Experimental sequence Each interrogation beginsonly after both ions have been subjected to Doppler cooling and prepared in the |g⟩ state with >99% probability using the conditional state preparation sequence described in ref. 53. Both ions are interrogated with the HA–HR sequence shown in Fig. 1d, with the parameters τL ≈ 4 ms, τ1 = τ2 ≈ 12 ms, and a total Ramsey time given
