High-accuracy measurement of the 87Rb ground-state hyperfine splitting in an atomic fountain

We describe the operation of a laser-cooled rubidium Rb-87 frequency standard. We present a new measurement of the Rb-87 hyperfine Frequency with a 1.3 x 10(-14) relative accuracy, by comparison with a Cs fountain primary standard. The measured 87Rb ground-state hyperfine splitting is v(s7Rb) = 6 834 682 610.90429(9) Hz. This value differs from previously published values (see ESSEN L., HOPE E. G. and SUTCLIFFE D., Nature, 189 (1961) 298; PENSELIN S.. MORAN T., COHEN W. and WINKLER G., Phys. Rev., 127 (1962) 524; ARDITI M. and CEREZ P., IEEE Trans. Instrum. Meas., IM-21 (1972) 391) by about 2-3 I-It and is 10(4) times more accurate. Because of the low collisional shift in 87Rb, future improvements may lead to a stability of 1 x 10(-14)tau(-1/2) and a relative accuracy in the 10(-17) range.