Non-Abelian effects in a quadrupole system rotating around two axes

The ³⁵Cl nuclear quadrupole resonance spectrum of a single crystal of sodium chlorate [R. Tycko, Phys. Rev. Lett. 58, 2281 (1987)] rotating about two axes reflects a non-Abelian gauge potential. This gauge potential is an example of Wilczek and Zee's [Phys. Rev. Lett. 52, 2111 (1984)] generalization of Berry's phase to the adiabatic transport of degenerate states.