rf Spectroscopy in an atomic fountain

Laser-cooled sodium atoms pushed up on a vertical trajectory by radiation pressure are observed to turn around due to gravity. The relatively long time the atoms spent freely falling in this 'atomic fountain' allowed the ground-state hyperfine splitting to be measured with a linewidth of 2 Hz. After a 1000-sec integration time, the center of the line was resolved to + or - 10 mHz. The absolute splitting was measured to be 1,771,626,129(2) Hz. (Author)