A Single-Ion Reservoir as a High-Sensitive Sensor of Electric Signals

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.