Doublet state

Essentially all occurrences of doublets in nature arise from rotational symmetry; spin 1/2 is associated with the fundamental representation of the Lie group SU(2).

The term "doublet" dates back to the early 19th century, when it was observed that certain spectral lines of an ionized, excited gas would split into two under the influence of a strong magnetic field, in an effect known as the anomalous Zeeman effect.

By applying both a magnetic field and carefully tuning a radio-frequency transmitter, the nuclear spins will flip and re-emit radiation, in an effect known as the Rabi cycle.

The strength and frequency of the emitted radio waves allows the concentration of such nuclei to be measured.

These materials have the advantage of having 100% theoretical quantum efficiency based on spin statistics whereas singlet systems and triplet systems have significantly lower efficiencies or rely on noble metals such as Pt and Ir to emit light.