Although narrow spectral lines were measured for the lower-energy transitions, the higher energy pair of lines were broader by nearly a factor of three. It is important to account for this extra width of lines A and B, and further information is provided by photoluminescence measured with a spectrometer. At 4 K, lines C and D are much brighter than the higher-energy lines (Fig. 2a), which correspond to transitions from the upper branch of the excited state (Fig. 2b). With increasing temperature, the high-energy lines gain relative intensity, indicating that thermal relaxation occurs in the SiV− excited state. The downward exchange rate Γ ↓ adds to the rate of decay to the ground state and reduces the effective lifetime of the upper branch. Consequently, lines A and B are broadened and lose intensity in photoluminescence. The upward exchange rate depends on the Boltzmann factor, making it small but still measurable at 4 K. This additional rate out of the lower branch accounts for half of the exta linewidth above the transform limit for lines C and D.

To explain the observed homogeneity between distinct centres, we reconsider the energy level scheme in Fig. 2b. The ground and excited states have E symmetry15,24,25 and are split due to spin–orbit interaction24. In general, strain and electric fields can perturb these states to result in line shifts and increased splittings. Electric fields may be produced by nearby charged impurities and, therefore, vary across small spatial scales. The precise correspondence between orientation and line position for each SiV− measured here suggests that strain, which can be nearly uniform over a 7 × 7-μm region, is more influential than electric fields.

The large spin–orbit splittings of 46.7 GHz (258.1 GHz) in the ground (excited) state help to make SiV− unresponsive to small transverse strains. This occurs because the effect of such strain is a small perturbation until strain splitting increases to about the magnitude of spin–orbit. We observed this effect in the ground-state splitting measured between lines C and D, which varied much less (±1 GHz) than line position across the 20 sites and showed no correlation with orientation. This implies that the observed line shift results from axial strain. The inversion symmetry of SiV− (refs 15, 24) reduces the influence of small axial strain, as inverting the strain direction does not change line shift. Our observations indicate that this shielding has a lower threshold than provided by spin–orbit for transverse strain. Despite the presence of residual strain in this sample region we were still able to find identical emitters.

The spectral properties we have presented establish SiV− as an attractive single-photon emitter. For such applications, it is usually of interest to have a high quantum yield, meaning that the probability of photon emission is high for each excitation event. Measuring the absolute quantum yield is difficult, because it is not possible to collect all of the photons emitted by a colour centre. Here we have reported saturation fluorescence, but the presence of poorly understood metastable states30 prevents a deduction of absolute quantum yield for SiV− from these data. It seems, however, that SiV− has a lower quantum yield than NV−, which has similar saturation flourescence but a longer excited-state lifetime of 13 ns12. We found the SiV− decay lifetime to be shorter (1.28±0.06 ns) at room temperature than at 4 K (Fig. 2d). This is consistent with a thermally activated non-radiative decay path17,26,31 and indicates an improvement of the SiV− quantum yield at low temperature. It may be possible to disable this non-radiative decay path and dramatically improve the quantum yield. This could explain how SiV− centres in nanodiamonds grown on iridium were able to produce flourescence rates up to 6.2 Mc s−1 (ref. 32).

Another nuance in interpreting saturation flourescence is that photons are measured across the entire emission band of a colour centre (including the phonon sideband). For sources of indistinguishable single photons, the absolute quantum yield is less important than the ‘effective’ yield of photons in the desired transform-limited spectral line. The strong ZPL of SiV− gives it a significant advantage in this respect, and here we have established that line C can contain up to 50% of the total flourescence. Our results show that a single SiV− centre can provide indistinguishable photons at a collectable rate on the order of hundreds of kc s−1.

In summary, we have demonstrated a uniform single-photon source in the solid state without requiring external tuning of the optical properties. We observe nearly transform-limited linewidths, without spectral diffusion, which would allow high spectral overlap between single photons emitted from distinct sources. The production of multiple, independent single-photon emitters with identical properties is essential to the scalability of a number of schemes that use entangled photons, including quantum computing with linear optics, and is expected to form a fundamental resource in quantum optics technologies. The SiV− centre is therefore promising for such applications.