Optical atomic clock


The recent availability of optical frequency-combs has made possible, for the first time, direct optical frequency measurements. This, in turn, opened the way to atomic clocks based on optical transitions which could be superior in accuracy and in stability compared with the actual microwave atomic standards. Among all possible atomic sources, a sample of neutral Sr atoms has been considered as one of the most interesting candidates because of the simple level scheme which presents a set of transitions well suited for laser cooling down to almost quantum degeneracy, as well as narrow linewidth clock-transitions (linewidth <1mHz for the fermionic 87Sr isotope). We intend to define a new frequency standard, referenced on visible transitions of atomic strontium.

Our first experiment was realized with a thermal atomic beam of strontium and a pre-stabilized diode laser tuned to the narrow 1S0-3P1 intercombination line (689 nm, natural linewidth 7.6 kHz) of the bosonic 88Sr isotope [G.Ferrari et al. Proc. SPIE 5478, 5.3 (2004)]. We report the first absolute frequency measurement of the 1S0-3P1 intercombination line of 88Sr [G.Ferrari et al. Phys. Rev. Lett. 91, 243002 (2003)], obtained with a relative uncertainty of 2.3 x 10-11, which represents an improvement by more than 4 orders of magnitude with respect to previous data. In the future we plan to perform spectroscopy on ultracold atoms. Beside improving the signal to noise ratio and hence the short-term stability of the clock, a large sample of cold atoms will also reduce the dominant contribution of the 1st and 2nd order Doppler effect in the error budget of the frequency reference, improving the long-term stability and giving access to overall accuracy on the region of one part in 10-17.

The research group is involved in an European project for experiment to be done in space. In this direction our group is collaborating with CNES and ESA, within the ACES project (Atomic Clock Ensemble in Space), whose aim is to launch a Cesium atomic clock in space that should reach a level of accuracy of 10-16. The development of a strontium optical frequency reference could be used in this frame to perform ultra-precise test of relativity and of variation of the fine structure constant.