Fig. 6: Coherence figures at low magnetic field in the hyperfine sweet spot.

a, Free induction decay coherence as measured through a Ramsey experiment. The data constitute of an average of ten traces, for a total integration time of approximately 5 min (12 h dataset, Extended Data Fig. 9), and we find a coherence time of \({T}_{2}^{* }\) = 17.6 μs. b, CPMG dynamical decoupling coherence, as measured for a sequence with 250 refocusing pulses. We find a coherence time of \({T}_{2}^{{{{\rm{DD}}}}}\) = 1.3 ms. c, Randomized benchmarking of the performance of Q2, at base temperature T = 20 mK. The solid line is a fit of the data to P = a exp(−(1 − 2F_C)N_C), with a the visibility, F_C the Clifford gate fidelity, and N_C the number of Clifford gates, from which we extract a single-qubit gate fidelity of F_g = 99.94%. The reduced visibility for larger N_C is caused by the readout being affected by the large number of pulses applied to the gate, but does not affect the extracted fidelity (Methods), as indicated by the dotted line where we fix a = 1. d, Randomized benchmarking at a fridge temperature of T = 1.1 K for Q2. We now operate in the joint Q1–Q2 hyperfine sweet spot at ϕ_B = 97.5°. We extract a single-qubit gate fidelity of F_g = 99.7%.