The world would look radically different without rulers and measuring tapes that fit into a pocket. Carpenters, fashion designers and engineers rely on these trusty tools to check the size of everything from a wooden board to a fabric swatch. But physicists who work with light lack that same convenience for one of the basic measurements of their craft. They routinely need to measure and compare the frequencies—colors—of the light waves they are using.
To date, they have lacked a similarly pocket-sized tool for routine measurements, and they make do with cumbersome equipment that crowds their lab space and is far from portable.
The standard tool for measuring the frequency of light is called an optical frequency comb. This device produces a rainbow of different frequencies of light, all spaced at regular intervals like the tick marks on a measuring tape. Frequency combs let researchers measure the difference between distinct frequencies and are crucial for many experiments and measurements that use light.








