  We propose a scheme to perform single-shot quantum non-demolition (QND)
readout of the spin of an electron trapped in a semiconductor quantum dot (QD).
Our proposal relies on the interaction of the QD electron spin with optically
excited, quantum well (QW) microcavity exciton-polaritons. The spin-dependent
Coulomb exchange interaction between the QD electron and cavity polaritons
causes the phase and intensity response of left circularly polarized light to
be different to that of the right circularly polarized light, in such a way
that the QD electron's spin can be inferred from the response to a linearly
polarized probe. We show that, by careful design of a sample with coupled QD
and QW, it is possible to eliminate spin-flip Raman transitions. Thus, a QND
measurement of the QD electron spin can be performed within a few 10's of
nanoseconds with fidelity $\sim 99.9$%. This improves upon current optical QD
spin readout techniques across multiple metrics, including speed and
scalability.
