Auto-cancellation of motional frequency shifts in optical ion clocks

Motion-induced frequency shifts significantly contribute to the uncertainty budgets of state-of-the-art optical ion clocks. For clock transitions with second-order Doppler and quadratic Stark shifts of opposite signs it is possible to engineer electrodynamic ion confinement such that these two shift effects become anticorrelated causing zero net shift. Here we introduce a spectroscopic interrogation protocol which leads to first-order auto-cancellation of motional frequency shifts without requiring the opposite sign configuration. Particularly for ultranarrow clock lines like the electric octupole transition in singly-charged ytterbium, this novel method helps to reach total systematic uncertainties at or below the 1e-18 level.