Diffusion-induced Ramsey narrowing
  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. 
Yanhong Xiao, Irina Novikova, David F. Phillips, Ronald L. Walsworth