  A single-ion reservoir has been tested, and characterized in order to be used
as a highly sensitive optical detector of electric signals arriving at the
trapping electrodes. Our system consists of a single laser-cooled $^{40}$Ca$^+$
ion stored in a Paul trap with rotational symmetry. The performance is observed
through the axial motion of the ion, which is equivalent to an underdamped and
forced oscillator. Thus, the results can be projected also to Penning traps. We
have found that, for an ion oscillator temperature $T_{\scriptsize{\rm
axial}}\lesssim 10$~mK in the forced-frequency range $\omega_z =2\pi \times
(80,200$~kHz), the reservoir is sensitive to a time-varying electric field
equivalent to an electric force of $5.3(2)$~neV/$\mu $m, for a measured quality
factor $Q=3875(45)$, and a decay time constant $\gamma_z=88(2)$~s$^{-1}$. This
method can be applied to measure optically the strength of an oscillating field
or induced (driven) charge in this frequency range within times of tens of
milliseconds. Furthermore the ion reservoir has been proven to be sensitive to
electrostatic forces by measuring the ion displacement. Since the heating rate
is below $0.3$~$\mu$eV/s, this reservoir might be used as optical detector for
any ion or bunch of charged particles stored in an adjacent trap.
