Multiparameter quantum sensing and magnetic communication with a hybrid dc and rf optically pumped magnetometer

We introduce and demonstrate a hybrid optically pumped magnetometer (HOPM) that simultaneously measures one dc field component and one rf field component quadrature with a single atomic spin ensemble. The HOPM achieves sub-$\mathrm{pT}/\sqrt{\mathrm{Hz}}$ sensitivity for both dc and rf fields, and is limited in sensitivity by spin projection noise at low frequencies and by photon shot noise at high frequencies. We demonstrate with the HOPM an alternative application of multiparameter quantum sensing: background-canceling spread spectrum magnetic communication. We encode a digital message as rf amplitude, spread among 16 channels from 29 to 33 kHz in a noisy magnetic environment, and observe quantum-noise-limited rf magnetic signal recovery enabled by quantum-noise-limited dc noise cancelation, reaching noise rejection of 15 dB at 100 Hz and more than 20 dB at 60 Hz and below. We measure signal fidelity versus signal strength and extrinsic noise in communication of a short text message. The combination of high sensitivity, quantum-noise-limited performance, and real-world application potential makes the HOPM ideally suited for the study of high-performance multiparameter quantum sensing.