A single electron in a magnetic field is like a tiny top that spins and weakly "squeaks" at its own frequency. Hearing that squeak directly is nearly impossible: it gets lost in the noise. Scientists have devised a resonator: a superconducting microwave strip that works like a tuning fork tuned to the same note. When an electron comes near, its squeak makes the tuning fork ring louder, and the signal becomes noticeable.

Such a resonator has been made in several ways. Its quality factor (how many oscillations it can sustain without loss) exceeds 20,000 at the single-photon level — meaning it barely dissipates energy. The resonator works stably even in magnetic fields up to 500 millitesla, which is important for controlling spins. Thanks to this amplification, a single spin can be detected either by directly counting photons or by observing a shift in the resonator's frequency, as if the tuning fork changed its pitch slightly when the top spins nearby.