Multiparameter quantum sensing and magnetic communications 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-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 a new application of multiparameter quantum sensing: background-cancelling spread spectrum magnetic communication. We encode a digital message as rf amplitude, spread among sixteen channels from \SI{29}{\kilo\hertz} to \SI{33}{\kilo\hertz} in a noisy magnetic environment, and observe quantum-noise-limited rf magnetic signal recovery enabled by quantum-noise-limited dc noise cancellation, reaching noise rejection of \SI{15}{\decibel} at \SI{100}{\hertz} and more than \SI{20}{\decibel} at \SI{60}{\hertz} 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 study of high-performance multiparameter quantum sensing.