Scientists have successfully cooled molecular nuclear spin chains using parahydrogen at hypogeomagnetic fields, i.e., below Earth's magnetic field. This breakthrough is crucial for quantum computing based on nuclear spin networks, which has been limited by the initialization of high-entropy states. The method, known as Signal Amplification by Reversible Exchange (SABRE) assisted by parahydrogen, allowed for the hyperpolarization of a 12-spin chain of [U-13C,15N]-butyronitrile.

Through this technique, percent-level polarizations were generated for 13C and 15N, preparing non-equilibrium multi-spin orders across the network. A von Neumann entropy analysis of the hyperpolarized system showed that, at the optimal transfer field of 0.52 μT, the full spin system could reach S/k = 8.274, compared to S/k = 8.318 for the unpolarized reference. Experimentally, nuclear spin temperatures of 52 mK for 15N and 257 mK for 13C subensembles were achieved, representing significant cooling. The larger entropy deficit of the full network than of individual subsystems indicates correlated multi-spin order beyond single-spin polarizations.

The ability to cool these systems and generate low-entropy states is fundamental for the development of molecular quantum simulators. The precisely determined coupling network provides an experimentally benchmarked Hamiltonian, useful for testing quantum simulation, quantum control, and Hamiltonian learning protocols. Rapid field cycling to 9.4 T enabled site-resolved NMR readout, opening new avenues for the characterization and manipulation of these complex quantum systems.