  We introduce the spin dissymmetry factor, a measure of the spin-selectivity
in the optical transition rate of quantum particles. This spin dissymmetry
factor is valid locally, including at material interfaces and within optical
cavities. We design and numerically demonstrate an optical cavity with
three-fold rotational symmetry that maximizes spin dissymmetry, thereby
minimizing the spin dephasing of a cavity-coupled quantum particle. Our
approach emphasizes the difference between spin and chirality in the nearfield
and reveals a classical parameter for designing more efficient quantum optical
devices.
