New Absolute Frequency Measurement of the 1S - 2S Transition in Atomic Hydrogen: Preliminary Results M. Fischer 1 , N. Kolachevsky 1,2 , M. Zimmermann 1 , T. Udem 1 , and T.W. H¨ ansch 1,3 1 Max-Planck-Institut f¨ ur Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany 2 P.N. Lebedev Physics Institute, Leninsky prosp. 53, 119991 Moscow, Russia 3 Ludwig-Maximilians-Universit¨ at, Geschwister-Scholl-Platz 1, 80539 Munich,Germany M. Abgrall 4 , I. Maksimovic 4 , J. Gr¨ unert 4 , G. Santarelli 4 , P. Lemonde 4 , P. Laurent 4 , A. Clairon 4 , and C. Salomon 5 4 BNM-SYRTE, 61 Avenue de l’Observatoire, 75014 Paris, France 5 Laboratoire Kastler-Brossel, ENS, 24 rue Lhomond, 75005 Paris, France During the last century spectroscopic experiments performed on the hydrogen atom have been used as critical tests of fundamental theories and have stimulated much progress of physics. An increasing accuracy of spectroscopic methods has not only allowed to improve the accuracy of the known theories and their consequences, but have also opened new horizons for further delicate tests. The measurement of the absolute frequency of the 1S - 2S transition in atomic hydrogen with its natural linewidth of 1.3 Hz has attracted the attention of experimental physicists due to its important role for the definition of such fundamental values as the Rydberg constant and the Lamb shift [1], and for tests of QED. A recent breakthrough in absolute frequency measurements [2] has brought this experiment on a level of accuracy of 10 -14 . The last measurement performed in the year 1999 has demonstrated a precision of 1.9 parts in 10 14 [3]. Such a level of accuracy along with the 3-years time period passed since this measurement allows to discuss a possibility to observe the drift of fundamental constants in this laboratory experiment. We report on preliminary results of a new measurement of the absolute frequency of the 1S - 2S transition in atomic hydrogen performed in February 2003. The hydrogen spectrometer used for these experiments is described in [4]. Important improvements have been introduced which significantly improve the signal-to-noise ratio and reduce the linewidth of the transition as well as the drift of the laser frequency. The unique transportable Pharao Cs fountain with specified Allan standard deviation of 1.2 × 10 -13 within one second and an accuracy of about 2 × 10 -15 has been installed in Garching and served as primary frequency standard. With the help of an optical frequency comb generator based on a Ti:sapphire femtosecond laser [2], the frequency of the 1S - 2S transition has been phase coherently compared with the frequency of the Cs standard. The data recorded during 12 days of measurement require a delicate processing for the elimination of systematic effects. Up to now we have treated 4 days, and the preliminary result of averaging is equal to 2 466 061 102 474 807(67) Hz. Assuming that the measurements performed in 1999 and 2003 years are equivalent, one can evaluate a possible drift of the 1S - 2S frequency as -6.9(8.8) × 10 -15 Hz/year. The final analysis of the data will be made in the near future. References [1] F. Biraben et al. In: The Hydrogen Atom. Precision Physics of Simple Atomic Systems, ed. by S.G. Karshenboim, F.S. Pavone, G.F. Bassani, M. Inguscio, and T.W. H¨ ansch (Springer, Berlin Heidelberg 2001), p. 17 [2] R. Holzwarth et al., Phys. Rev. Lett. 85, 2264 (2000) [3] M. Niering et al., Phys. Rev. Lett. 84, 5496 (2000) [4] A. Huber et al., Phys. Rev. A 59, 1844 (1999)