No, the CSAC is a Rubidium standard. It measures the ~6.8 GHz ground-state hyperfine splitting in 87Rb using coherent population trapping with a modulated 780 n

No, the CSAC is a Rubidium standard. It measures the ~6.8 GHz ground-state hyperfine splitting in 87Rb using coherent population trapping with a modulated 780 nm diode laser. This signal is used to lock the local oscillator to the atomic transition frequency. And all that in a tiny package with 120 mW power consumption. Pretty cool stuff! Edit: At second glance, it seems liks they are actually using Cesium now (some other demonstrators use Rubidium). The principle is the same, but the hyperfine splitting is 9.192631770 GHz (by definition) and the laser wavelength is 894 nm.