  We propose a method for sensitive parallel detection of low-frequency
electromagnetic fields based on the fine structure interactions in paramagnetic
polar molecules. Compared to the recently implemented scheme employing
ultracold $^{87}$Rb atoms [B{\"o}hi \textit{et al.}, Appl. Phys. Lett.
\textbf{97}, 051101 (2010)], the technique based on molecules offers a 100-fold
higher sensitivity, the possibility to measure both the electric and magnetic
field components, and a probe of a wide range of frequencies from the dc limit
to the THz regime.
