Fig. 4: Spectral properties of individual SPEs at low temperature (5 K).

a High-resolution spectrum of SPE1 ZPL, showing a resolution-limited line at 435.73 nm. b Spectral diffusion of SPE1 ZPL during 600 s. The standard deviation of the centre wavelength over time is found to be 45 μeV, as determined by Lorentzian fits of the data. c Spectral diffusion of another SPE (SPE2) and d spectral diffusion of an ensemble of three SPEs, with uncorrelated fluctuations of different magnitudes. All SPEs are excited with 1 mW cw laser light at 405 nm.