Chip-scale packaged in-line polarization-resolved detector for optically pumped magnetometers

Optically pumped magnetometers (OPMs) have emerged as promising quantum sensors for neuroscience, geophysical exploration, and non-destructive testing. Their performance relies on detecting Faraday rotation of a probe beam interacting with spin-polarized alkali atoms. Conventional detection schemes based on polarizing beam splitters and discrete balanced photodetectors remain bulky and limit system miniaturization. Here, we present a chip-scale packaged in-line polarization-resolved detector (CSP-iPRD) that integrates a wire grid polarizer (WGP) and a bi-cell photodiode into a compact form factor. The integrated WGP exhibits a polarization extinction ratio (PER) of 25.3 dB at 795 nm. The assembled CSP detector, with overall dimensions of 3.5 × 3.5 × 1.8 mm³, achieves a common-mode rejection ratio (CMMR) of 29.6 dB and an angular resolution of 8.4 × 10^-5 degree/Hz^1/2. When incorporated into a spin-exchange relaxation-free (SERF) OPM, it achieves a magnetic sensitivity of 33.5 fT/Hz^1/2 at 10 Hz. These results highlight a scalable and CMOS-compatible detection platform that paves the way for next-generation miniaturized OPMs and field-deployable quantum sensing systems.