Fig. 4. Measurement of classical spin-photon correlations.

a Level scheme showing the resonant entanglement/readout laser, the (optional) rotation pulse and the blue and red decay paths. b Pulse sequence used for the entanglement measurements. c Second-order autocorrelation measurement to evaluate the purity of our source. Two different time windows, [1.15 ns, 1.7 ns] marked in green, and [1 ns, 4 ns] marked in purple, in d, e, are considered which correspond to part or all of entanglement pulse decay, respectively. d Measured arrival times of red/blue photons after the entanglement pulse, conditioned on a photon being present in the readout pulse (spin projection to the $∣\downarrow ⟩$ state). Measurements were carried out at 9 T (dark red and dark blue curves) and 5 T (light red and light blue curves). e The same measurement as in d, but with a π pulse inserted before the readout pulse. The readout, therefore, projects the spin into the $∣\uparrow ⟩$ state.