Accurate Magnetic Field Measurement Method Could Advance Quantum Sensing

Optically pumped magnetometers (OPMs) are used to measure magnetic fields in biosensing, contraband testing, and magnetic communications. They also aid in dark matter searches and serve as promising platforms for quantum-enhanced measurements.

Accurate vector magnetometry, however, remains a challenge for OPMs due to the OPM’s inherent scalar operation. Scalar OPMs require an external reference to extract directional information. While scalar measurements are often sufficient, robust calibration of vector OPMs is increasingly important for applications requiring high accuracy as well as precision.

Artist’s depiction of a new strategy for measuring the direction of magnetic fields by exposing a cell containing roughly one hundred billion rubidium atoms to a microwave signal. Courtesy of Steven Burrows/JILA.

Researchers at JILA, a joint research institute of the University of Colorado Boulder and the National Institute of Standards and Technology, demonstrated a vector OPM that uses Rabi oscillations driven between the manifolds of rubidium atoms to measure the direction of a magnetic field against the polarization ellipse structure of a microwave field.

The researchers exposed a cell containing roughly one hundred billion rubidium atoms, in vapor form, to a microwave signal. They hit the chamber with a magnetic field, which caused the atoms inside the chamber to shift. Using a laser, the researchers measured the shift in the atomic energy.

“Atoms can tell you a lot,” professor Cindy Regal said. “We’re data mining them to glean simultaneously whether magnetic fields are changing by extremely small amounts and what direction those fields point.”

Regal said that if an atom is hit with a microwave signal, its internal structure will “wiggle.” This “atomic dance” can tell physicists a lot.

“Ultimately, we can read out those wiggles, which tell us about the strength of the energy transitions the atoms are undergoing, which then tells us about the direction of the magnetic field,” Regal said.

In the current study, the team observed the shifts in the atomic energy — the atomic “dancing” — to pinpoint the orientation of a magnetic field to an accuracy of nearly one-hundredth of a degree.

By relying only on atomic measurements, the researchers were able to detect drift in the microwave vector reference and compensate for systematic shifts. To enable deadzone-free operation, the researchers introduced a Rabi measurement that used dressed-state resonances.

A child wears a helmet manufactured by FieldLine Inc. made up of more than 100 optically pumped magnetometer (OPM) sensors. Courtesy of FieldLine Inc.

These measurements, performed within the vapor cell platform, achieved an average vector accuracy of 0.46 milliradians and vector sensitivities down to 11 micro-radians per square root hertz, for geomagnetic field strengths near 50 micro-teslas. This performance surpassed the accuracy threshold of several existing OPM methods that use atomic vapors with an electromagnetic vector reference.

“You can think of each atom as a compass needle, and we have a billion compass needles, which could make for really precise measurement devices,” researcher Dawson Hewatt said.

In the future, the findings could be used to build quantum sensors to map brain activity, for example, or help airplanes navigate.

“What magnetic imaging allows us to do is measure sources that are buried in dense and optically opaque structures,” professor Svenja Knappe said. “They’re underwater. They’re buried under concrete. They’re inside your head, behind your skull.”

Unlike mechanical devices with internal parts that can change over time, atoms are always the same, Regal said.

The team plans to further improve the precision of its vector OPM before introducing it for practical use. The researchers hope that one day airplane pilots can use the vector OPM based on atomic vapors to navigate the plane by following local changes in Earth’s magnetic field, much like migratory birds use their innate biological magnetic sensors.

“It’s now a question of how far can we push these atomic systems,” Knappe said.

The research was published in Optica (www.doi.org/10.1364/OPTICA.542502).