Diamonds sometimes have a defect: instead of two adjacent carbon atoms, there is one nitrogen atom and an empty spot. This defect acts like a tiny antenna that responds to magnetic fields. Scientists aimed a laser and microwaves at such a diamond, and the defect started spitting out electrons. A weak electric current emerged, just a few picoamperes — a billion times smaller than the current in a light bulb.

That current is the signal. The stronger the magnetic field, the more the current changes. The researchers picked up these changes in various ways, like a tuner homing in on a note amid the noise. The best result: a sensitivity of 397 nanoteslas per square root of hertz. That means the sensor can detect a field hundreds of times weaker than Earth's magnetic field, if you wait a second.

For the first time, such a sensor was put to work: it tracked a varying magnetic field in real time, with an error of just 1.5 microteslas. It's like listening to the pulse of a magnetic field without missing a beat. The scientists also tested how laser and microwave power affect the signal, and theory matched experiment down to the details.

Now the diamond sensor is not just a lab curiosity but a working tool. Such sensors could be built into a chip and carried around to measure magnetic fields in your phone, at a hospital, or in a factory. The magnetic field is no longer invisible: the diamond turns it into a current you can see on a screen.