[gardarh]

One thing I'm really curious about: How can you measure the accuracy of the world's most accurate clock?

By definition there would be no more accurate timing device to benchmark it against so is the accuracy cited in the article only theoretical?

[yetanotherphd]

They built two of them, and found that they agreed to within their nominal accuracy.

[Daniel_Newby]

Build several identical clocks and watch the ensemble drift relative to each other.

The lattice clock in this story has numerous clusters of atoms. They could fill only a few clusters and measure the performance relative to a good clock, then statistically derive how much it would improve by using the full set of clusters.