A study published in PNAS successfully demonstrated the first noninvasive, real-time magnetic measurement of single-neuron action potentials in living marine invertebrates using a quantum sensor utilizing nitrogen-vacancy (NV) defects in diamond. The system precisely reads the extremely minute magnetic fields at the nanotesla (nT) level generated during neural signal transmission, detecting changes in the quantum spin state of the NV centers via optically detected magnetic resonance (ODMR) fluorescence signals. Overcoming the limitations of traditional electrode insertion methods, this approach presents a new paradigm for next-generation brain-computer interfaces and precision medical diagnostics by mapping neural network activity with a millimeter-scale field of view and micrometer-level (corrected from the original article's "nanometer" claim to reflect the ~1 µm resolution cited in the study) resolution without damaging biological tissue. [Quantum Biology Society] One of the core challenges in brain science is to precisely measure action potentials, the electrical signals exchanged by brain nerve cells (neurons), at the individual cell level. However, widely used existing methods, such as microelectrode insertion or patch-clamp techniques, require piercing the cell membrane, which fatally damages biological tissue and disrupts the natural activity of the cells. Recently, a remarkable empirical study was published that completely overcomes these limitations by noninvasively reading the minute magnetic field changes of neurons from outside the cell using a quantum sensor. The paper, titled "Optical magnetic detection of single-neuron action potentials using quantum defects in diamond," published in the international journal PNAS, reported an innovative neural measurement technology from a Harvard University research team. This joint research team, led by Professors Ronald L. Walsworth, Mikhail D. Lukin, and Hongkun Park of Harvard University, utilized nitrogen-vacancy (NV) centers—atomic-level quantum defects existing inside a diamond crystal—as an ensemble sensor. ■ The Ultimate Quantum Sensor Created by Diamond Defects The research team placed intact giant axons extracted from living marine invertebrates (Myxicola infundibulum and squid) close to the surface of the diamond sensor. When an action potential signal is transmitted from a single neuron, an extremely minute magnetic field change at the nanotesla (nT) level occurs outside the organism and is captured by the diamond sensor. ■ Reading Changes in Spin States with Light This groundbreaking measurement utilizes changes in the quantum mechanical spin state. The minute magnetic field generated by the action potential of a neuron perturbs the quantum spin of electrons within the diamond NV centers. The research team successfully read these minute changes in quantum spin accurately in real-time by converting them into fluorescence signals through the optically detected magnetic resonance (ODMR) technique. This is the first achievement to accurately visualize the activity of a single neuron using only magnetic fields and light, without the need to pierce the cell or stain it with toxic fluorescent substances. ■ The Future of Next-Generation Brain-Computer Interfaces This noninvasive quantum sensor technology holds the potential to map the activity of intact neural networks in two dimensions with a millimeter-scale field of view while maintaining the organism's normal physiological activity. By tracking the minute magnetic fields emitted from individual neurons in high resolution without causing damage, this technology is expected to contribute to perfectly decoding the brain's complex information-processing mechanisms in the future. Furthermore, it is evaluated as a paradigm shift that will accelerate the commercialization of precision medical devices diagnosing human brain neurological diseases in real-time and next-generation brain-computer interface (BCI) technologies that directly read brain signals. https://www.pnas.org/doi/10.1073/pnas.1601513113