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Magnetometers, in the form of compasses, have existed for nearly two millennia. Initially used for spiritual purposes, they found practical application in navigation about 800 years later and have since become indispensable for this purpose. Carl Gauss later developed an early version of the modern magnetometer and formulated Gauss’s law, which is one of the essential building blocks for Maxwell’s equations, the cornerstone of electromagnetic theory. Today, magnetometry is performed daily by billions of people through their smartphones for navigation — a truly remarkable innovation that has enabled precise positioning.
The primary magnetometers in modern smartphones are anisotropic magnetoresistive (AMR) sensors. AMR sensors measure changes in the electrical resistivity of a magnetic material under a magnetic field. These sensors are compact, inexpensive, and sufficiently sensitive to detect Earth’s magnetic field (approximately 50 µT), functioning as digital compasses with heading accuracy of about 1 degree. Although highly useful for many applications, these magnetometers do not achieve the ultimate sensitivity to magnetic fields. That level of precision is attainable through quantum magnetometry, which leverages quantum principles to measure magnetic fields. While smartphone magnetometers typically offer sensitivities in the nanotesla range, quantum magnetometers can achieve sensitivities at least a million times greater—reaching levels on the order of 1 fT or even beyond. Such extraordinary sensitivity opens the door to groundbreaking discoveries and measurements previously unattainable, including detecting magnetic signals from brain activity (magnetoencephalography, MEG), visualizing magnetic phenomena at the nanoscale, and mapping subtle variations in Earth’s magnetic field for geological studies.
One of the most prominent quantum magnetometers, which has long dominated the field, is based on superconducting quantum interference devices (SQUIDs). SQUID magnetometers consist of a superconducting loop incorporating two Josephson junctions and exploit quantum behavior on a macroscopic scale, a breakthrough recognized with the Nobel Prize in Physics in 2025. Despite their exceptional sensitivity, SQUID magnetometers require cryogenic temperatures, making their integration into everyday applications highly challenging. Achieving similar sensitivity at room temperature has for many years remained out of reach. However, recent advances in diode laser technology and fabrication methods have driven significant progress in optically pumped magnetometers, such as alkali vapor cells and color-center defects in solids. These emerging technologies have demonstrated performance approaching that of SQUID magnetometers, signaling a promising shift toward practical, room temperature, and high-sensitivity quantum magnetometry.
Atomic vapor magnetometry was first pioneered in the late 1950s. Its operating principle relies on creating long-lived orientation in the atomic ground state of an alkali vapor using laser light tuned near the atomic optical transition. These oriented states then undergo Larmor precession in an applied magnetic field, which alters the optical absorption or dispersion of the medium. This change is detected through a sensing transmitted light. The ultimate sensitivity of atomic magnetometers is fundamentally limited by quantum mechanics and depends on factors such as the atomic magnetic moment, the number of participating atoms, and the spin coherence time. Additional noise sources include photon shot noise and AC Stark shifts.
Because the output of an atomic magnetometer is directly related to the magnetic field through fundamental physical constants, these devices require no calibration. Furthermore, unlike SQUID magnetometers, they do not rely on magnetic flux pickup loops and are immune to low-frequency 1/f noise, as their sensing element does not generate intrinsic 1/f noise. They are inherently scalar magnetometers, and measure only the magnitude of magnetic field. This is a critical advantage for mobile platforms where insensitivity to magnetic field orientation is essential. The vector magnetometers provide both the magnitude and direction of the field. Typical examples of vector magnetometers are 3 axis fluxgate (or AMR magnetometers in mobile phones) which are crucial for navigation and geophysical measurements. Standard techniques do exist to convert scalar atomic magnetometers into vector instruments, though this comes at the expense of sensitivity.
One of the most extensively studied color-center defect magnetometers is the nitrogen-vacancy (NV) centers in diamonds. A NV center functioning like an artificial atom and consists of a nitrogen atom that substitutes a carbon atom in the diamond lattice, and an adjacent vacant site. Under green laser excitation, electrons are promoted to an excited state and subsequently decay either radiatively (producing photoluminescence) or non-radiatively via a singlet shelving state. In the absence of an external magnetic field, the ground state comprises a zero-spin level and two degenerate ±1 spin levels, separated by 2.87 GHz energy level. When a magnetic field is applied, the Zeeman effect lifts the degeneracy of ±1 spin levels, and the energy splitting between these levels become directly proportional to the magnetic field strength, a relationship readily detected using electron spin resonance spectroscopy.
The sensitivity of NV-based magnetometers is strongly dependent on spin coherence time, which is limited by interactions between the NV electron spin and impurities in the diamond lattice. Advances in material science have enabled the production of diamonds with low nitrogen concentrations and isotopically engineered diamonds. Employing ensembles of NV centers in such high-purity diamonds enhances fluorescence signals and reduces shot noise by an inverse of the square root of the number of NV centers. Photo-collection efficiency also plays a critical role in determining sensitivity. Perhaps the greatest advantage of NV magnetometers is their nanoscale spatial resolution, making them ideal for imaging ferromagnetic structures and detecting exotic magnetic phenomena. Their exceptional magnetic sensitivity further enables nanoscale NMR and MRI, opening pathways for molecular-level structural imaging.
The two magnetometers discussed above are exceptionally versatile, operating under room temperature and covering applications that range from probing nanoscale magnetic phenomena to measuring interstellar magnetic fields. NV centers can be integrated with waveguides or photonic cavities, enabling seamless incorporation into photonic systems and enhancing performance through improved light collection. Chip-scale atomic magnetometers have been under active development and successfully demonstrated for more than two decades, allowing them to mature into state-of-the-art instruments now available commercially. These advancements have significantly reduced their cost and improved accessibility, opening new opportunities in space science, electric circuit analysis, navigation, medical and biomedical technologies, as well as fundamental research.
The future adoption of quantum magnetometers will have transformative impacts across society, industry, and science. These sensors already enable breakthroughs in healthcare through non-invasive diagnostics such as magnetoencephalography, enhance navigation in GPS-denied environments, and advance research from nanoscale imaging to space exploration. Such capabilities will open new markets in medical technology and industrial sensing, providing significant competitive advantages. As costs decline and accessibility improves, quantum magnetometers will democratize high-precision sensing, accelerating innovation and technological progress across multiple sectors.
Författare: Sobhan Sepehri, Jakob Blomgren och Christer Johansson, RISE
