Earth's magnetic field, the one that swings a compass needle, is strong. The new sensor picks up fields a hundred thousand times weaker, and it needs neither liquid helium nor bulky setups. Everything fits on a circuit board, just like the one inside your smartphone.

The secret lies in a pair: a copper resonator and a crystal of yttrium iron garnet. The resonator is a cavity where microwaves bounce around like echoes in an empty hall. The crystal can soak up that echo and give it back. When an alternating magnetic field appears nearby, it gives the crystal a gentle nudge, and side tones emerge in the signal, like overtones on a string. Those tones give the weak field away.

Previously, such measurements required cooling to near absolute zero or huge coils. Now everything fits in a palm-sized box and works at room temperature. Magnetic hearing becomes cheap and portable — for heart cardiograms, finding flaws in metal, or underground communication.