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URSI Asia-Pacific Radio Science Conference

The preliminary evaluation of the new fountain clock NIM6

Fang Fang, Weiliang Chen, Kun Liu, Nianfeng Liu, Lei Han, Tianchu Li

Published Mar 1, 2019• 4 citations
Publisher

Abstract

In this paper, we introduce the design of our new fountain clock NIM6. Besides some improvements on the vacuum system, Ramsey cavity to reduce the Type B uncertainty, NIM6 collects atoms from a MOT loading optical molasses to get a better signal to noise ratio. The atom density will be more uniform compared to a 2D MOT loading optical molasses, and the diameter of the cloud can be adjusted by the intensity and detuning of lights during the post cooling after MOT to keep the collisional shift low. Cs atoms are collected in the lower MOT chamber for 0.5 second, and then the MOT magnetic field is turned down. Atoms are 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.

DOI
10.23919/ursiap-rasc.2019.8738417

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In this paper, we introduce the design of our new fountain clock NIM6. Besides some improvements on the vacuum system, Ramsey cavity to reduce the Type B uncertainty, NIM6 collects atoms from a MOT loading optical molasses to get a better signal to noise ratio. The atom density will be more uniform compared to a 2D MOT loading optical molasses, and the diameter of the cloud can be adjusted by the intensity and detuning of lights during the post cooling after MOT to keep the collisional shift low. Cs atoms are collected in the lower MOT chamber for 0.5 second, and then the MOT magnetic field is turned down. Atoms are 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.
We report the design of a new cesium fountain clock NIM6, which is under construction in NIM. Besides some improvements on the vacuum system, Ramsey cavity and microwave synthesizer to reduce the Type B uncertainty. Another major improvement on NIM6 is to collect more atoms from a MOT loading optical molasses and optical pumping to get a better signal to noise ratio at the detection. The atom distribution will be more uniform compared with a 2D MOT loading optical molasses, and the diameter of the cloud can be adjusted by the intensity and detuning of lights during the post cooling to keep the collisional-induced frequency shift low. The atom numbers can be further increased by a new de-pumping - optical pumping procedure to pump atoms to the |F=3, mF=0> clock state directly. With a new cryogenic sapphire oscillator (CSO) based frequency synthesizer, NIM6 is aiming to reach the quantum projection noise, thus leading to a reduced Type A uncertainty compared with NIM5.
A new fountain clock NIM6 is under construction. Besides some improvements on the vacuum system, a new Ramsey cavity and a microwave synthesizer are made to reduce the Type B uncertainty. Another feature of NIM6 is collecting atoms from a MOT loading optical molasses to get more atoms with a more uniform density distribution. With a new ultra-stable microwave frequency synthesizer based on cryogenic sapphire oscillator (CSO) or ultra-stable laser, NIM6 is aiming to reach the quantum projection noise, thus leading to a reduced Type A uncertainty.
We propose an atomic fountain clock with a truncated cold atomic beam to achieve both a low collisional frequency shift and high frequency stability. In this clock, the launching velocity of a cold atomic beam can be swept to reduce the atomic density in the interrogation region for the Ramsey resonance and to increase the atomic density in the detection region. Before the top of the beam arrives at the interrogation region, the cold atomic beam is truncated by turning off the cooling laser beams to remove the unnecessary light shift. The atomic density in the interrogation region is theoretically evaluated to be 0.04 times that in an ordinary atomic fountain with optical molasses for the same number of detected atoms. The frequency stability limit due to quantum projection noise is calculated to reach $6.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ in 1 s from the number of detected atoms while the fractional collisional shift is estimated to be $~\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$.
The realisation of cold and slow atomic beams has opened the way to a series of precision measurements of high scientific interest, as atom interferometry, Bose-Einstein Condensation and atomic fountain clocks. The latter are used since several years as reference clocks, given the high performance that they can reach both in accuracy and stability. The common philosophy in the construction of atomic fountains has been the pulsed technique, where an atoms sample is launched vertically and then falls down under the effect of the gravity. The Observatoire de Neuchâtel had a different approach and has built a fountain clock (FOCS1) operating in a continuous mode. This technique offers two main advantages: the diminution of the undesirable effects due to the atomic density (e.g. collisions between the atoms and cavity pulling) and to the noise of the local oscillator (intermodulation effect). To take full advantage of the continuous fountain approach however, we need to increase the atomic flux. The techniques chosen to reach this goal are an efficient transverse collimation together with more atoms to begin with. Here we report on a study of different transverse collimation techniques performed in a two-dimensional phase stable optical lattice (namely gray molasses and magnetically induced laser cooling) as well as the development and characterisation of a 2D-magneto-optical trap used to load the fountain. Best performances are reached in a 2D+-MOT configuration of such a pre-source, for which we detect an atomic flux 20 times greater than the one measured when the fountain is loaded by a Cs vapour. This gain could improve the atomic clock stability by a factor 6. An even better stability is expected after introducing a pre-cooling stage before performing the transverse collimation. This new configuration is currently under investigation and will be implemented in a second continuous fountain (FOCS2).