NPL Cs fountain frequency standards and the quest for the ultimate accuracy

• Published in: Journal of physics. Conference series 2016-06, Vol.723 (1), p.12003
• Main Authors: Szymaniec, K, Lea, S N, Gibble, K, Park, S E, Liu, K, G owacki, P
• Format: Article
• Language: English
• Subjects: Cesium; Clocks; Cold atoms; Fountains; Frequency shift; Frequency standards; Optical traps; Physics; Uncertainty
• ISSN: 1742-6588; 1742-6596
• DOI: 10.1088/1742-6596/723/1/012003

NPL operates a system of two primary caesium fountain clocks consisting of a fully characterised standard NPL-CsF2 together with a new standard NPL-CsF3, which has recently become operational. Both fountains feature a single-stage vapour-loaded magneto-optical trap as the source of cold atoms and an approximate cancellation of the potentially large cold collision frequency shift. As a result, the collision-shift type-B uncertainty is less than 10-16. Subsequently, more subtle systematic effects, including the frequency shifts from distributed cavity phase, microwave lensing and collisions with background gas have also been evaluated at the level of 10-16 or below. Now, as several systematic effects contribute to the fountains' uncertainty budgets similarly, further significant improvement of their accuracies is expected to be even more difficult. The short-term stability of these standards is also a significant factor limiting the overall precision as many days or even weeks of averaging is required for the type-A statistical uncertainty to approach the declared type-B systematic uncertainty. Going forward, further improvements in the reliability and robustness of operation of fountain standards is one of our priorities.