Answer: Lasers

Since the mid 1950s, when the first atomic clock radically increased the accuracy of timekeeping beyond simply trying to synchronize quartz-crystal clocks to the movement of the Earth around the Sun, there has been a continual drive to refine and improve atomic timekeeping.

For decades atomic clocks relied on the use of microwave radiation to excite electrons and measure the oscillation of electrons in hydrogen-1, cesium-133, and rubidium-87 atoms and many of the atomic clocks around the world still use this reliable and time-proven mechanism. Despite significant improvements to the microwave model over the last sixty years (the microwave-based atomic clocks get better by a factor of ten roughly every decade), they have been surpassed by new technology.

Recent refinements, building on research efforts started back in the 1990s, have yielded an atomic clock that uses cooling and lasers (known as optical-lattice clocks) to increase the precision in which the atoms can be monitored and interacted with. How much more precision? The best optical clocks researchers have created to date are so precise that if run for the entire age of the universe the clock would lose no more than a second.