  Ultra-cold atom-ion mixtures are gaining increasing interest due to their
potential applications in quantum chemistry, quantum computing and many-body
physics. Here, we studied the dynamics of a single ground-state cooled ion
during few, to many, Langevin (spiraling) collisions with ultra-cold atoms. We
measured the ion's energy distribution and observed a clear deviation from
Maxwell-Boltzmann to a Tsallis characterized by a power-law tail of high
energies. Unlike previous experiments, the energy scale of atom-ion
interactions is not determined by either the atomic cloud temperature or the
ion's trap residual excess-micromotion energy. Instead, it is determined by the
force the atom exerts on the ion during a collision which is then amplified by
the trap dynamics. This effect is intrinsic to ion Paul traps and sets the
lower bound of atom-ion steady-state interaction energy in these systems.
Despite the fact that our system is eventually driven out of the ultra-cold
regime, we are capable of studying quantum effects by limiting the interaction
to the first collision.
