A compact cold-atom coherent population trapping clock in which laser-cooled atoms are interrogated with highly coherent coherent population trapping fields under free fall is presented. The system achieves fractional frequency instability at the level of $3\times {10}^{-13}$ on the time scale of an hour. The clock may lend itself to portable applications since the atoms typically fall only 1.6 mm during the typical interrogation period of 18 ms.

• Received 2 August 2017

DOI:https://doi.org/10.1103/PhysRevApplied.8.054001