PROBLEMS WITH MANUSCRIPT: 

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Report of the Referee -- AT11669/Cere 
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The present paper by A. Cere and colleagues reports on a detailed 
experimental characterization of a photon pair source based on 
four-wave mixing in cold atomic ensemble. The group has used this 
method in a variety of previous demonstrations, as well as other 
groups did over the recent years. The focus of the present paper is to 
provide a summary of the different figures of merit and to confirm how 
experimental parameters (number of atoms, detunings, powers) governed 
these figures. These observations are compared to a well-established 
3-level model. This detailed study will be certainly useful for future 
implementations as this technique is a powerful method for single 
photon generation and subsequent light-matter interfacing. I therefore 
recommend publication in Physical Review A after the authors address 
the following important points. 

- Additional details should be given for the implementation. I did not 
find for instance the temperature of the atoms or, more importantly, 
the diameter of the beams used for the FWM. Also, the authors should 
precise if the residual magnetic fields are canceled or not. 

- In section IV, only coincidence histograms are given. It would be 
interesting to provide the correlation function of the heralded single 
photons. 

- Concerning the heralding efficiency, i.e., a major figure of merit, 
it seems to saturate in the best case around 20% when the OD is 
increased. The authors should detail this saturation and discuss the 
fundamental limitation in this number. Also, as various experiments 
are developing with very large OD, above 500, what will be the 
behavior for such systems? 

- In figure 8, the model is in good agreement for large powers but it 
seems not adequate anymore for small values. Can the authors comment 
on this discrepancy? Also, still on figure 8, the powers should be 
labeled by the color of the corresponding curve. 

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