  Ultracold gases provide micrometer size atomic samples whose sensitivity to
external fields may be exploited in sensor applications. Bose-Einstein
condensates of atomic gases have been demonstrated to perform excellently as
magnetic field sensors \cite{Wildermuth2005a} in atom chip
\cite{Folman2002a,Fortagh2007a} experiments. As such, they offer a combination
of resolution and sensitivity presently unattainable by other methods
\cite{Wildermuth2006a}. Here we propose that condensates of Fermionic atoms can
be used for non-invasive sensing of time-dependent and static magnetic and
electric fields, by utilizing the tunable energy gap in the excitation spectrum
as a frequency filter. Perturbations of the gas by the field create both
collective excitations and quasiparticles. Excitation of quasiparticles
requires the frequency of the perturbation to exceed the energy gap. Thus, by
tuning the gap, the frequencies of the field may be selectively monitored from
the amount of quasiparticles which is measurable for instance by
RF-spectroscopy. We analyse the proposed method by calculating the
density-density susceptibility, i.e. the dynamic structure factor, of the gas.
We discuss the sensitivity and spatial resolution of the method which may, with
advanced techniques for quasiparticle observation \cite{Schirotzek2008a}, be in
the half a micron scale.
