  Radio communication through attenuating media necessitates the use of
very-low frequency (VLF) and ultra-low frequency (ULF) carrier bands, which are
frequently used in underwater and under-ground communication applications.
Quantum sensing techniques can be used to circumvent hard constraints on the
size, weight and noise floor of classical signal transducers. In this
low-frequency range, an optically pumped atomic sample can be used to detect
carrier wave modulation resonant with ground-state Zeeman splitting of alkali
atoms. Using a compact, self-calibrating system we demonstrate a resonant
atomic transducer for digital data encoded using binary phase- and
frequency-keying of resonant carrier waves in the 200 Hz -200 kHz range. We
present field trial data showing sensor noise floor, decoded data and received
bit error rate, and calculate the projected range of sub-sea communication
using this device.
