One hundred years after the discovery of electron spin, the proportionality of the electron’s magnetic dipole moment and its spin makes possible what is arguably the most stringent test of the Standard Model (SM) and sensitive probes for physics beyond the SM. The electron magnetic moment, in natural units of Bohr magnetrons, is the most precise prediction of the SM and the most precisely measured quantity of any elementary particle. The quantum methods used for a recent measurement of the size of a single electron’s magnetic moment will be discussed. To improve beyond the precision of 1 part in 10^{13}, a quantum limited detector is being employed in a new apparatus and measurement, with special relativity providing a quantum non-demolition coupling of the electron’s spin state to the detector. Comparing the electron and positron moments provides the opportunity to test the fundamental CPT invariance of the SM with leptons at an even higher precision.