  A novel form of Ramsey narrowing is identified and characterized. For
long-lived coherent atomic states coupled by laser fields, the diffusion of
atoms in-and-out of the laser beam induces a spectral narrowing of the atomic
resonance lineshape. Illustrative experiments and an intuitive analytical model
are presented for this diffusion-induced Ramsey narrowing, which occurs
commonly in optically-interrogated systems.
