High-purity solid parahydrogen.

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 the 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 <10. We also find that, at sufficiently low temperatures, the cryogenic catalyst provides isotopic purification, reducing the HD fraction.