Fig. 1: Main idea of the readout scheme.

a Energy levels (simplified) of an NV⁻ center in diamond. Initially, the NV⁻ center is in its (optical) ground state with spin state either |0〉 or |±1〉. If the spin state is |0〉, a gated laser tuned to a |0〉 transition (637 nm; thin light red arrow) can excite the NV⁻ center, while spin |±1〉 is protected against excitation. A second gated high-power laser (642 nm; bold dark red arrow) can ionize from the excited state. The blue arrows indicate the microwave (MW) transitions used below. CB/VB denote the conduction/valence band of the diamond host material. b Schematic of the setup. Three individually gated lasers can illuminate the sample, which is mounted in a flow cryostat. In addition, the sample can be driven by two MW frequencies. c Final pulse sequence. Red, dark red, and green correspond to gated 637 nm resonant, 642 nm, and 517 nm laser excitation. Blue refers to MW drive. During photon acquisition (‘APD’; rose shade) for postselection and final data acquisition, cw MW excitation at both ground-state transitions is added to constantly mix the spin state during charge-state readout. d Lower panel: photoluminescence excitation (PLE) spectrum of the deep NV⁻ center that is also used for Figs. 2 and  3. Detuning is denoted from 637.20 nm. Off-axial strain is estimated to be 1.7 GHz. The inset shows the used pulsed sequence; ‘df’ indicates a change of the laser detuning. Upper panel: simulated spectrum according to Doherty et al.¹⁰. Red highlighting indicates the transition used below.