English: The vapor cell of a chip scale rubidium atomic clock developed by the US NIST in 2019, shown next to a coffee bean for scale. This is the next generation version of the first chip scale atomic clock (CSAC), a caesium chip developed at NIST in 2011. The clock is based on an atomic transition of rubidium atoms vibrating at an optical frequency of 384.6 THz absorbing infrared light at 780 nm wavelength The higher optical frequency enables the clock to have an accuracy of 1.7 x 10^-13 about 100 times higher than the first chip scale atomic clock which used a caesium transition of 9 GHz in the microwave range. (Source: NIST Team Demonstrates Heart of Next-Generation Chip-Scale Atomic Clock, 17 May 2019, NIST website)