Can you build an atomic clock at home? (quora.com)

for time enthusiasts, rubidium oscillators were one approach as second-hand ones became available from CDMA shutdown even to the point of hacking one http://phk.freebsd.dk/hacks/HP5065A/index.html

or see https://hackaday.com/2016/09/25/rubidium-disciplined-real-ti... https://deltafabri.wordpress.com/2015/03/08/under-60-ns-an-a... I'm not sure the TAPR clock block is still available

several modules are listed on https://www.microsemi.com/product-directory/timing-synchroni... both for a rubidium oscillator & a Chip Scale Atomic Clock https://en.wikipedia.org/wiki/Chip-scale_atomic_clock and from Jackson Labs http://www.jackson-labs.com/index.php/products/csac

some links on https://www.raspberrypi.org/forums/viewtopic.php?t=212429