[Blackbeard_]

That would be more than a 5 sigma difference which would definitely be significant but that number you're quoting for normal hydrogren is the 1S-2S line.

This measurement is the 1S-2P line which has an average frequency of ~2,466,051.625 GHz (there's actually two levels). Which agrees to within 1 sigma - i.e. good agreement.

[gus_massa]

Thanks for the correction. I just assumed that the other article was about the same transition and I forget to check the details.

PS: If someone else wants to know why the 1S-2S and the 1S-2P transition have almost the same energy, but no exactly the same energy, it's an interesting weird story https://en.wikipedia.org/wiki/Lamb_shift