A Diamond Chip Smaller Than Half a Cubic Millimeter Read the Magnetic Field of a Human Heartbeat at Room Temperature, With No Electrodes and No Skin Contact. Mainz Physicists Say It Took Ten Years and Could Reach Burn Patients ECG Can't.

Every heartbeat is an electrical event, and every electrical current makes a magnetic field. Physicists at Johannes Gutenberg University Mainz have now measured that field from a living human heart using a quantum sensor carved from diamond, at room temperature, without touching the patient's skin. The work, published in Science Advances, is the product of more than a decade of development and a German flagship project called DIAQNOS that is aimed squarely at getting the technology out of the lab and into operating rooms.

The sensor relies on nitrogen-vacancy centers, defects that form when a nitrogen atom replaces a carbon atom in the diamond lattice right next to a missing atom. The energy levels of those centers shift in response to magnetic fields, electric fields, temperature and mechanical stress, and reading them out with light gives a precise measurement. The Mainz group, led by Dmitry Budker of the PRISMA++ Cluster of Excellence and the Helmholtz Institute Mainz, built its version as a truncated diamond pyramid with a volume of less than half a cubic millimeter, mounted on an optical fiber as a portable endoscope. Muhib Omar, a doctoral student in Budker's group, developed the device and is the paper's coordinating author.

The point of comparison is the electrocardiogram, which measures the heart's electrical activity through electrodes stuck to the skin. ECG is cheap and everywhere, but the signal it sees is distorted by the differing conductivity of body tissues, and there are patients, burn victims among them, on whom electrodes cannot be placed at all. Magnetocardiography, which reads the magnetic field instead, is barely affected by tissue and needs no contact. Its problem has always been the hardware: it has required superconducting quantum interference devices cooled with liquid helium, or optically pumped magnetometers, both expensive and bulky. A diamond that works at room temperature and can be laid directly on the chest is a different proposition.

"These results are the product of over 10 years of development work," said Arne Wickenbrock, who coordinates DIAQNOS, an acronym for diamond-based quantum sensing for neurosurgery. "We work closely with neurosurgeons to ensure that our technologies do not remain confined to the laboratory but find clear practical applications." The project tested three independently built systems on the heart's magnetic field, from Mainz, the Universities of Stuttgart and Freiburg, and the startup Q.ANT. The Mainz endoscope is the only one that works without a magnetic bias field, which the other two use to filter out environmental interference.

The researchers are candid about the gap that remains. SQUIDs and optically pumped magnetometers still offer higher sensitivity and better signal-to-noise ratios. But the diamond's tiny volume is also its route to closing that gap. A flux concentrator, a magnetic structure that funnels field lines into the crystal, can amplify the signal by more than a factor of 100, and building concentrators that work at room temperature is Omar's main focus. "Adapting magnetic structures to optimally concentrate the magnetic field lines from a source within the diamond is the path to bringing these quantum technologies into practical use," he said.

Beyond the clinic's cardiology wing, the group sees three-dimensional mapping of the heart's conduction system, separation of a fetus's heartbeat from its mother's for prenatal monitoring, nerve monitoring during surgery in unshielded rooms, and gradiometers that compare two sensors to cancel background noise for cancer surgery. The furthest-reaching goal is a portable, room-temperature magnetoencephalography system for the brain, which could feed diagnostics for conditions such as epilepsy and, eventually, brain-computer interfaces. "NV magnetometers are characterized by fast initialization, excellent biocompatibility and stable operation over a wide temperature range," Wickenbrock said. "This makes them particularly attractive for biomedical applications."

The short version: a speck of diamond with a few engineered flaws can now hear the magnetic whisper of a heartbeat without cooling and without touching the patient. It is not yet as sharp as the room-sized machines it aims to replace, but it is small enough to be aimed exactly where a doctor wants it.