Optical frequency standard hydrogen atom

Abstract. A suitable femtosecond (fs) laser system can provide a broad band comb of stable optical frequencies and thus can serve as an rf/optical coherent link. In this way we have performed a direct comparison of the IS — 2S transition in atomic hydrogen at 121 nm with a cesium fountain clock, built at the LPTF/Paris, to reach an accuracy of 1.9 x 10-14. The same comb-line counting technique was exploited to determine and recalibrate several important optical frequency standards. In particular, the improved measurement of the Cesium Di line is necessary for a more precise determination of the fine structure constant. In addition, several of the best-known optical frequency standards have been recalibrated via the fs method. By creating an octave-spanning frequency comb a single-laser frequency chain has been realized and tested. 

The two-photon 243 nm lS S-i/2 transition and other transitions in the hydrogen atom have recently been included in a new list of approved radiations for the practical realisation of the metre [2]. The particular interest of these transitions lies in the fact that, uniquely among present optical frequency standards, they may be calculated in terms of the Rydberg constant with an accuracy approaching that with which they have been measured. The latest measurement of... 

The rapid progress in recent years in the spectroscopy of the hydrogen atom has renewed pressure for a much better optical frequency standard. This in itself would not be enough to solve the measurement problem. New techniques of comparing optical frequencies are needed. He have developed methods of modulating lasers which can be used for frequency differences in excess of 2THz. 

For almost three decades, the S-2S two-photon transition in atomic hydrogen with its natural linewidth of only 1.3 Hz has inspired advances in high-resolution spectroscopy and optical frequency metrology. This resonance [the 1S-2S transition] has become a de facto optical frequency standard. More importantly, it is providing a cornerstone for the determination of fundamental constants and for stringent tests of quantum electrodynamic theory. In the future, it may unveil... 

It is now recognized that cold collision frequency shifts [32] is a crucial issue for every high precision atomic frequency standard, microwave or optical. For hydrogen at a density of 109 cm-3 the shift of the 1S-2S transition is about 0.4 Hz, [8], or a fractional shift of 1.7 x 10-16. For a rubidium hyperfine standard operating at the same density, the shift is about 6 xlO-14 [45,46]. 

Isotope effects in atoms are on the order of p 10 and the magnetic hyperfine structure scales roughly as a pZgnuc 10 Zgnuc, where gnnc is the nuclear g-factor. One has to keep in mind that g uc also depends on p and the quark parameters Xq. This dependence has to be considered when comparing, for example, the frequency of the hyperfine transition in Cs (Cs frequency standard) [5] or the hydrogen 21 cm hyperfine line [30,31] with various optical transitions [5]. 

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