Frequency metrology

The absolute frequency of the fundamental IS — 2S transition in atomic hydrogen has now been measured to 1.8 parts in 1014, an improvement by a factor of 104 in the past twelve years. This improvement was made possible by a revolutionary new approach to optical frequency metrology with the regularly spaced frequency comb of a mode locked femto-second multiple pulsed laser broadened in a non-linear optical fiber. Optical frequency measurement and coherent mixing experiments have now superseded microwave determination of the 2S Lamb shift and have led to improved values of the fundamental constants, tests of the time variation of the fine structure constant, tests of cosmological variability of the electron-to-proton mass ratio and tests of QED by measurement of g — 2 for the electron and muon. 

Precision frequency metrology is now compatible for the search of a variation of the constants [38]. A new generation of frequency chains [8] allows to easily do two kind of frequency measurements which were hardly available previously ... 

In addition to precision frequency metrology, 2S-nX spectra can provide a wealth of information about scattering lengths for excited atoms. Metatable collision processes, and photoassociation processes, should also be observable. In summary, there are lots of scientific opportunities for trapped ultracold hydrogen. 

A New Type of Frequency Chain and Its Application to Fundamental Frequency Metrology... 

In this case the electric field would be repetitive with the round trip time. Therefore C(t) is a constant and its Fourier transform is a delta function centered as uc = 0. If it becomes possible to build a laser able to produce a stable pulse train of that kind, all the comb frequencies would become exact harmonics of the pulse repetition rate. Obviously, this would be an ideal situation for optical frequency metrology. 

We summarize hoe some considerations affecting the potential accuracy of a spectroscopic measurement of the IS ->2S transition. We shall not dwell on the challenging problems of laser stabilization and optical frequency metrology, but only on the atomic considerations. In particular, we shall consider the major sources of line broadening and possible systematic shifts. We discuss below some of the factors which govern the accuracy of IS —>2S spectroscopy in the hydrogen trap. 

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... 

Despite all of the above-mentioned limitations in accuracy of optical interferometry, it is still widely used in the determination of the wave-numbers of atomic transitions, since optical frequency metrology (synthesis chains, optical frequency combs, etc, 4) does not yet have the wide spectral coverage provided by the broad-band interferometers. As an example, a recent absolute wave-number determination of the Cs D2 resonance line at 852 nm is with a Fabry-Perot interferometer, saturated absorption and a grating-eavity semiconductor laser [76]. These results are of interest to various Cs atomic fountain measurements and lead to better determinations of fundamental constants, such as h/mp and a, [77] as well as of the acceleration due to gravity, g [78,79]. 

This chapter is organized as follows. First, the need for still further enhancement in the precision with which time is measured will be justified, and the concept of atomic clocks and their properties will be described in detail. Then, the properties required for such an atomic system suitable for time and frequency metrology will be developed as well as the conditions necessary to attain them, following schemes involving either ions or neutral atoms. For the utilization of ions in atomic clocks, the well-known technique of ion trapping is used. The next part of the discussion will be devoted, therefore, to both a panoramic view of the ion trap geometries used... 

T. Udem, R. Holzwarth, T.W. Hansch, Optical frequency metrology. Nature 416, 233-237... 

The method used is quite general and can be applied to other molecular species. Furthermore, in the near absence of background gas collisions it allows one to directly relate the rotational temperature of the ions to the temperature of the ambient black-body radiation. This feature (among others, see Refs. [101,105]) suggests the use of molecular ions, such as HD+ or CO" ", for BBR thermometry with possible applications in frequency metrology that is, it may help to improve the accuracy of frequency standards based on trapped ions [104]. 

Roth, B., Koelemeij, J., Schiller, S., Hilico, L., Karr, J.-R, Korobov, V., and Bakalov, D., Precision spectroscopy of molecular hydrogen ions Towards frequency metrology of particle masses, Precision Physics of Simple Atoms and Molecules, Lect. Notes Phys., 745, 205, 2008. 

Roth, B., Daerr, H., Koelemeij, J., Nevsky, A., and Schiller, S., Ultracold molecular hydrogen ions in a linear radiofrequency trap Novel systems for molecular frequency metrology, Proc. 20th European Frequency an Time Forum EFTF, Braunschweig, Germany, 2006. 

In time and frequency metrology, the mean value is usually the accuracy (mean time or mean fiequency offset), and the deviation in the mean is usually calculated using one of the statistics listed in Table IV. For example, if a device has a frequency offset of 2 x 10 and a la stability of2 x 10 °, there is a 95.4% probability thatthe fiequency offset will be between 1.8 and 2.2 parts in 10°. 

As an aside, one notes that frequency measurements or, more specifically, frequency ratio measurements have no limitations of the sort emphasized above. Nor, for that matter, is our knowledge of any fundamental constant limited by frequency measurement technology. An often stated goal is that we should make serious efforts to reduce all measurement to the problem of frequency metrology, thereby eliminating all the above difficulties. ... 

MC Wicks, PR Haycocks, JR Birch. Frequency metrology to 3 THz using thin film niomium nitride Josephson tunnel junctions. Dig CEPM 1992, pp 26-27. 

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