Figure 6. Polarization and readout of the nuclear spin qubit.

(a) Pulse sequence for polarizing and reading out the nuclear qubit. The polarization and readout schemes were adapted from the methods in Gurudev Dutt et al.33, with an additional ‘kick out’ pulse to remove the unwanted population in the |↑ state into the m=+1 subspace to increase the effective polarization. An additional 532-nm pulse was applied to initialize the centre spin if it was not in the |0 state before readout. (b,c) Selective Rabi oscillations driven by weak MW channels. The symbols are experimental data, and the lines are fitting to a superposition of two cosine functions corresponding to the in- and off-resonance oscillations. P_|0› denotes the population in the |0 state. (b) Polarizing the nuclear qubit to |↓ without using the ‘kick out’ pulse. Large amplitude Rabi nutations driven by both MW₁ (green) and MW₂ (magenta) indicate that the polarization is incomplete. (c) Effect of the ‘kick out’ pulse. By employing the ‘kick out’ pulse, the population in the |0↑ state is removed, and an effective polarization of 90% is achieved in the m_I=0 subspace. The residual oscillation driven by MW₂ is caused by the off-resonance excitation in the m_I=±1 subspaces, which also contributes as a dominant source of errors in the state tomography experiments.