# A Single-Cell Optically Pumped Intrinsic Gradiometer

## TL;DR

A new single-cell sensor was developed to detect biomagnetic fields with high sensitivity and reduced complexity.

## Contribution

A single-cell intrinsic optically pumped gradiometer was demonstrated with high sensitivity and simplified design.

## Key findings

- Achieved 267 pT/cm/√Hz sensitivity and 50 dB common mode rejection ratio.
- Recorded cardiac-synchronous magnetic measurements using an optical pulse sensor.
- Simplified sensor design reduces cost and complexity compared to multi-cell gradiometers.

## Abstract

What are the main findings?
Demonstrated a single-cell intrinsic optically pumped gradiometer.Achieved 267 pT/cm/√Hz sensitivity and 50 dB common mode rejection ratio.

Demonstrated a single-cell intrinsic optically pumped gradiometer.

Achieved 267 pT/cm/√Hz sensitivity and 50 dB common mode rejection ratio.

What are the implications of the main findings?
Enables simplified biomagnetic sensing.Reduces sensor complexity for future OPM systems.

Enables simplified biomagnetic sensing.

Reduces sensor complexity for future OPM systems.

Optically pumped magnetometers (OPMs) provide a non-cryogenic alternative to superconducting quantum interference devices (SQUIDs) for detecting weak biomagnetic fields. We report the design, construction, and characterization of a single-cell intrinsic OPM gradiometer. The gradiometer employs a rubidium-87 vapor cell in an orthogonal pump and probe beam configuration. The pump beam was split to illuminate two parallel sensing regions of the cell, separated by a baseline of 3 cm, with opposing circular polarization. A linearly polarized probe beam propagated through both regions and was captured by a balanced polarimeter whose output directly measured the spatial magnetic gradient. This prototype achieved a common-mode rejection ratio exceeding 50 dB and a sensitivity of 267 pT/cm/√Hz without passive magnetic shielding, using active ambient-field coils. As a proof of concept, we recorded preliminary cardiac-synchronous magnetic measurements using an optical pulse sensor for beat segmentation. After bandpass filtering and ensemble averaging, a cardiac-synchronous waveform was observed, consistent with cardiac timing. Unlike many multi-cell gradiometers that require complex calibration, modulation, and passive shielding, this single-cell design reduces cost and complexity.