Optical physics: When is a second not a second?

Measurements of the accuracy of two optical lattice based atomic clocks are presented in Nature Communications this week. The results suggest that this type of clock could present a more accurate definition of the second, improving the precision with which frequencies and times can be measured.

In the International System of Units, the second is defined by the frequency of light emitted from a caesium atom undergoing a specific transition. Atomic clocks currently used to measure this are based on atomic fountains, which throw atoms upwards much like a regular fountain. An alternative type of atomic clock, which traps atoms using light and keeps them held for long enough to make detailed measurements, has recently been proposed and tested. Jerome Lodewyck and colleagues present results on the accuracy of two such optical lattice clocks using an atomic transition in strontium atoms instead of caesium. They build two of these strontium optical lattice clocks and find that they are able to keep time with each other much more accurately than when they measure with three caesium atomic fountain clocks.

The findings represent a step towards a possible new system for defining the SI second.