  We demonstrate the selective coupling of a single quantum dot exciton spin
state with the cavity mode in a quantum dot-micropillar cavity system. By
tuning an external magnetic field, the Zeeman splitted exciton spin states
coupled differently with the cavity due to field manipulated energy detuning.
We found a 26 times increase in the emission intensity of spin-up exciton state
with respect to spin-down exciton state at resonance due to Purcell effect,
which gives rise to the selective enhancement of light emission with the
circular polarization degree up to 93%. A four-level rate equation model is
developed and quantitatively agrees well with our experimental data. Our
results pave the way for the realization of future quantum light sources and
the quantum information processing applications.
