  We present an optical scheme to detect the oscillations of a two-ion string
confined in a linear Paul trap. The motion is detected by analyzing the
intensity correlations in the fluorescence light emitted by one or two ions in
the string. We present measurements performed under continuous Doppler cooling
and under pulsed illumination. We foresee several direct applications of this
detection method, including motional analysis of multi-ion species or coupled
mechanical oscillators, and sensing of mechanical correlations.
