High-accuracy measurement of the

We describe the operation of a laser-cooled rubidium 87Rb frequency standard. We present a new measurement of the 87Rb hyperfine frequency with a $1.3\times10^{-14}$ relative accuracy, by comparison with a Cs fountain primary standard. The measured 87Rb ground-state hyperfine splitting is $\nu_{^{87}\mbox{\scriptsize Rb}}=6~834~682~610.90429(9)$ 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 Wscinkler G., Phys. Rev., 127 1962 524; Arditi M. and Cerez P. IEEE Trans. Instrum. Meas. IM-21 1972 391) by about $2\mbox{-}3$ Hz and is 104 times more accurate. Because of the low collisional shift in 87Rb, future improvements may lead to a stability of $1 \times 10^{-14}\tau^{-1/2}$ and a relative accuracy in the 10-17 range