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\bibitem{Shelkovnikov_etal2001} Shelkovnikov, A., Grain, C., Nguyen, C. T., Butcher, R. J., Amy-Klein, A., \& Chardonnet, C. (2001). 500-Hz two-photon Ramsey fringes with a SF$_{6}$ beam: towards a new frequency standard in the 30-THz spectral region. \textit{Appl. Phys. B: Lasers and Optics}, \textit{73}, 93--98.

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