Cooling of a zero-nuclear-spin molecular ion to a selected rotational state

We demonstrate rotational cooling of the silicon monoxide cation via optical pumping by a spectrally filtered broadband laser. Compared with diatomic hydrides, ${\mathrm{SiO}}^{+}$ is more challenging to cool because of its smaller rotational interval. However, the rotational level spacing and the large dipole moment of ${\mathrm{SiO}}^{+}$ allows for direct manipulation by microwaves, and the absence of hyperfine structure in its dominant isotopologue greatly reduces demands for pure quantum state preparation. These features make $^{28}\mathrm{Si}^{16}{\mathrm{O}}^{+}$ a good candidate for future applications such as quantum information processing. Cooling to the ground rotational state is achieved on a 100 ms timescale and attains a population of 94(3)%, with an equivalent temperature $T=0.53(6)\text{ }\text{ }\mathrm{K}$. We also describe a novel spectral-filtering approach to cool into arbitrary rotational states and use it to demonstrate a narrow rotational population distribution ($N\ifmmode\pm\else\textpm\fi{}1$) around a selected state.