Brain-Computer-Interface: Focus on diamond-based quantum sensors

Much has been developed in the field of prostheses and exoskeletons in recent years. However, the possibilities for a truly intuitive and, above all, non-invasive control systems are still lacking. The sensors currently in use are not sensitive enough for this. A consortium from research and industry wants to change that with the NeuroQ project. Dr. Jan Jeske from the Fraunhofer Institute for Applied Solid State Physics IAF told us what role artificial diamonds play in this.

Thanks to high-quality sensors, electrodes and actuators, today's prostheses are already capable of precisely grasping and holding objects. Muscle signals are recognized and movement sequences calculated and adapted independently by the prosthesis. An exoskeleton, though, is not a 1:1 replacement of a body part, but more a kind of assistance system. The fitting is easier and the suits have proven themselves not only in logistics as "carrying aids". In rehabilitation – especially in the field of neurorehabilitation – their effects are substantial.

Whether prosthesis or exoskeleton, non-invasive control through a brain-computer interface (BCI), i.e. a human-computer interface, is not yet possible. The problem: Today's sensors are not sensitive enough to precisely and reliably pick up the complex "neural data" required for this. With an implant, this would be feasible, but the risks of such an intervention are enormously high.

At the Friedrich Alexander University of Nuremberg-Erlangen (FAU), an interdisciplinary research group is addressing the question of how the brain controls muscles and how these signals can ideally be tapped. Here, too, they are familiar with the problem that sensors on the skin do not measure the signals in the muscles precisely enough. Alessandro Del Vecchio, Professor of Neuromuscular Physiology and Neural Interfaces at FAU, explains exactly where efforts are being focused.

Experience with BCI systems has already been gained at Charité Berlin. Leading the field is Prof. Dr. Surjo R. Soekadar, Einstein Professor of Clinical Neurotechnology at Charité and also a speaker at the COMPAMED Innovation Forum 2022 on this topic. "Despite the considerable progress made [in the field of BCI], it has not yet been possible to control complex hand movements with such a non-invasive system," Dr. Soekadar is quoted as saying in the press release on the NeuroQ project launch.

The problem: Currently, researchers only have the methodology of electroencephalography (EEG) available to capture brain signals sensorially. The human skull attenuates or distorts the electrical signals. Precise control is therefore not possible.

Dr. Jan Jeske of Fraunhofer IAF in Freiburg suggests that measuring neuromagnetic fields could be superior to relying on electric fields. This is because our nerve cells generate not only tiny electric currents but also magnetic fields. Therefore, Magnetoencephalography (MEG), which measures magnetic fields, can offer significant benefits over an Electroencephalogram (EEG). He explains, "Magnetic fields pass through our body with less disturbance, providing much clearer signals than electrical sensors."

For a magnetocardiogram (MKG), for example, sensitive magnetic field sensors are already being used – so-called SQUID sensors (superconductive quantum interference device).

"The disadvantage of SQUIDS is that they only work at cryogenic temperatures – about four Kelvin (- 452,5 degrees Fahrenheit). Liquid helium is needed to cool them down to this range. This is not only technologically complex, but also very expensive," says Dr. Jeske. In addition, these sensors cannot be placed so close to the patient's head because of the low-temperature cooling. With the development of new, sensitive and precise quantum magnetometers that can also be used at room temperature, it would then be possible to read out even the smallest magnetic fields or brain activities and translate them into more complex control signals.

This  is where diamonds come into play. Jeske is an expert in this field. The project manager wants to use so-called NV diamonds to optimize highly sensitive magnetic field resonance sensors.

"The use of NV diamonds as magnetometers is first based on the optical activity of the color centers. The diamonds emit red light. At the same time, you can send not only light but also radio frequencies to them. Now quantum physics plays a role here. Because these electrons that are flying around this atomic system, it’s their electron spin that you can manipulate using radio frequency." An electron-spin transition can be addressed by hitting the corresponding resonant frequency of that same transition. And that would be 2.87 gigahertz without a magnetic field present. At the same time, the light emission becomes darker. Depending on how close one comes to this resonance frequency, this has an influence on the brightness of the diamond. "And that means that if the magnetic field now changes or is newly added, then the brightness of the luminous NV centers also changes as a result. And so you can draw conclusions about the strength of a magnetic field directly from the brightness of the diamonds," Dr. Jeske said.

But this is not new. These NV centers are already used extensively – including in sensor technology. Another advantage is biocompatibility, so nanodiamonds can also be used as biomarkers.

"What’s special about our research is: we want to use the NV diamond as a laser medium and use the laser cavity to get a stronger signal."

Last year, researchers at Fraunhofer IAF made a real breakthrough. For the first time, they were able to measure a magnetic field via stimulated emission. "So we have been able to show a magnetic field-dependent cavity signal, and for the first time." The project leader, Jeske, is pleased to report. "Otherwise, we have only measured the magnetic field via fluorescence." Compared to the fluorescence measurement, the laser signal was not only significantly stronger, but also much richer in contrast.

Now the consortium must transfer this laser system, including the measurement methodology, into a sensor system and then integrate it into an industrially usable module. "We have to further improve the sensor system scientifically. And then we work on the integration into a usable module," Jeske says. "The integration has very different aspects: there's the electronics, the optics and the laser and all the peripherals around it. We have to translate all of this from the lab and lab equipment, so to speak, into something more compact, well-movable that can also be used on patients," the project manager continues. And at the very end, "we don't just want to see a signal, we also want to translate it into a signal for the exoskeleton control system. And this is where Prof. Soekadar comes in again, who then wants to test the new sensors with patients."

While NeuroQ is still in its infancy, Dr. Jeske is optimistic: "This is a really new technology that we're taking a big step forward." The group has five years to do so and is funded by the German Federal Ministry of Education and Research (BMBF) with nearly nine million euros.