Next-generation clocks and redefining the second

5 problems

• Blackbody radiation shift of lattice clocks at the $10^{-19}$ level

  Open

  Heat radiation from the room shifts atomic clock frequencies, and for strontium this is one of the largest uncertainties. Controlling it ten times better requires both temperature control and better atomic theory.

• Can optical clocks meet the redefinition criteria by 2029?

  Open

  Before the second is redefined, several optical clocks around the world must agree, be compared across continents and feed international time routinely. Whether that will be achieved in time for 2030 is not settled.

• Intercontinental optical clock comparison at $10^{-18}$

  Open

  Clocks on different continents must be compared to check that they agree, but satellite links are not yet good enough. Height differences also matter, since a clock 1 cm higher runs faster by about one part in $10^{18}$.

• What limits the accuracy of a solid-state thorium nuclear clock?

  Open

  Thorium-229 nuclei embedded in a transparent crystal can be laser-excited, which could make a clock that is compact and insensitive to many disturbances. The crystal environment broadens and shifts the line, and the ultimate accuracy is unknown.

• Laser excitation of the Th-229 nuclear isomer in a trapped ion

  Open

  Thorium-229 nuclei have been excited by laser inside crystals, but a single thorium ion held in a trap would avoid crystal disturbances and could make a clock accurate to about one part in $10^{19}$. Laser excitation of the nucleus in a trapped ion has not yet been achieved.