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Europhysics Letters (EPL)

Ramsey Resonance in a Zacharias Fountain

Andr  Clairon, Ch Salomon, Saïda Guellati-Khélifa, William D. Phillips

Published Sep 7, 1991• 223 citations
Publisher

Abstract

We report a realization of Zacharias's 1953 proposal for observing a Ramsey resonance in an atomic fountain. Launched upward from a moving optical molasses where they have been cooled to ∼ 5 μK, cesium atoms pass once through a microwave cavity, continue to the summit of their trajectory, then fall again through the same cavity, completing the separated-fields interaction. The atoms spend 0.25 s in free flight above the cavity. Linewidth (2 Hz) and S/N imply a stability of 3·10 -12 τ -1/2 , at least as good as in existing Cs clocks, with eventual expected improvements of 10 2 .

DOI
10.1209/0295-5075/16/2/008

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We report a realization of Zacharias's 1953 proposal for observing a Ramsey resonance in an atomic fountain. Launched upward from a moving optical molasses where they have been cooled to ∼ 5 μK, cesium atoms pass once through a microwave cavity, continue to the summit of their trajectory, then fall again through the same cavity, completing the separated-fields interaction. The atoms spend 0.25 s in free flight above the cavity. Linewidth (2 Hz) and S/N imply a stability of 3·10 -12 τ -1/2 , at least as good as in existing Cs clocks, with eventual expected improvements of 10 2 .
We have demonstrated the possibility for a compact frequency standard based on a sample of cold cesium atoms. In a cylindrical microwave cavity, the atoms are cooled and interrogated during a free expansion and then detected. The operation of this experiment is different from conventional atomic fountains, since all the steps are sequentially performed in the same position of space. In this paper we report the analysis of a Ramsey pattern observed to present a 47 Hz linewidth and a stability of 5x10^-13 for an integration time longer than 100s. Some of the main limitations of the standard are analyzed. This present report demonstrates considerable improvement of our previous work (S. T. Muller et al, J. Opt. Soc. Am. B, v.25, p.909, 2008) where the atoms were in a free space and not inside a microwave cavity.
We have demonstrated the possibility for a compact frequency standard based on a sample of cold cesium atoms. In a cylindrical microwave cavity, the atoms are cooled and interrogated during a free expansion and then detected. The operation of this experiment is different from conventional atomic fountains since all the steps are sequentially performed in the same position of space. In this paper we report the analysis of a Ramsey pattern observed to present a (47±5) Hz linewidth and a stability of (5±0.5)x10−13τ−1/2 for an integration time longer than 100 s. Some of the main limitations of the standard are analyzed. This present report demonstrates considerable improvement of our previous work [J. Opt. Soc. Am. B25, 909 (2008)JOBPDE0740-322410.1364/JOSAB.25.000909] where the atoms were in a free space and not inside a microwave cavity.
In our Cs atomic fountain, we laser cooled and trapped 10(exp 10) Cs atoms in 0.4 s. By shifting the frequencies of the laser beams, the atoms were launched upwards at 2.5 m/s and a fraction of the atoms were optically pumped into the F=3 ground state. The unwanted atoms in the F=4 ground state were removed from the fountain with radiation pressure from a laser beam tuned to excite only those atoms. The Cs atoms in the F=3 state traveled ballistically upwards, were excited by the microwave cavity, and then returned back through the same cavity in the atomic fountain configuration.
We present a progress report on laser-cooled cesium atoms at NRC, work which was started a few years ago in order to realize a cesium fountain frequency standard. We selected an acceleration with two-dimensional moving molasses, which can increase both the force and the interaction time by a factor of /spl radic/2 over the more common 1-D molasses scheme. This configuration also allows for trapping and cooling of atoms while previously launched atoms are still moving in the clock section of the system. We report measurements on the number of atoms, temperature, friction coefficient and spring constant of our magneto-optical trap and 1-D molasses for various conditions.
National Physical Laboratory India (NPLI) has been developing a cesium atomic fountain primary frequency standard. We have succeeded in trapping about 107Cs atoms, cool them to about 5 μK by both magneto-optical trap (MOT) and polarization gradient cooling (PGC) and launch them up by moving molasses method. We have demonstrated the fountain action by launching the atoms up to 72.9 cm up and detection of the return signal by measuring the fluorescence. Work is underway to interrogate the launched atoms with microwaves and observe Ramsey fringes.