a, The atomic structure and relevant energy levels. The green ‘bond’ depicts the N–V symmetry axis. b, The spatial image of IPL on a linear colour scale, showing the emission from several N–V centres; NV1–NV4 (left to right) are marked. c, The intensity correlation function g(2)(τ) versus τ for NV1, indicating single photon emission. d, Optically detected ESR for NV1. IPL versus microwave frequency νMW (circles) with a lorentzian fit (solid line). The laser power is 1 mW for b and 370 μW for c and d.