Magnetic-Dipole Transitions in Highly Charged Ions as a Basis of Ultraprecise Optical Clocks

We evaluate the feasibility of using magnetic-dipole ($M1$) transitions in highly charged ions as a basis of an optical atomic clockwork of exceptional accuracy. We consider a range of possibilities, including $M1$ transitions between clock levels of the same fine-structure and hyperfine-structure manifolds. In highly charged ions these transitions lie in the optical part of the spectra and can be probed with lasers. The most direct advantage of our proposal comes from the low degeneracy of clock levels and the simplicity of atomic structure in combination with negligible quadrupolar shift. We demonstrate that such clocks can have projected fractional accuracies below the $10^{-20}–10^{-21}$ level for all common systematic effects, such as blackbody radiation, Zeeman, ac-Stark, and quadrupolar shifts.

Figure 1

Illustrative examples of magnetic-dipole clock transitions in monovalent [panels (a) and (b)] and divalent (c) highly charged ions. The ordering of the hyperfine levels and values of hyperfine intervals depends on the signs and relative sizes of hyperfine structure constants. A particular choice of nuclear spins in panels (b) and (c) and hyperfine states forming clock transitions eliminates quadrupolar shifts.