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 opticallyexcited, quantum well (QW) microcavity exciton-polaritons. The spin-dependentCoulomb exchange interaction between the QD electron and cavity polaritonscauses the phase and intensity response of left circularly polarized light tobe different to that of the right circularly polarized light, in such a waythat the QD electron's spin can be inferred from the response to a linearlypolarized probe. We show that, by careful design of a sample with coupled QDand QW, it is possible to eliminate spin-flip Raman transitions. Thus, a QNDmeasurement of the QD electron spin can be performed within a few 10's ofnanoseconds with fidelity $\sim 99.9$%. This improves upon current optical QDspin readout techniques across multiple metrics, including speed andscalability.
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