Optical pumping of rubidium atoms in a parahydrogen matrix*

Building on prior work with rubidium atoms in a cryogenic argon matrix [\textit{Phys. Rev. A} \textbf{88}, 063404 (2013)], we have grown solid parahydrogen crystals doped with rubidium atoms. Typical rubidium densities are on the order of $10^{17}$~cm$^{-3}$. We have demonstrated optical pumping of the atomic spin of the implanted rubidium atoms; the measured spin polarization signals are roughly one order of magnitude larger than what was achieved in argon matrices. The combination of high atomic densities and optical addressability make this a promising experimental platform for applications such as magnetometry and fundamental physics measurements. Spin lifetimes ($T_1$) on the order of 1 second have been observed. Progress towards measuring coherence times ($T_2$) will be discussed.