“Milestone in quantum sensor technology”

In medical diagnostics, sensitive sensors are needed to measure, for example, the weak magnetic fields of heart and brain activity (MCG, MEG) of the human body and create images of the body via magnetic resonance imaging (MRI), which enables the detection of diseases at an early stage.

However, only a few highly sensitive magnetic field sensors achieve the necessary precision and each of them presents major technical obstacles for clinical application. The already established squid sensors require complex cryogenic cooling of about -270 °C. Vapor cell magnetometers (OPMs) are another option. Although these achieve the highest sensitivities even without cryogenic cooling, they have the disadvantage that they require absolute shielding of all background fields, including the earth’s magnetic field, and thus place massive structural requirements on rooms and buildings. Due to this, the more inaccurate electric measurements (ECG, EEG) continue to be common in everyday clinical practice.

At the Fraunhofer Institute for Applied Solid State Physics IAF in Freiburg, a project team is already researching a more suitable alternative: “Our goal is to develop an extremely sensitive magnetic field sensor that works at room temperature as well as in the presence of background fields and is thus useful for clinical implementations,” explains Dr Jan Jeske, project manager at Fraunhofer IAF.

In the project NV-doped CVD diamond for ultra-sensitive laser threshold magnetometry (short: DiLaMag), which is funded by the German Federal Ministry of Education and Research, Jeske and his team are researching a worldwide unique approach for highly sensitive quantum magnetic field sensors. For the first time, they use diamond in a laser system, thus enabling considerably more precise magnetic field measurements.

First experimental demonstration of laser threshold magnetometry

After several years of research effort, Jeske’s team has reached an important milestone: It has demonstrated the world’s first measurement of magnetic-field-dependent stimulated emission. In the process, the researchers made an interesting discovery: “We observed a very relevant and previously unknown physical process in NV diamond: the absorption of red light induced by green laser irradiation,” Jeske reports.

Using NV diamond as a laser medium, they not only achieved a 64 % amplification of the signal power by stimulated emission. The project team was even able to set a worldwide record: The magnetic-field-dependent emission shows a contrast of 33 % and a maximum output power in the milliwatts regime. This is a new contrast record in magnetometry with NV ensembles.

Stimulated emission is responsible for this. “We were able to show that this record would not have been possible with spontaneous emission. Thus, we have experimentally demonstrated the theoretical principle of laser threshold magnetometry for the first time,” Jeske emphasizes.

These results also show the advantages of diamond-based laser threshold magnetometry over conventional methods.

Reference: Hahl et al.: Magnetic-Field-Dependent Stimulated Emission from Nitrogen-Vacancy Centres in Diamond, Sci. Adv. 8, eabn7192 (2022); DOI: 10.1126/sciadv.abn7192

Further reading: Measuring the smallest magnetic fields with diamond and laser, WileyIndustryNews, 11.4.2022 • Quantum diamond biomarker detection – targeting simpler, faster and ultrasensitive medical diagnostics; (Element Six, QDTI), PhotonicsViews 19(1), February/March 2022