  A polarization degenerate microcavity containing charge-controlled quantum
dots (QDs) enables equal coupling of all polarization degrees of freedom of
light to the cavity QED system, which we explore through resonant laser
spectroscopy. We first measure interference of the two fine-split neutral QD
transitions and find very good agreement of this V-type three-level system with
a coherent polarization dependent cavity QED model. We also study a charged QD
that suffers from decoherence, and find also in this case that availability of
the full polarization degrees of freedom is crucial to reveal the dynamics of
the QD transitions. Our results pave the way for postselection-free quantum
devices based on electron spin-photon polarization entanglement.
