The clock works as follows: The rubidium atoms tick at an optical frequency in the terahertz (THz) band. This ticking is used to stabilize an infrared laser, called a clock laser, which is converted to a gigahertz (GHz) microwave clock signal by two frequency combs acting like gears. One comb, operating at a THz frequency, spans a broad enough range to stabilize itself.
The THz comb is synchronized with a GHz frequency comb, which is used as a finely spaced ruler locked to the clock laser. The clock thus produces a GHz microwave electrical signal – which can be measured by conventional electronics – that is stabilized to the rubidium's THz vibrations.
In the future, the chip-based clock's stability may be improved with low-noise lasers and its size reduced with more sophisticated optical and electronic integration.
The work is funded by the Defense Advanced Research Projects Agency and the NIST on a Chip program.
Paper: Z.L. Newman, V. Maurice, T.E. Drake, J.R. Stone, T.C. Briles, D.T. Spencer, C. Fredrick, Q. Li, D.A. Westley, B.R. Illic, B. Shen, M.-G. Suh, K.Y. Yang, C. Johnson, D.M.S. Johnson, L. Hollberg, K. Vahala, K. Srinivasan, S.A. Diddams, J. Kitching, S.B. Papp and M.T. Hummon. Architecture for the photonic integration of an optical atomic clock. 2019. Optica. DOI: 10.1364/OPTICA.6.000680.