Peer review of: Spin-controlled generation of indistinguishable and distinguishable photons from silicon vacancy (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
This manuscript from N. Morioka et al. reports on the emission of indistinguishable photons from SiC 
defects. The authors show that single-photon emission can be efficiently controlled with a combination 
of laser and RF pulses, obtaining good experimental values for spin manipulation, g2 and 
indistinguishability. The paper is well written, the results presented and discussed clearly. I think the 
topic and the quality of the paper are suitable for publication in Nature Communications. Nevertheless, 
a few minor issues must be addressed before publication. 
 
• I could not find in the main text any comment on the statistics of these experiments. Have these 
experiments been performed on only one emitter? How many emitters have been investigated in total? 
How repeatable are the results? 
• In the conclusion paragraph, the authors compare the spectral stability of SiC emitters with other 
platforms. It would be useful to have a similar comparison also for the other selling points of the 
papers, for example indistinguishability and single-photon purity (g2). 
• At line 88, the authors talk about the deconvoluted linewidth with high-power of excitation. From the 
text, it is not clear how the resonances are deconvoluted and respect to what? 
• For some reason the quality of Fig.3 is poor. It needs to be improved. 
• In Fig. 4c the left axis is missing. 
 
 
 
Reviewer #2 (Remarks to the Author): 
 
This manuscript presents experimental measurements of two photon interference from photons 
sequentially emitted from the zero phonon lines (ZPLs) of a single V1 silicon vacancy in 4H-SiC. By 
controlling the spin state of the defect, the degree of indistinguishability between photons can be 
varied, and beating between photons can be observed. The authors also perform theoretical 
calculations of how pure dephasing and spectral diffusion affect the results. The results indicate that 
the ZPL can produce highly indistinguishable photons even when exciting non-resonantly and even 
take a step toward generating spin-photon entanglement using the frequency of the photons. I find 
this work to be very high quality, quite impressive, and the first of its kind for this defect. I also find 
the detailed analysis of the effects of pure dephasing and spectral diffusion quite useful. This 
manuscript is of sufficient importance and interest to the solid state quantum information science 
community that it should be published in Nature Communications. 
 
While I do support publication, I have one main concern that should be addressed, and a few lesser 
points to bring up. The main concern is about the count rate of photons into the ZPLs. For quantum 
networks or cluster state generation, this seems like a very important figure of merit, and I can’t find 
anything in the main text and only hints in the supplementary information. In particular, the authors 
make comparisons to other emitters like defects in diamond, quantum dot, and single molecules in 
terms of spectral stability. Another important feature of some of these systems is that most of the 
photons are emitted into the ZPL and have high photon count rates. It seems important to know what 
percentage of photons are emitted into the ZPLs and what count rate this results in. This seems like a 
concern for silicon vacancies in SiC. 
 
Lesser points: 
 
1. The discussion of coincidence window gating and how it relates to pure dephasing is a little 
confusing. My understanding is that this is used to better eliminate photons emitted during the laser 
pulse, but I feel like I’m not following the concept very well or what Fig. 4(c) is getting at. The 
supplementary information helped a little, but not too much. 
2. Fig. 4(c) is missing the vertical axis label, which I believe should be the corrected HOM visibility. 
3. The Rabi oscillations shown in Supplementary Figure 5(a,b) look a little strange. The authors 
comment on this, saying that a detailed theoretical model will be given later. I did find the model, but 
I couldn’t find an explanation. Is it related to off-resonant driving of the transitions?