  Alkali atoms trapped in solid hydrogen matrices have demonstrated ultralong
electron spin coherence times, and are promising as quantum sensors. Their spin
coherence is limited by magnetic noise from naturally-occurring orthohydrogen
molecules in the parahydrogen matrix. In the gas phase, the orthohydrogen
component of hydrogen can be converted to parahydrogen by flowing it over a
catalyst held at cryogenic temperatures, with lower temperatures giving a lower
orthohydrogen fraction. In this work, we use a single cryostat to reduce the
orthohydrogen fraction of hydrogen gas and grow a solid matrix from the
resulting high-purity parahydrogen. We demonstrate operation of the catalyst
down to a temperature of 8 K, and we spectroscopically verify that
orthohydrogen impurities in the resulting solid are at a level < 1e-6. We also
find that, at sufficiently low temperatures, the cryogenic catalyst provides
isotopic purification, reducing the HD fraction.
