Since the 1950's, NRC has used cesium atomic clocks, which are the world's best timekeepers. They use the exquisite reproducibility of spinning atoms of the element cesium.
Pure cesium is a beautiful silver-gold coloured metal that melts just above room temperature.
It is uncommon only because it combines so easily with other common elements. A gram of cesium could be found in about a cubic foot of ordinary granite. Cesium-133 atoms are sent from end to end in the vacuum tank of an atomic clock, as shown in this cutaway view. Some internal parts are illustrated below. Some magnetized atoms are set spinning by microwaves in the Ramsey cavity.