Sur, S; Latha, K. V. P; Chaudhuri, R. K; Das, B. P; Mukherjee, D
(World Scientific Publishing Co, 2007)
Demands from several areas of science and technology have lead to a worldwide search for accurate optical clocks with an uncertainty of 1 part in $10^{18}$, which is $10^{3}$ times more accurate than the present day cesium ...