911勛圖

Time is a fundamental physical quantity and the second is one of the seven base units in the International System of Units. In 1967, the second was redefined as ���the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.��� Except for a little rewording, that definition still remains in place. The most accurate timekeeping devices are atomic clocks. The current cesium fountain clocks would neither gain nor lose a second in more than 100 million years; yet, scientist are trying to make more accurate clocks. That is because more accurate time measurement would lead to valuable scientific and engineering applications ��� examples include: placing bounds on the variation of fundamental constants of Nature and developing GPS systems with sub-centimeter accuracy. In this talk, I���ll present an overview of how 3 common types of atomic clocks work.