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 $T=0.53(6)\mathrm{K}$. We also describe a novel spectral-filtering approach to cool into arbitrary rotational states and use it to demonstrate a narrow rotational population distribution ($N\pm 1$) around a selected state.

• Received 19 May 2020
• Accepted 10 August 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.113201