Sub-Poisson, two-level behavior of three-level atoms prior to quantum jumps

An experimental technique which permits the study of single three-level atoms prior to quantum jumps to a shelving state has been used to study the second-order photon statistics of laser-excited barium atoms. Measured values of Mandel's Q parameter agree with the theory derived for two-level atoms. This result illustrates the extent to which the presence of additional levels can be ignored in certain systems: Although the third level in this system is responsible for the quantum jumps, prior to each jump the system's behavior is within experimental errors, exactly like that of a two-level atom. The negative values of Q obtained for a range of laser intensities demonstrate the sub-Poisson character of the emission.