  Two-electron charged self-assembled quantum dot molecules exhibit a
decoherence-avoiding singlet-triplet qubit subspace and an efficient
spin-photon interface. We demonstrate quantum entanglement between emitted
photons and the spin-qubit after the emission event. We measure the overlap
with a fully entangled state to be $69.5\pm2.7\,\%$, exceeding the threshold of
$50\,\%$ required to prove the non-separability of the density matrix of the
system. The photonic qubit is encoded in two photon states with an energy
difference larger than the timing resolution of existing detectors. We devise a
novel heterodyne detection method, enabling projective measurements of such
photonic color qubits along any direction on the Bloch sphere.
