Laser linewidth effects in quantum state discrimination by electromagnetically induced transparency

We discuss the use of electromagnetically modified absorption to achieve selective excitation in atoms: that is, the laser excitation of one transition while avoiding simultaneously exciting another transition whose frequency is the same as or close to that of the first. The selectivity which can be achieved in the presence of coherent population trapping (CPT) is limited by the decoherence rate of the dark state. We present exact analytical expressions for this effect, and also physical models and approximate expressions which give useful insights into the phenomena. When the laser frequencies are near-resonant with the single-photon atomic transitions, CPT is essential for achieving discrimination. When the laser frequencies are far detuned, the “bright” two-photon Raman resonance is important for achieving selective excitation, while the “dark” resonance (CPT) need not be. The application to laser cooling of a trapped atom is also discussed.