Figure 1: Spin initialization and read-out.

(a) Energy level scheme for a SiV⁻ under magnetic field. Excited-state levels are labelled from A to D. States labelled 3 and 4 correspond to the upper orbital branch of the ground state. The dotted grey box highlights the Zeeman-split lower orbital branch, on which we focus in this work and where state 1 (in red) corresponds to a spin-up state and state 2 (in blue), to a spin-down state. The ground state upper orbital branch lies approximately at ΔE=50 GHz above the lower one. A tunable laser at a wavelength of ∼737 nm (red double arrow) drives transition D1 resonantly. Grey dashed arrows show fast decay paths from excited state D to excited states A and B. Solid grey arrows identify the measured fluorescent transitions. (b) A first 500 ns-long optical pulse resonant with transition D1 causes optical pumping into the spin-down state, as evidenced by the exponential decay of the fluorescence. The height of the leading edge of the fluorescence due to the second 500 ns-long pulse indicates the recovery of the spin-up population, and thus acts as a read-out. The dark grey areas under the leading edges of the initialization and read-out pulses correspond to a duration of 30 ns during which the signal is integrated to calculate the peak ratio. (c) Recovery of the ratio between the leading edge of the read-out pulse with respect to the initialization pulse as a function of the time interval between the pulses at 3.5 K (purple dots). The purple curve is an exponential fit with 1/e value T_1,spin= 350±11 ns.