"The 1967 General Conference on Weights and Measures defined the of time, the second, based on an atomic transition—specifically, between two hyperfine levels of the ground state of cesium-133. (See Physics Today, August 1968, page 60.) Although Cs atomic clocks remain the standard, their time might be running out. Their underlying atomic transition is excited by radiation with a microwave frequency around 9 × 109 Hz, and after decades of advances, a Cs clock’s frequency can be measured with a fractional uncertainty Δν/ν0 of about one part in 1016. But clocks based on optical transitions operate at frequencies around 1014 Hz, which gives them an advantage in the push for lower uncertainty. (See the article by James Bergquist, Steven Jefferts, and David Wineland, Physics Today, March 2001, page 37.) The current record, 9.4 × 10−19, was set in 2019 by an aluminum ion–based optical atomic clock at ."