Some recent discussions about atomic clocks:

New atomic fountain clock joins group that keeps the world on time (nist.gov) | 118 points | 76 days ago | 33 comments | https://news.ycombinator.com/item?id=43831792

Major leap for nuclear clock paves way for ultraprecise timekeeping (nist.gov) | 12 points | 7 months ago | 10 comments | https://news.ycombinator.com/item?id=42362215

I left a comment on the first that summarizes the second one, which describes how they're working on a new type of atomic "nuclear" clock based on the atomic nucleus instead of electron orbitals. It doesn't mention the accuracy, I wonder how it would compare to this "ion" clock.