Fig. 2. Initialisation of the spin-qubit state.

a Excitation pulse sequence, as described in the main text. b Time-resolved measurement of the $∣\downarrow \uparrow \Downarrow ⟩-∣\uparrow ⟩$ transition. Emission during the resonant pump pulse exhibits Rabi oscillations with a period 0.864 ns (1.158 GHz). Inset: Trion level schemes with relevant trasnsitions. c Normalised integrated emission intensity of the readout pulse (blue) as a function of spin-pumping laser duration. The blue data points are fitted with a rate equation model to extract the evolution dynamics of the $∣\uparrow ⟩$ state and hence the spin initialisation fidelity F_init (orange). Poissonian errors for the measured data points are contained within the square markers, and for the spin initialisation fidelity the 95% CI is illustrated by the shaded region.