Adiabatically controlled motional states of a CaO plus and Ca plus trapped-ion chain cooled to the ground state

Control of the external degree of freedom of trapped molecular ions is essential for their promising applica-tions to spectroscopy, precision measurements of fundamental constants, and quantum information technology. Here, we demonstrate near ground-state cooling of the axial motional modes of a calcium mono-oxide ion via sympathetic sideband cooling with a cotrapped calcium ion. We also show that the phonon state of the axial out-of-phase mode of the ion chain is maintained while the mode frequency is adiabatically ramped up and/or down. The adiabatic ramping of the motional mode frequency is a prerequisite for searching for the proposed molecular dipole-phonon interaction [W. C. Campbell and E. R. Hudson, Phys. Rev. Lett. 125, 120501 (2020); M. Mills et al., Phys. Chem. Chem. Phys. 22, 24964 (2020)].