A team from Fraunhofer Institute for Applied Solid State Physics (Fraunhofer IAF), with diamond sensing technology experts from RMIT University, demonstrated that magnetic-field-dependent emission from nitrogen-vacancy (NV) centers in diamond deliver a measurement of magnetic fields with 10× more precision than current state-of-the-art techniques. This boost in sensitivity could improve existing techniques for magnetically sensing and mapping brain activity in disorders such as concussion, epilepsy, and dementia. Negatively charged NV centers in diamond have shown potential as magnetic field quantum sensors. Laser threshold magnetometry predicts that, in theory, NV center ensemble sensitivity could be improved through increased signal strength and magnetic field contrast. To demonstrate laser threshold magnetometry in practice, the team used a laser cavity containing a NV-doped, low-absorbing diamond gain medium. The NV centers were pumped at 532 nm, and the cavity was seeded at 710 nm, which led to a 64% signal power amplification through stimulated emission. The researchers tested the magnetic field dependency of the amplification to demonstrate magnetic field-dependent stimulated emission from an NV center ensemble. The resulting emission showed an ultrahigh contrast of 33% and a maximum output power in the milliwatt regime. Marco Capelli, a co-researcher on the work at the Australian Research Council (ARC) Centre of Excellence for Nanoscale BioPhotonics laboratories at RMIT University. An international team has team developed a laser-based diamond sensor that can measure magnetic fields up to 10× more precisely than standard techniques. The advancement supports increasingly portable instruments that can be used to detect concussion, epilepsy, and dementia. Courtesy of RMIT University. According to the researchers, the ensemble contrast achieved in this experiment is a new record for NV centers and is higher than what can be achieved with spontaneous emission. It shows the advantages of coherent cavity readout for sensing and the principle of laser threshold magnetometry, reportedly for the first time. Diamond is already being used for sensing magnetic fields. The amount of light that comes from quantum defects in the diamond changes with the strength of the magnetic field, and most of that light is lost. “Our breakthrough was to make a laser from the defects,” said RMIT professor Andrew Greentree. “By collecting all the light — instead of just a small amount of it — we can detect the magnetic field 10 times more precisely with our sensor compared with current best practice.” Magnetic field quantum sensors are used to monitor and diagnose medical conditions through the examination of neuronal activity (magnetoencephalography) or cardiological signals (magnetocardiography). Magnetoencephalography (MEG) technology is bulky and expensive, and requires ultracold temperatures with liquid helium to operate. The patient must remain still while the system is in use. MEG technology based on the diamond-laser sensor would be much smaller than today’s MEG devices, would operate at room temperature, and could be fitted onto the patient, the team said. “We really want to have something that we can place on a patient’s head and we want them to be able to move around — and there’d be no need for expensive liquid helium to operate such a device,” Greentree said. Clinicians want to be able to monitor the progression of a disease like Alzheimer’s and the effect of treatment. Similarly, they want to be able to measure the effect of an injury like concussion on the brain. “With this MEG technology we envisage, you might be able to pick up early-onset dementia. With epilepsy, you could find out where it’s occurring, and that would help you to better target interventions,” Greentree said. A laser interaction with diamond materials in the Australian Research Council (ARC) Centre of Excellence for Nanoscale BioPhotonics laboratories at RMIT University. Courtesy of RMIT University. Diamond NV magnetic field sensors could also improve sensors used for the geological exploration of magnetic minerals and magnetic anomaly detection, the researchers said. In the mining sector, it could lead to improved mineral exploration methods. With sufficient funding and collaboration with industry partners, they said that a proof-of-concept device using the new sensor could be developed within five years. Researchers at the National Institutes for Quantum and Radiological Science and Technology in Japan and The College of Staten Island also collaborated on the work. The research was published in Science Advances (www.doi.org/10.1126/sciadv.abn7192).