Prospects for using laser-prepared atomic fountains for optical frequency standards applications

We consider the problems and techniques of high-resolution microwave and optical spectroscopy of atoms on free-flight gravitational orbits in an atomic fountain. We consider choice of species and tabulate some relevant characteristics of several attractive atomic systems, including possible clock transitions and their estimated radiative linewidths. We illustrate one possible scheme for appoaching Zacharias’s dream of an atomic fountain and discuss a powerful two-stage laser cooling technique to reduce unwanted velocity spread. We present the status of measurements on slow atoms and on stabilized laser sources for the atomic-clock applications in both rf and optical domains.