A new proposal for the measurement of blackbody radiation shift in a Cesium fountain (the Politecnico di Torino method)

The AC Stark frequency shift induced by blackbody radiation (BBR) is presently a limiting factor toward the accuracy evaluation of Cs fountain frequency standards [1] [2]. In fact, at laboratory temperature, the AC Stark relative frequency shift is of the order of 10^-14 and therefore the black-body coefficient must be evaluated to the percent level if an accuracy below 10^-15 is desired. The BIPM recommended coefficient is -1.69(4) · 10^-14 for Cs atoms while recent BBR shift measurements performed at IEN (now INRIM) [3] [4] yield a value which differs by 18% from the accepted value. The Politecnico di Torino is now developing a new method for the measurement for the BBR shift, with the aim of resolving the discrepancy on the coefficient. In this paper the Politecnico di Torino method is analyzed with a full description of the new apparatus. The method is based on rapidly varying thermal cycles.