First uncertainty evaluation of the FoCS-2 primary frequency standard

We report the uncertainty evaluation of the Swiss continuous primary frequency standard FoCS-2 (Fontaine Continue Suisse). Unlike other primary frequency standards which are working with clouds of cold atoms, this fountain uses a continuous beam of cold caesium atoms bringing a series of metrological advantages and specific techniques for the evaluation of the uncertainty budget. Recent improvements of FoCS-2 have made possible the evaluation of the frequency shifts and of their uncertainties in the order of 1 × 10⁻¹⁵. When operating in an optimal regime a relative frequency instability of 8 × 10⁻¹⁴ (<i>τ</i>/<i>s</i>) ^−1/2 is obtained. The relative standard uncertainty reported in this article, 1.99 × 10⁻¹⁵, is strongly dominated by the statistics of the frequency measurements.