RAMSEY RESONANCE IN A ZACHARIAS FOUNTAIN

We report a realization of Zacharias's 1953 proposal for observing a Ramsey resonance in an atomic fountain. Launched upward from a moving optical molasses where they have been cooled to approximately 5-mu-K, cesium atoms pass once through a microwave cavity, continue to the summit of their trajectory, then fall again through the same cavity, completing the separated-fields interaction. The atoms spend 0.25 s in free flight above the cavity. Linewidth (2 Hz) and S/N imply a stability of 3.10(-12)-tau--1/2, at least as good as in existing Cs clocks, with eventual expected improvements of 10(2).