Steering of the French time scale TA(F) towards the LNE-SYRTE primary frequency standards

Abstract

The French atomic time scale TA(F), which is computed with data from about 20 industrial caesium standards located in nine French institutions, aims to provide a stable national frequency reference to the contributing institutions. To improve its stability, it was decided a few years ago to steer the time scale, which up to that date was free running, on the LNE-SYRTE primary frequency standards (PFS). The frequency of TA(F) was first slowly corrected monthly by an arbitrary given frequency offset with respect to TAI, to compensate the drift without disturbing the 30 d relative frequency stability of the time scale. Once close enough to the SI second, the time scale was steered monthly to the frequency data issued from the LNE-SYRTE PFS, in that way providing a more stable reference. We describe the steering applied to TA(F) and show the results in terms of relative stability with respect to TAI, or by comparing TA(F) with the SI second on the geoid as published monthly by the BIPM in its Circular T. When applying this steering during recent years, the departure over 30 d intervals of TA(F) from the SI second on the geoid was maintained inside the limits ±3 × 10 −15 . Within these limits, the TA(F) scale unit interval is in agreement with the SI second, a result which was made possible thanks to the four PFS currently in operation in the LNE-SYRTE.

Progress in atomic fountains at LNE-SYRTE

“…Following the early demonstration of the low cold collision shift in 87 TABLE IV SUMMARY OF CS FOUNTAIN COMPARISONS OVER THE NOVEMBER 2006 TO APRIL 2011 PERIOD. THE FIRST COLUMN GIVES THE CUMULATED high accuracy absolute measurements of the 87 Rb hyperfine frequency against Cs fountain clocks [64][79] [3].…”

“…TA(F) is a timescale based on an ensemble of com-mercial Cs clocks located in France and elaborated at LNE-SYRTE. This steering has improved the long-term stability of the timescale with respect to TAI [87].…”

“…3) 87 Sr and 88 Sr optical lattice clocks measured at SYRTE: LNE-SYRTE is developing two Sr optical lattice clocks. During their development, several absolute frequency measurements of the Sr clock frequency were made [93][105][106] [107].…”

“…The second, FOM, was originally a prototype for the PHARAO 1 cold atom space clock [2] and was later modified to be a transportable fountain clock. The third, FO2, is a dual fountain which operates with 87 Rb and 133 Cs simultaneously [3]. Development for more than a decade has continuously improved each of these.…”

“…Similar fibercoupled collimators provide the other laser beams for state selection and detection. In all three fountains, including the 87 Rb part of FO2, the laser system is on a separate optical bench and the light is coupled to the vacuum system with optical fibers. The lasers are home-built extended cavity diode lasers, most of which use a narrow band interference filter for wavelength selection [5].…”