An apparatus and method for measuring CPT is disclosed. The apparatus includes a quantum absorber that is irradiated by radiation from an electromagnetic radiation source. The quantum absorber includes a material that exhibits CPT. The electromagnetic radiation source generates electromagnetic radiation having first and second CPT-generating frequency components. The first CPT-generating frequency component has a frequency .nu.L-.nu., and a first CPT component amplitude. The second CPT generating frequency component has a frequency .nu..sub.L+.nu. and a second CPT component amplitude. The apparatus also includes a detector for generating a detector signal related to the power of electromagnetic radiation that leaves the quantum absorber. The detector signal exhibits an asymmetry as a function of frequency .nu. in a frequency range about a frequency .nu..sub.0. The apparatus includes an asymmetry servo loop that alters one of .nu..sub.L, the first CPT component amplitude, and the second CPT component amplitude to reduce the asymmetry.