Mercury yields best measure of a second so far.

The clock is proof that optical clocks, which count miniscule fractions of a second using visible or ultraviolet laser light, can outperform the current generation of atomic timepieces. It could also open the door to a new era of precision measurements of fundamental constants, according to Jim Bergquist, a physicist at the National Institute of Standards and Technology in Boulder, Colorado, who headed the study published in the 14 July issue of Physical Review Letters.

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