Measuring Magnetic Fields with Atoms
QuSpin develops compact atomic magnetometers that use the quantum properties of atoms to measure magnetic fields with extraordinary precision.
QuSpin develops compact atomic magnetometers that use the quantum properties of atoms to measure magnetic fields with extraordinary precision.
Magnetic fields carry information that is often difficult or impossible to obtain in any other way. They can reveal electrical activity in the brain and heart, identify objects and structures beneath the Earth’s surface, support navigation, characterize materials, and enable new forms of scientific measurement.
Magnetic fields are produced by electrical currents and magnetic materials. Because magnetic measurements do not require physical contact with the source, they provide a powerful, noninvasive way to observe otherwise hidden processes.
The challenge is that many of the most interesting magnetic signals are extremely small. Detecting them requires sensors that are highly sensitive, stable, and capable of separating the desired signal from the surrounding magnetic environment.
Atomic magnetometers meet this challenge by using atoms themselves as the sensing element.
An atomic magnetometer measures magnetic fields by observing how atoms respond to the surrounding magnetic environment.
QuSpin’s sensors are a class of atomic magnetometer known as optically pumped magnetometers, or OPMs. They use laser light to prepare and observe atoms contained inside a small glass vapor cell.
Different versions of this technology are described using different names depending on how they operate and what they measure. These include:
Although these instruments operate in different magnetic environments, they are based on the same underlying interaction between light, atoms, and magnetic fields.
At the center of the sensor is a small glass vapor cell containing atoms such as rubidium. A laser illuminates the atoms and prepares them in a controlled quantum state through a process called optical pumping.
When a magnetic field is present, it changes the behavior of the atomic spins. This change affects the light passing through the vapor cell. A photodetector measures the resulting optical signal, and the sensor electronics convert it into a precise measurement of the magnetic field.
Laser light optically pumps the atoms, aligning their spins and establishing a well-defined starting state.
The surrounding magnetic field changes the orientation or precession of the atomic spins.
The changing atomic state modifies the transmitted light. A photodetector and control electronics translate this optical response into a magnetic-field measurement.
Because the measurement is based on well-defined atomic properties, atomic magnetometers can provide exceptional sensitivity without requiring cryogenic cooling.
The QuSpin Zero-Field Magnetometer (QZFM) is based on zero-field resonance with a single laser beam. Light from a precisely tuned semiconductor laser passes through a glass vapor cell containing rubidium atoms and is captured by a photodetector. When the background magnetic field is zero, the rubidium atoms become largely transparent. A magnetic field perpendicular to the light path causes the atoms to absorb more light—the photodetector senses this change in transparency, converting a magnetic signal into an electric signal.
Sweeping the field through zero traces out a Lorentzian response called the zero-field resonance, typically about 30 nT wide. A small 1 kHz modulation field and lock-in detection convert this into a dispersion curve with maximum slope at zero field—the output of the magnetometer. The direction of the modulation field defines the sensitive axis; applying two orthogonal modulation fields makes the sensor simultaneously sensitive along two axes.
In the triaxial variant, a beam splitter creates two perpendicular beams that pass through the same vapor cell without overlapping—forming two orthogonal, independent dual-axis magnetometers inside the same sensor head. Together they allow the sensor to function as a triaxial magnetometer.
Three-axis coils integrated in the sensor head automatically cancel residual fields up to 50 nT inside a magnetically shielded room—no large external coils needed. The automated process takes a few seconds.
Bandwidth of ~150 Hz, set by the physics of the zero-field resonance. The vapor cell runs at ~150 °C to increase vapor density; multi-sensor arrays share a common modulation drive to eliminate cross-talk.
QTFM Gen‑2 is a pulsed rubidium OPM based on an optical detection scheme called Free Induction Decay (FID). Light from a 795 nm laser is circularly polarized by a quarter waveplate, passes through a rubidium vapor cell heated to ~80 °C, and is captured by a photodetector. The laser wavelength is electronically locked to the rubidium D1 optical transition.
Operation repeats every millisecond—one field measurement per cycle:
A strong polarizing field and the laser spin‑polarize the rubidium atoms.
The polarizing field switches off almost instantly; measurement begins.
Atoms precess about Earth’s field, modulating the transmitted light in real time.
A high‑resolution counter reads the precession frequency—6.998 Hz per nT, an absolute measure of the field.
The precession frequency depends only on the magnitude of the background field—√(Bx² + By² + Bz²)—not on its individual vector components. For this reason the QTFM is a scalar or “total‑field” magnetometer, unlike the vector QZFM or a fluxgate.
Although the QTFM is intrinsically a scalar sensor, an add‑on three‑axis coil attachment converts it into a hybrid vector‑scalar magnetometer. The coils sequentially apply known Bx, By, and Bz offset fields in interleaved measurement cycles, and the vector components of the background field are calculated from the resulting shifts in the total field—giving total‑field and vector outputs from a single device, with the drift‑free stability of a scalar measurement.