Performance of the PTB reconstructed primary clock CS1 and an estimate of its current uncertainty

Abstract

The paper describes the current performance of the primary clock CS1 of the Physikalisch-Technische Bundesanstalt (PTB). The clock employs a thermal atomic beam and magnetic state selection. After major reconstruction during 1995 and 1996, routine clock operation was restarted in May 1997. Results of comparisons with UTC(PTB) and other internationally recognized time scales are presented. An evaluation of the CS1 Type B uncertainty yielded 7 10-15 (1). The CS1 is thus the most accurate atomic clock in continuous operation worldwide .

Power dependence of distributed cavity phase-induced frequency biases in atomic fountain frequency standards

“…T he subject of frequency shifts in atomic frequency standards caused by distributed cavity phase is not new; it goes back to the earliest days of the thermal beam standards [1], [2] and has been the subject of continuing work both theoretical and experimental over the last 50 years [3]- [5]. Laser-cooled fountain frequency standards pose significantly different problems with respect to distributed cavity phase than the thermal beam standards.…”

Comparisons of the PTB primary clocks with TAI in 1999

“…Clock operation of the CS1 has to be interrupted for about 8 hours whereas only 30 minutes are needed for this action on the CS2 and the CS3. After each beam reversal, the strength of the magnetic quantization field (C-field) has to be changed by about 1 part in 10 3 , as explained in detail in [6].…”

“…The determination of the relative frequency correction comprises measurements of the mean C-field strength and its inhomogeneity. The determination of the inhomogeneity has been documented for the CS1 [6], the CS2 [11] and the CS3 [8] and has not been repeated in 1999. The figures reported in Table 3 can still be taken as conservative estimates of this uncertainty contribution.…”

“…A discussion follows of the uncertainty contributed by the time transfer method employed to link the PFS to EAL and TAI, respectively, together with information on TAI instability. Detailed descriptions of how the clocks' uncertainties were evaluated have been published for the CS3 [8] and more recently for the rebuilt CS1 [6]. No such refereed publication exists as yet for the CS2, therefore Section 5 explains the evaluation of the uncertainty with particular emphasis on the CS2.…”

On the power dependence of extraneous microwave fields in atomic frequency standards

“…[3][4][5][6][7][8]. Laser-cooled fountain frequency standards pose problems with respect to both distributed cavity phase and microwave leakage different from those associated with thermal beam standards.…”