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arXiv:1601.04852 · Physical Review A

Loading a fountain clock with an enhanced low-velocity intense source of atoms

G. Dobrev, Vladislav P. Gerginov, S. Weyers

Published Apr 27, 2016• 6 citations
arXivHTMLPDF

Abstract

We present experimental work for improved atom loading in the optical molasses of a cesium fountain clock, employing a low-velocity intense source of atoms [Lu et al., Phys. Rev. Lett 77, 3331 (1996)], which we modify by adding a dark-state pump laser. With this modification the atom source has a mean flux of $4\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms/s at a mean atom velocity of $8.6$ m/s. Compared to fountain operation using background gas loading, we achieve a significant increase of the loaded and detected atom number by a factor of 40. Operating the fountain clock with a total number of detected atoms ${N}_{\mathrm{at}}=2.9\ifmmode\times\else\texttimes\fi{}{10}^{6}$ in the quantum projection noise-limited regime, a frequency instability ${\ensuremath{\sigma}}_{y}(1s)=2.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ is demonstrated.

DOI
10.1103/physreva.93.043423

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Physical Review Letters•Jun 7, 1999•568 citations
We present experimental work for improved atom loading in the optical molasses of a cesium fountain clock, employing a low-velocity intense source of atoms [Lu et al., Phys. Rev. Lett 77, 3331 (1996)], which we modify by adding a dark-state pump laser. With this modification the atom source has a mean flux of 4\ifmmode×\else\texttimes\fi1084\ifmmode\times\else\texttimes\fi{}{10}^{8}4\ifmmode×\else\texttimes\fi108 atoms/s at a mean atom velocity of 8.68.68.6 m/s. Compared to fountain operation using background gas loading, we achieve a significant increase of the loaded and detected atom number by a factor of 40. Operating the fountain clock with a total number of detected atoms Nat=2.9\ifmmode×\else\texttimes\fi106{N}_{\mathrm{at}}=2.9\ifmmode\times\else\texttimes\fi{}{10}^{6}Nat​=2.9\ifmmode×\else\texttimes\fi106 in the quantum projection noise-limited regime, a frequency instability \ensuremathσy(1s)=2.7\ifmmode×\else\texttimes\fi10\ensuremath−14{\ensuremath{\sigma}}_{y}(1s)=2.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\ensuremathσy​(1s)=2.7\ifmmode×\else\texttimes\fi10\ensuremath−14 is demonstrated.
We demonstrate the efficient capture of cesium atoms from a low-velocity intense source (LVIS) of atoms into an optical molasses. The high load rates that are achievable with the technique can potentially improve the stability of and reduce collisional shifts in atomic fountain clocks. An LVIS is an atomic beam created from a small hole in one of the retroreflectors in an otherwise ordinary magnetooptical trap (MOT). Our typical LVIS flux was 10/sup 10/ atoms/s. The asymptotic value for the number of atoms captured in the optical molasses was 1.1(1) /spl times/ 10/sup 9/ atoms, and the fill time constant was /spl tau/ = 290(30) ms. The initial molasses fill rate was R/sub t=0/ = 3.8(5) /spl times/ 10/sup 6/ atoms/ms. At this rate, it would require 24 ms to capture and launch 10/sup 7/ state-selected atoms in an atomic fountain. The fill rate at short times indicates that approximately 40% of the LVIS atoms were being captured in the optical molasses.
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 cesium fountain clock is operated utilizing a microwave oscillator that derives its frequency stability from a stable laser by means of a fiber-laser femtosecond frequency comb. This oscillator is based on the technology developed for optical clocks and replaces the quartz-based microwave oscillator commonly used in fountain clocks. As a result, a significant decrease in the frequency instability of the fountain clock is obtained, reaching $7.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}{[\ensuremath{\tau}\text{ }(\text{s})]}^{\ensuremath{-}1/2}$. We could demonstrate that for a significant range of detected atom numbers the instability is limited by quantum projection noise only, and that for the current status of this fountain clock the microwave source poses no limit on the achievable frequency instability.
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 $~\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$.
We present results on the performance of two important subsystems for NIST-F2 - the second-generation atomic fountain clock at NIST. Firstly, we demonstrate the efficient capture of cesium atoms from a low-velocity intense source (LVIS) of atoms into an optical molasses. Our typical LVIS flux was 10/sup 10/ s/sup -1/. The initial molasses fill rate was R/sub t=0/ = 3.8(5)/spl times/10/sup 9/ s/sup -1/, which is consistent with 100% atom capture given the lifetime of our vacuum system. Secondly, recent results for our launch performance in the (1,1,1) fountain geometry are presented. We were able to cool the atoms to a temperature of 1.5 /spl mu/K with our current design. Our plans for improving the laser cooling performance are also presented.
We describe the operation of a laser cooled cesium fountain clock in the quantum limited regime. An ultrastable cryogenic sapphire oscillator is used to measure the short-term frequency stability of the fountain as a function of the number of detected atoms Nat{N}_{\mathrm{at}}Nat​. For Nat{N}_{\mathrm{at}}Nat​ varying from $4\ifmmode\times\else\texttimes\fi{}{10}^{4}$ to $6\ifmmode\times\else\texttimes\fi{}{10}^{5}$ the Allan standard deviation of the frequency fluctuations is in excellent agreement with the ${N}_{\mathrm{at}}^{\ensuremath{-}1/2}$ law of atomic projection noise. With $6\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms, the relative frequency stability is $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}{\ensuremath{\tau}}^{\ensuremath{-}1/2}$, where \ensuremath{\tau} is the integration time in seconds. This is the best short-term stability ever reported for primary frequency standards, a factor of 5 improvement over previous results.