The frequency standard comprises a quantum absorber, a source of incident electro-magnetic radiation, a detector, a frequency difference controller, a spectrum controller and a frequency standard output. The quantum absorber has transitions including a first transition between a first lower quantum state and an upper quantum state, and a second transition between a second lower quantum state and the upper quantum state. The first transition and the second transition have energies that correspond to frequencies of &ohgr;1 and &ohgr;2, respectively. The lower quantum states differ in energy by an energy difference subject to a total a.c. Stark shift. The source of incident electro-magnetic radiation is arranged to irradiate the quantum absorber. The incident electro-magnetic radiation includes main frequency components at frequencies of &OHgr;1 and &OHgr;2, equal to &ohgr;1 and &ohgr;2, respectively, and additionally includes additional frequency components collectively having a spectrum. The detector is arranged to receive electro-magnetic radiation from the quantum absorber and generates a detection signal in response to the received electro-magnetic radiation. The frequency difference controller controls the source to generate the main frequency components with a difference in frequency that obtains an extremum in the detection signal. The extremum indicates that the difference in frequency corresponds to the energy difference. The spectrum controller sets the spectrum of the additional frequency components to reduce the magnitude of the total a.c. Stark shift. The frequency standard output a frequency standard signal related in frequency to the difference in frequency.