  We actively stabilize the harmonic oscillation frequency of a laser-cooled
atomic ion confined in a rf Paul trap by sampling and rectifying the high
voltage rf applied to the trap electrodes. We are able to stabilize the 1 MHz
atomic oscillation frequency to better than 10 Hz, or 10 ppm. This represents a
suppression of ambient noise on the rf circuit by 34 dB. This technique could
impact the sensitivity of ion trap mass spectrometry and the fidelity of
quantum operations in ion trap quantum information applications.
