The magnetic moment of the electron has been measured using a one-electron quantum cyclotron and quantum nondemolition spectroscopy. The spin frequency of the electron is determined by the g -factor. ν ν s = g 2 ν ν c g 2 = ν ν ¯ ¯ c + ν ν ¯ ¯ a ν ν ¯ ¯ c