Fig. 1. Optical properties of single silicon vacancies h-V_Si.

a Crystallographic structure of 4H-SiC and position of h-V_Si centre (highlighted by a pink sphere symbolising a missing Si atom). b Level structure of the h-V_Si centre at zero magnetic field. Ground state (GS) and V1 excited state show degenerate sublevels $m_{\mathrm{S}}=\pm \frac{1}{2}$ and $m_{\mathrm{S}}=\pm \frac{3}{2}$. Optical transitions between V1 and GS are spin conserving and associated with the transitions A₁ and A₂. Emission from the second excited state, V1’, is not observed owing to ultrafast relaxation (dashed arrows). c Single h-V_Si centre emission spectrum under off-resonant excitation. d ZPL emission fine structure recorded over 20 min. The red dots are raw data. The two emission lines associated with A₁ and A₂ transitions are clearly resolved. The blue dashed line is a Lorentzian fit to the raw data, giving linewidths (FWHM) of 86 ± 6 and 77 ± 6 MHz for the A₁ and A₂ lines, respectively. The black line is data after deconvolution correction for the finite linewidth of the scanning Fabry–Pérot cavity (Lorentzian FWHM of 29 ± 2 MHz), the resulting real emission linewidths are 57 ± 6 and 48 ± 6 MHz, respectively, which is very close to the Fourier transform limit. e Second-order autocorrelation function recorded for a single h-V_Si centre under pulsed off-resonant excitation (pulse energy: 5.7 pJ). We observe g² (τ = 0) = 0.12 ± 0.01, clearly indicating single-photon emission. f g² (τ = 0) as a function of laser pulse energy. The line is a guide to the eye. Error bars correspond to one standard error.