frequency comb

Clocks on the cusp of a nuclear age

You need three things to build a clock: an energy source, a resonator, and a counter. In an analog wrist watch, for example, a small battery is the energy source that sends a small electric signal to a quartz crystal, which, in response, oscillates at a specific frequency (piezoelectric effect)

atomic clocks

The clocks that used atoms and black holes to stay in sync

You’re familiar with clocks. There’s probably one if you look up just a little, at the upper corner of your laptop or smartphone screen, showing you what time of day it is, allowing you to quickly grasp the number of daytime or nighttime hours, depending on your needs.