NIM5 Cs fountain clock and its evaluation

Abstract

The cesium fountain primary frequency standard NIM5 has been developed at the National Institute of Metrology in China. The NIM5 loads atoms in an optical molasses from the background Cs vapor directly. Atoms are then cooled to a temperature of about 2 μK and launched to a height of 81 cm. The fringes of the Ramsey pattern have a width of 0.98 Hz. The NIM5 operates for more than 300 d a year, operating nearly continuously for 15 d at a time. By stabilizing the 9.19 GHz microwave frequency to the center of the central Ramsey fringe, a typical fractional frequency instability of 3 × 10 −13 (τ/s) −1/2 is obtained when running at high atom density, and a combined uncertainty, including Type A and B uncertainties, is typically 1.6 × 10 −15 . Comparisons of data between NIM5 and 5 other fountain clocks were carried out in May 2013 via two-way satellite time and frequency transfer (TWSTFT), and the results show good agreement within the uncertainties. Six groups of NIM5 data from January to June 2014 have been published in Circular T 319 and 320.

Operation of NIM5 fountain with 1.5x10-15uncertainty and design of new NIM6 in NIM

“…The detailed description of NIM5 design is in the reference [1]. In a routine operation, atoms are collected in the optical molasses for 600 ms, then accelerated for 1 ms in a moving molasses, and further cooled for another 1.5 ms by adiabatically reducing the cooling beam intensities and red offsetting the frequencies by 60 MHz.…”

“…We found NIM5 has a relatively large microwave leakage, and an interferometric RF switched is applied to reduce this effect. The detailed evaluations of the NIM5 are stated in detail in [1].…”

Closely interleaved self-comparison method applied to precise measurement

“…Furthermore, in pursuit of better determination of type-B uncertainties with a particularly small effect and uncertainties hard to obtain directly, the self-comparison method, as a direct evaluation, is widely used. The self-comparison is fulfilled by the same clock running in different states [1,6,[9][10][11][12][13][14][15][16] . Since the method is significant in the removal of systematic errors, it has succeeded in the evaluation of cold collisional frequency shifts of 133 Cs and 87 Rb atomic fountain clocks [1,9,10] .…”

“…The self-comparison is fulfilled by the same clock running in different states [1,6,[9][10][11][12][13][14][15][16] . Since the method is significant in the removal of systematic errors, it has succeeded in the evaluation of cold collisional frequency shifts of 133 Cs and 87 Rb atomic fountain clocks [1,9,10] . Besides, self-comparison is also used in the evaluation of optical lattice clocks, partially due to the lack of comparable clocks [6,[11][12][13][14][15][16] .…”

“…These error signals are used to calculate the relative frequency shift of the physical effect. Our CISC method can bypass the reference clock and extra comparison loops required in other methods [1,9,10] , and leads to better applicability for high precise clocks. The method can also evaluate the uncertainties without interrupting the running of the clock, and provides more information for the Allan deviation at short averaging times due to the close interleaving.…”