Coherent Quantum Control of a Nuclear-Spin-Isomer Superposition in a Nitrogen Molecular Ion

Preserving quantum coherence becomes increasingly challenging as systems grow in size and internal complexity. Molecules, with their rich and highly structured spectra, offer a unique platform for both high-resolution control and for investigating how quantum coherence persists in complex systems. Here [1], we present a scheme to coherently couple two distinct nuclear-spin isomers of the same molecule, namely the I = 0 and I = 2 isomers of the nitrogen molecular ion. Our approach exploits a magnetic-field-tunable avoided crossing in the molecular spectrum, where an electric-quadrupole hyperfine interaction mixes the two otherwise uncoupled isomeric manifolds. By operating near this avoided crossing, we show how to engineer a strong and coherent coupling between initially unmixed nuclear-spin–isomer states, enabling the creation of a nuclear-spin–isomer qubit based on a long-lived, weakly perturbing internal molecular degree of freedom. This scheme highlights the potential of nuclear-spin isomers as robust resources for molecular quantum control and for probing quantum coherence in complex systems.

[1] T. Levin and Z. Meir, Phys. Rev. Research 7, 013274 (2025).