Molecular hydrogen exists in two nuclear spin configurations: para-H2, in which the spins of the two hydrogen atoms cancel, and ortho-H2, in which they add together. Ortho-H2 has three substates defined by the rotational direction of the nucleus. As hydrogen cools, ortho-H2 naturally tends to convert to the lower-energy para-H2 state. The UMD team found that when H2 is frozen inside dry-ice crystals, the crystalline symmetry of the surrounding solid blocks that conversion for two of the three ortho-H2 substates while leaving one accessible.
"The big finding is that, depending on what ice we put an H2 molecule into, its quantum dynamics are entirely dependent on the surrounding environment," said Nathan McLane, a chemical physics graduate student and the paper's lead author.
The mechanism is rooted in the geometry of crystalline carbon dioxide. Its molecular structure imposes a set of symmetry-based selection rules that govern which quantum transitions are permitted. Senior author Leah Dodson, an assistant professor in UMD's Department of Chemistry and Biochemistry, framed the broader significance: "We show experimentally that when molecular hydrogen - the simplest molecule - is confined within different molecular crystals, the symmetry of the surrounding solid determines which quantum spin states can interconvert and which remain protected."
The team also showed those rules can be deliberately relaxed. Introducing nitrogen dioxide into the dry-ice crystal lattice alters its crystalline properties, enabling all three ortho-H2 substates to convert to para-H2. That ability to tune the permitted transitions represents the core experimental advance.
The U.S. Department of Energy, which funded the research, has a direct interest in one near-term application: hydrogen fuel storage. Because different nuclear spin states of hydrogen require different amounts of energy to heat up, enriching specific spin states while protecting others could make storage systems more efficient and stable. When ortho-H2 converts to para-H2 it releases heat, a factor that fuel managers must account for carefully. The team plans to extend the approach to methane as a next step.
A second application involves astrochemistry. NASA currently estimates the formation temperatures of comets by measuring the proportions of ortho and para water the comets release - a calculation that rests on assumptions about how nuclear spins evolve in cometary environments that have not been experimentally verified. Dodson, who also works in astrochemistry, said the new laboratory technique could be used to test those assumptions directly.
The quantum computing angle is more speculative but conceptually notable. Protecting quantum states from decoherence is a central engineering challenge for qubit systems, and the ability to isolate specific spin substates using only crystal geometry suggests that materials design alone, rather than complex electromagnetic apparatus, could in principle offer a route to state protection. McLane acknowledged the current setup is not a practical qubit platform - "it's just H2 in dry ice" - but Dodson described the work as foundational. "This work is setting out the foundational rules for how quantum states might become protected," she said.
UMD chemistry major LeAnh Duckett co-authored the paper with McLane and Dodson. The research was supported by the U.S. Department of Energy Office of Science Early Career Research Program under Award Number DE-SC0024262.
Research Report: Environment-Imposed Selection Rules for Nuclear-Spin Conversion of H2 in Molecular Crystals
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