COLD CESIUM MYSTERIES SOLVED BY NIST.

Researchers at NIST have solved a long-standing problem by constructing a quantitative model of collisions of ultracold cesium atoms. Atomic cesium is an important species that has been utilized in numerous cooling and trapping experiments; for example, laser-cooled cesium forms the basis of the new NIST-F1 cesium fountain atomic clock. A detailed understanding of the collision processes involving ultracold cesium atoms is necessary since collisions induce shifts in the cesium transition frequency that can adversely affect the performance of the fountain clock. In spite of many studies, a quantitative understanding of collisions of cold cesium atoms has proved elusive.

The researchers set out to explain all existing data on cold cesium collisions, including new data from experiments performed at Stanford University. The resulting quantitative model not only accounts for all known data on ground state cold cesium atom collisions, but also accurately predicted locations for a number of resonances in the Stanford experiment prior to measurements having been made. Contrary to previous expectations, the new model predicts that the collisional shift in a cesium fountain clock could be greatly reduced if the clock could be operated at a much lower temperature, in the range of 50 nK. The model also makes specific predictions for achieving Bose-Einstein condensation in cesium.