Knowledge Bank

The complexity of molecular spectra prevents direct laser cooling of most molecules. Molecular ions can be indirectly laser-cooled by a Columbic interaction with a neighboring atomic ion. This sympathetic cooling method has been used to lower the temperature of molecular ions ranging from CaO$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ to C$\mathrm{<mrow\; data-mjx-texclass="ORD">60</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$ to less than 100 mK by Doppler cooling the atomic ions}rensen \textit{Phys.~Rev.~Lett.} \underline{\textbf{93}}, 243201 (2004). V.~L.~Ryjkov, XZ.~Zhao and H.~A.~Schuessler \textit{Phys.~Rev.~A} \underline{\textbf{74}}, 023401 (2006). A.~Ostendorf, C.~B.~Zhang, M.~A.~Wilson, D.~Offenberg, B.~Roth, and S.~Schiller \textit{Phys.~Rev.~Lett.} \underline{\textbf{97}}, 243005 (2006).}. Starting with two ions of Ca$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ in the trap, we introduce oxygen gas until one CaO$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ is produced. The motion of a CaO$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ molecular ion and a Ca$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ atomic ion are coupled by the Coulomb interaction in the same trap. The frequencies of the normal modes of the atom-molecule crystal are first measured by observing the fluorescence quenching of the Ca$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ atoms. The normal modes can be cooled to the motional ground state by addressing the sidebands of the Ca$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ quadrupole transition. The result is an atom-molecule crystal with a translational temperature in the $\mu$K regime.