Journal article (paywall): <a href="http:&#x2F;&#x2F;journals.aps.org&#x2F;prl&#x2F;abstract&#x2F;10.1103&#x2F;PhysRevLett.116.063001" rel="nofollow">http:&#x2F;&#x2F;journals.aps.org&#x2F;prl&#x2F;abstract&#x2F;10.1103&#x2F;PhysRevLett.116...</a><p>There&#x27;s a team working at JILA in colorado who&#x27;ve achieved something similar with Strontium: <a href="http:&#x2F;&#x2F;arxiv.org&#x2F;abs&#x2F;1412.8261" rel="nofollow">http:&#x2F;&#x2F;arxiv.org&#x2F;abs&#x2F;1412.8261</a><p>Edit: One of the reasons to build these clocks is to find out if the fundamental constants really are unchanging. So they build two clocks that rely on different  relations of fundamental constants, and see if the &quot;ticking&quot; of the clocks changes with respect to each other. See: <a href="http:&#x2F;&#x2F;arxiv.org&#x2F;pdf&#x2F;1407.4408.pdf" rel="nofollow">http:&#x2F;&#x2F;arxiv.org&#x2F;pdf&#x2F;1407.4408.pdf</a> for info on that.