Extended Data Fig. 6: B0 dependence.
From: High-fidelity spin qubit operation and algorithmic initialization above 1 K
a, T1 and the decay amplitude as a function of B0 at T = 1 K. In (1, 3), T1 exhibits a notable drop at low B0 and near the hot spot induced by excited state crossings. The reduced decay amplitude is caused by the degraded spin readout around the hot spot, and additionally the small qubit energy relatively to the thermal energy at low B0. b, Measured T1 decay curves as a function of B0 at T = 1 K. The curves are fitted with the same method as described in Extended Data Fig. 4b. c, T2 as a function of B0 down to 25 mT at T = 1 K in several charge configurations. \({T}_{2}^{* }\) and \({T}_{2}^{{\rm{Hahn}}}\) are almost B0-invariant in the three- and five-electron configurations, but experience a drop around the hot spot in the one-electron configuration. The effect is highly local and the qubit performance is consistent across configurations at low B0 until 50 mT. d, Rabi oscillations in (5, 3) at ultra-low B0 of 25 mT and 85 mT, where the qubit energy is only 3.3 % and 11.4 % of the thermal energy. Due to the small ΔEZ, crosstalk and deviation from the standard parity basis \(\{\left|\downarrow \downarrow \right\rangle ,\left|\downarrow \uparrow \right\rangle ,\left|\uparrow \downarrow \right\rangle ,\left|\uparrow \uparrow \right\rangle \}\) become severe. e, Simultaneously driven Rabi oscillations on both qubits showing the alternation of the four parity basis states. f, Resonant Rabi oscillation of Q1 as a function of microwave power at B0 = 85 mT and T = 1 K. The decay envelops are fitted to \(a{e}^{-{(t/{T}_{2}^{{\rm{Rabi}}})}^{c}}+d\), where a and c are the decay amplitude and exponent and d is around 0.5. In general, we notice a reduction in the decay exponent at lower B0, especially with faster driving. Possible causes are off-resonance driving on the ancilla qubit, decoherence during off-resonance driving, or a greater effect from the microwave-induced noise46. All of these can contribute to the reduced oscillation amplitude. The coherence does not appear to be affected and the quality factor of the Rabi oscillation is improved with faster driving. Error bars represent the 95 % confidence level.