https://www.gov.uk/government/news/cutting-edge-timing-techn...
The US has ROCkN https://www.darpa.mil/research/programs/robust-optical-clock... but that seems to be just for military use
alt Hacker News
The US has ROCkN https://www.darpa.mil/research/programs/robust-optical-clock... but that seems to be just for military use