Resolving magnetometer vector light shift — and turning it into a tool

Important instruments for neuroscience and medical diagnostics, optically pumped magnetometers measure responses of polarized atoms to produce information about the low-frequency external field. They can measure weak magnetic fields, such as those generated by the human brain, retina, and musculature.

In contrast to traditional double-resonance magnetometers, a simpler magnetometer scheme, with a multi-pass cavity and a single detuned and elliptically polarized beam, has emerged in recent years. But the new system’s single beam causes an intrinsic vector light shift, which alters atomic energy levels and reduces the tool’s accuracy.

Using a compact double-resonance scalar magnetometer with a single detuned elliptically polarized laser beam, Xie et al. explored the vector light shift issue and showed how it can be significantly mitigated.

“We built a compact, highly sensitive magnetometer and solved a key performance-limiting problem,” said author Dong Sheng. “While a multi-pass cavity boosts the signal for miniaturization, it also creates an intrinsic ‘light shift’ error — where the inhomogeneous beam patterns of the probing laser inside the cavity distort the atomic response — producing a predictable signal asymmetry. We successfully identified this problem and demonstrated that simply adding a half-wave plate inside the cell can suppress this error dramatically.”

Furthermore, the researchers suggested that the signal asymmetry itself provides a clear “fingerprint” of the light shift, offering a way to quantify and correct the inhomogeneous beam pattern inside the multipass cavity.

“In essence, we turned a measurement problem into a diagnostic tool,” said Sheng. “The asymmetry we studied is not just noise — it’s a new metric that can be used to evaluate how well future designs control laser inhomogeneity, turning a systematic error into a useful engineering benchmark.”

Source: “Light shift effects in a double-resonance magnetometer using a single elliptically polarized beam,” by Ziping Xie, Zengli Ba, Qianqian Yu, and Dong Sheng, Journal of Applied Physics (2025). The article can be accessed at: https://doi.org/10.1063/5.0304818 .