Scientists have explored a novel operating mode for quantum magnetometers based on nitrogen-vacancy (NV) centers in diamond. Instead of operating these sensors in their usual stable regime, they deliberately forced a feedback loop system to operate beyond its stability point. This approach allowed the magnetometer to become a self-sustained oscillator, whose limit cycle is generated by the control loop itself. This method could enhance the sensitivity and detection capabilities of these devices.

The experiment focused on a continuous-wave magnetometer based on an ensemble of NV centers, whose probe frequency was frequency-modulation (FM) locked to one flank of its optically detected magnetic resonance (ODMR). By increasing the software loop gain, G, the system was driven through a flip (period-doubling) bifurcation of its discrete feedback map. Beyond a critical gain, Gc, the lock became unstable and started to self-oscillate, generating a limit cycle intrinsic to the system.

A threshold condition, Gc = 2Dcal/Dtrue, was derived, relating the critical gain to the ratio of the transduction slope of the ODMR lock-in signal at calibration time (Dcal) to its current value (Dtrue). Experimental results on the prototype show a bifurcation with onset at Gc ≈ 2, consistent with predictions for a self-calibrated loop. Furthermore, sub-threshold critical fluctuations following a √(G/(2-G)) divergence were observed, validating the theoretical model. This new operating mode opens avenues for more robust and potentially more sensitive quantum sensing, especially in environments where sensor stability is crucial.