Magnetic-field sensors based on nitrogen-vacancy (NV) centers in diamond are among the leading solid-state quantum sensors. They operate at room temperature, can be integrated into compact devices and enable vector magnetometry—the measurement of both the magnitude and direction of a magnetic field. Recent advances have improved their sensitivity into the sub-pT/ range, a regime relevant for detecting cardiac signals. Until now, however, researchers have had to make a trade-off: sensors with very high sensitivity offer only a very limited dynamic range, while more robust variants with a large dynamic range were considerably less sensitive.

To address this limitation, researchers at the Fraunhofer Institute for Applied Solid State Physics IAF are pursuing laser threshold magnetometry, an approach that mitigates this trade-off by combining high sensitivity with a large dynamic range. In this approach, NV centers in diamond are directly integrated into a laser cavity as an optical element, where they act as a magnetic-field-dependent loss channel. Rather than measuring the NV fluorescence, the researchers use the system’s laser signal as the readout.

First demonstration of 100 percent optical contrast

In their latest publication, the researchers demonstrate such a two-media laser threshold magnetometer. The strong coupling of the NV centers to the cavity field enhances the sensor’s response to changes in the magnetic field. The laser system thus achieves 100 percent optical contrast, rather than the few percent typical for NV magnetometry. It also delivers a coherent laser readout signal with an output power of up to 50 mW, substantially higher than that of conventional NV-fluorescence readout.

If photon shot noise is the dominant noise source, this combination of high contrast and a strong signal yields a sensitivity below 400 fT/ for each of the three components of the magnetic-field vector. The study also provides the first experimental demonstration that the attainable sensitivity improves more than linearly as optical contrast approaches 100 percent.

Up to a 590-fold improvement over the state of the art

The ratio of the sensitivity limit to the dynamic range is crucial, since improvements in one typically come at the expense of the other. Compared with conventional fluorescence-based NV sensors and vapor-cell magnetometers, the NV-diamond laser system improves this ratio by up to a factor of 590 while maintaining vector magnetometry capability.

“Our approach could resolve a trade-off that has long limited the development and application of magnetic-field sensors. It could pave the way for a new generation of mobile, highly sensitive magnetic-field sensors,” says Florian Schall, first author of the paper and a quantum-sensing researcher at Fraunhofer IAF.

A decade ago, Dr. Jan Jeske, head of the Quantum Sensing Group at Fraunhofer IAF, was the first to develop a theoretical model for a magnetic-field-dependent laser system based on a diamond containing NV centers. “The prospect of substantially improving sensing performance motivated us to pursue laser threshold magnetometry over the long term. I am very pleased that we have now experimentally demonstrated the magnetic-field-dependent shift in the lasing threshold,” says Jeske.

Potential applications for these quantum sensors include detecting biomagnetic signals from the heart, muscles, and brain; enabling magnetic-field-based navigation; and identifying magnetic anomalies, for example, in resource exploration and security applications.