Radium Ion Optical Clock

We report the first operation of a Ra⁺ optical clock, a promising high-performance clock candidate. The clock uses a single trapped ²²⁶Ra⁺ ion and operates on the 7 s ²S_1/2 →6 d ²D_5/2 electric quadrupole transition. By self-referencing three pairs of symmetric Zeeman transitions, we demonstrate a frequency instability of 1.1 ×10^-13/√{τ } , where τ is the averaging time in seconds. The total systematic uncertainty is evaluated to be Δ ν /ν =9 ×10^-16. Using the clock, we realize the first measurement of the ratio of the D_{5 /2} state to the S_{1 /2} state Landé g -factors: g_D/g_S=0.598 805 3 (11 ) . A Ra⁺ optical clock could improve limits on the time variation of the fine structure constant, α ˙/α , in an optical frequency comparison. The ion also has several features that make it a suitable system for a transportable optical clock.