3 Optical-lattice clocks
Optical clocks can be built from atoms that have been confined in an "optical lattice" - a crystal-like structure created by interfering laser beams. Because the atoms are held in the lattice, they can be probed for relatively long periods. To prevent the clock-transition frequency from being
perturbed by the strong optical fields that create the lattice, the laser that creates the lattice has to operate at a "magic" wavelength. The light shifts that are exerted on the ground and upper states of the clock transition are then exactly equal.