  We study cold inter-species collisions of Caesium and Rubidium in a strongly
imbalanced system with single and few Cs atoms. Observation of the single atom
fuorescence dynamics yields insight into light-induced loss mechanisms, while
both subsystems can remain in steady-state. This significantly simplifies the
analysis of the dynamics, as Cs-Cs collisions are effectively absent and the
majority component remains unaffected, allowing us to extract a precise value
of the Rb-Cs collision parameter. Extending our results to ground state
collisions would allow to use single neutral atoms as coherent probes for
larger quantum systems.
