between two levels at a fixed rate; count the switches and you have a clock.
- Atom interferometers measure gravity, tilt and acceleration. They split clouds of cooled atoms into two paths and recombine them.
- Magnetometers measure magnetic fields. Some use gas atoms warmed in a small glass cell; others use tiny flaws in diamond called NV centres.
- Quantum light sensors use carefully prepared light to measure with less noise. The giant LIGO detectors use this "squeezed light" to hear gravitational waves.
Sensor to job
Clock
Atom interferometer
Magnetometer
Quantum light sensor
Time a phone network
The answers, in words
- Time a phone network: Clock. Atomic clocks time phone networks, the internet and financial markets.
- Find a tunnel under a road: Atom interferometer. University of Birmingham, 2022: a quantum gravity sensor found a hidden 2 m tunnel under a road.
- Record a child's brain activity: Magnetometer. Wearable brain scanners use small quantum magnetometers; a UK children's epilepsy charity got one in 2021.
- Navigate a submarine: Clock. October 2025: an optical clock dived on a Royal Navy uncrewed submarine (company claim). Atom interferometers that track motion help too.
- Map magma under a volcano: Atom interferometer. Since 2020 a quantum gravimeter has tracked magma on Mount Etna's summit by its weight.
- Detect a gravitational wave: Quantum light sensor. Since 2019 LIGO has used squeezed light to hear gravitational waves.
Part C · Where is quantum sensing used today?
How do atomic clocks run the modern world?
In 1955 Louis Essen built the first accurate caesium clock at the National Physical Laboratory in Teddington (page 12c). Since 1967 caesium has defined the second itself: 9,192,631,770 switches.
Today the best optical clocks, which use atoms switching at the much faster rate of visible light, beat the caesium standard by up to a hundred times. In 2025 NIST reported an aluminium-ion clock with an uncertainty of 5.5 parts in 10¹⁹. Over the age of the universe that's an error of well under a second. Scientists aim to redefine the second around these clocks in 2030 (page 12a).
- Pendulum clockDrift: 0.9 seconds
- Quartz watchDrift: 86 thousandths of a second
- Caesium clockDrift: less than a billionth of a second
- Optical clock (NIST, 2025)Drift: less than a billionth of a second
The pendulum, quartz and caesium figures are illustrative bands. The optical clock uses NIST's 2025 figure: 5.5 parts in 10¹⁹.
The drift after one year, in words
- Pendulum clock: 5.3 minutes
- Quartz watch: 32 seconds
- Caesium clock: 1.6 billionths of a second
- Optical clock (NIST, 2025): less than a billionth of a second
The pendulum, quartz and caesium figures are illustrative bands. The optical clock uses NIST's 2025 figure: 5.5 parts in 10¹⁹.
Can you navigate without GPS?
GPS works by timing signals from satellites. Jam or block those signals and ships, planes and phone networks lose their way. The UK government estimates a seven-day loss of satellite navigation would cost £7.6 billion.
Quantum navigation carries the reference on board instead: a precise clock plus atom sensors that track every turn and acceleration. Dated trials:
- May 2024: a UK consortium flew an optical clock and a cold-atom system on an RJ100 aircraft. These were parts of a navigator, not a complete one.
- 2024: Boeing and AOSense flew for four hours without GPS using a quantum inertial unit (company claim).
- April 2025: Q-CTRL navigated about 500 km by reading the Earth's magnetic map with quantum magnetometers, reporting up to 50 times less position error than standard methods (company claim).
- 2023 to 2025: Imperial College's quantum sensor sailed on a Royal Navy research ship, then went to the Arctic. Its team says it still needs to become "shock resistant".
The UK's target: quantum navigation on aircraft by 2030.
What else is in use or in trials?
- Seeing underground: in 2022 a University of Birmingham quantum gravity sensor found a hidden 2 m tunnel under a road, the first such sensor to work outside a lab (Nature). Its spin-out Delta.g delivered a sensor to the Department for Transport in 2025.
- Watching volcanoes: a quantum gravimeter from the French firm Exail went to the summit of Mount Etna in 2020 to track magma by its weight (page 12b).
- Scanning brains: wearable brain scanners built in Nottingham use small quantum magnetometers on a helmet instead of a fixed, fridge-cooled machine. Children can move during a scan. A UK children's epilepsy charity got one in 2021. They aren't routine in the NHS yet: the first two NHS clinical sites are due from June 2027 (page 12b).
- From space: since March 2026 a diamond-based quantum magnetometer has been in orbit for a US magnetic-mapping challenge.
Product, trial or lab?
In everyday use
Real-world use or trial
Lab demonstration
Atomic clocks in GPS satellites
The answers, in words
- Atomic clocks in GPS satellites: In everyday use. Everyday: every GPS satellite carries them.
- The quantum gravimeter on Mount Etna: Real-world use or trial. Real-world use: volcano research since 2020.
- The wearable brain scanner: Real-world use or trial. Real-world use in clinical research; the first NHS sites are due from June 2027.
- The RJ100 flight: Real-world use or trial. Trial: parts of a navigator, flown in May 2024.
- Q-CTRL's GPS-free flight: Real-world use or trial. Trial, and a company claim (April 2025).
- The Birmingham tunnel find: Real-world use or trial. Real-world use: the first field result, 2022.
- Optical clocks: Lab demonstration. Lab, moving into trials.
- Quantum radar: Lab demonstration. Lab: see the media check below.
Myths and media
Media check
Right, half right or wrong? Call it first.
Card 1 of 3
A headline: "Quantum compass will replace GPS."
Check
Three quick ones
Collect
Your aha card
Nature's own rulers.
Identical atoms, read by interference, measure time, gravity and magnetism better than anything else we have. Some of this is already in your pocket.
Go further
- 12a · Real-world caseAtomic clocks, from GPS to redefining the second
- 12b · A step deeperUK case studies: gravimeters and wearable brain scanners
- 12c · People storyLouis Essen and the first caesium atomic clock (NPL, Teddington, 1955)
Company information is for education only. It isn't investment advice. Figures carry dates.