Quantum magnetometers can detect incredibly small changes in magnetic fields by tapping into the strange and powerful features of quantum physics. These devices rely on the discrete nature and coherence of quantum particles—behaviors that give them a major edge over classical sensors. But how far can their sensitivity go? And what actually makes a magnetometer “quantum?”

A new study explores the theoretical boundaries of these devices, comparing multiple methods for defining their limits. The findings shed light not only on performance but also on what truly separates quantum sensors from their classical counterparts.

Quantum Magnetometers and Ultra-High Sensitivity.