  We present a method of sensing AC magnetic fields. The method is based on the
construction of a robust qubit by the application of continuous driving fields.
Specifically, magnetic noise and power fluctuations of the driving fields do
not operate within the robust qubit subspace, and hence, robustness to both
external and controller noise is achieved. We consider trapped-ion based
implementation via the dipole transitions, which is relevant for several types
of ions, such as the $^{40}{\rm{Ca}}^{+}$, $^{88}{\rm{Sr}}^{+}$, and the
$^{138}{\rm{Ba}}^{+}$ ions. Taking experimental errors into account, we
conclude that the coherence time of the robust qubit can be improved by up to
$\sim 4$ orders of magnitude compared to the coherence time of the bare states.
We show how the robust qubit can be utilized for the task of sensing AC
magnetic fields, leading to an improvement of $\sim 2$ orders of magnitude of
the sensitivity. In addition, we present a microwave based sensing scheme that
is suitable for ions with a hyperfine structure, such as the
$^{9}{\rm{Be}}^{+}$,$^{25}{\rm{Mg}}^{+}$,$^{43}{\rm{Ca}}^{+}$,$^{87}{\rm{Sr}}^{+}$,$^{137}{\rm{Ba}}^{+}$,$^{111}{\rm{Cd}}^{+}$,$^{171}{\rm{Yb}}^{+}$,
and the $^{199}{\rm{Hg}}^{+}$ ions. This scheme enables the enhanced sensing of
high frequency fields at the GHz level.
