  While it is straightforward to count laser-cooled trapped ions by
fluorescence imaging, detecting the number of dark ions embedded and
sympathetically cooled in a mixed ion crystal is more challenging. We
demonstrate a method to track the number of dark ions in real time with
single-particle sensitivity. This is achieved by observing discrete steps in
the amount of fluorescence emitted from the coolant ions while exciting secular
motional resonances of dark ions. By counting the number of fluorescence steps,
we can identify the number of dark ions without calibration and without relying
on any physical model of the motional excitation. We demonstrate the scheme by
detecting H$_2^+$ and H$_3^+$ ions embedded in a Be$^+$ ion Coulomb crystal in
a linear radio frequency trap. Our method allows observing the generation and
destruction of individual ions simultaneously for different types of ions.
Besides high-resolution spectroscopy of dark ions, another application is the
detection of chemical reactions in real time with single-particle sensitivity.
This is demonstrated in this work.
