Measuring tiny magnetic fields, such as those generated by brain waves, enables many new novel oppor­tunities for medical diag­nostics and treatment. The research team led by Jan Jeske at Fraunhofer Institute for Applied Solid State Physics IAF is working on a globally innovative approach to precise magnetic field measure­ments: Laser Threshold Magneto­metry. The researchers have now combined an NV diamond and a laser diode in a resonator, success­fully demonstrating the sensor system with two active media for the first time. These results represent a signi­ficant progress in the research project NeuroQ, funded by German Federal Ministry of Education and Research (BMBF).

Quantum sensors based on nitrogen-vacancy (NV) centers in diamond are already widely used for precise magnetic field measurements at room tempera­ture and in background magnetic fields. Laser Threshold Magneto­metry (LTM) is a novel research approach to measure extremely low magnetic fields in the femtotesla to picotesla range. In addition, LTM allows measure­ments with a high dynamic range without the need to suppress background fields. These features make laser threshold magneto­metry particularly useful for medical appli­cations, such as measuring bio­magnetic signals from the brain or heart.

The scientific principle of LTM has already been extensively studied in theory. Since then, the researchers have been working on the reali­zation of the first laser threshold magneto­meter. The basic concept is to develop a laser from NV centers and to use the laser light, which reacts to magnetic fields, to obtain precise information about the strength and direction of a magnetic field. The laser threshold is the point at which the laser starts or stops emitting light. As magnetic fields near the laser threshold have a very strong effect on the signal, they can be measured very precisely at this point. Compared to fluorescent light, laser signals can be measured much more accurately and over a wider dynamic range.

In 2022, they succeeded in demonstrating the world’s first magnetic field-dependent light amplification of NV centers. Due to the external laser source, however, the laser threshold of the NV centers could not yet be realized. In the current results, the researchers combined the NV diamond with a second laser medium, a laser diode for additional light ampli­fication, in an optical resonator. This enabled them to demonstrate the laser threshold for the first time: Depending on how strongly the NV centers were pumped, the laser system switched on or off.

“The results are a breakthrough for the development of laser threshold magneto­metry. On this basis, sensors with up to 100 percent contrast, strong light signals and a wide range of measurable magnetic field strengths can be realized in the future,” says Jan Jeske. The work of his colleague Lukas Lindner shows an early stage of the lighthouse project “Laser threshold magneto­meter for neuronal communication interfaces”, or NeuroQ for short. The NeuroQ project team is currently working on further developing the innovative NV diamond laser system, which is currently in the patent appli­cation process, and increasing its sensitivity.

The NeuroQ consortium is developing high-precision quantum sensors for medical applications: The quantum sensors will measure neuronal activity and transmit the signals to an exo­skeleton via a brain-computer interface. This techno­logy will enable paralyzed people to control an exo­skeleton with their thoughts and thus regain some of their mobility. (Source: Fh.-IAF)

Reference: L. Lindner et al.: Dual-media laser system: Nitrogen vacancy diamond and red semiconductor laser, Sci. Adv. 10, adj3933 (2024); DOI: 10.1126/sciadv.adj3933

Link: Project NeuroQ, Fraunhofer Institute for Applied Solid State Physics IAF, Freiburg im Breisgau, Germany