  We study the resonance fluorescence properties of an optically active spin
1/2 system, elucidating the effects of a magnetic field on the coherence of the
scattered light. We derive a master equation model for this system that
reproduces the results of a two level system (TLS) while also being applicable
to a spin system with ground state coupling. This model is then solved
analytically in the weak excitation regime. The inclusion of spin dynamics in
our model alters the properties of the coherently scattered light at a
fundamental level. For a TLS the coherence properties are known to be
determined by the input laser. We show that spin scattered light inherits the
coherence properties of the spin. This mapping allows us to measure spin
dynamics and coherence time through direct measurement of the scattered fields.
Furthermore, we show the ability to resolve sub-natural linewidth zeeman
splittings. Along with representing an invaluable tool for spin spectroscopy
understanding the coherence properties of the spin-scattered field will be
vital for spin-photon based quantum technologies.
