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Second Report of Referee B -- LC19350N/Tan
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The Phys. Rev. Applied version of the paper by Tan et al. has been
resubmitted with minimal changes compared to its previous (PRL)
version. Overall, my impression is the same as after reading the first
version: not very much novelty, but a very high technical level of the
experiment, and several ‘misinterpretations’ in the text. The latter
were already mentioned in my first review, but the authors insist on
keeping most of their formulations. I strongly disagree with this, and
below I give the reasons in more detail.

1. The title calls this technique ‘quantum sensing’. Note that the
other reviewer also commented on this: thermal light is not quantum;
there are neither quantum states nor quantum effects involved; this
phrase is therefore misleading. The authors reply to Reviewer A: ‘.
While the thermal fluctuations can be described as classical
fluctuations, the origin of the thermal light in our case is a level
of spontaneous emission in the laser system, and as such in our view
as "quantum" as the spontaneous parametric conversion in SPDC
sources.’ They further refer to this statement in replying to my
comment. I would like to stress: indeed, thermal light has quantum
origin, but so does any light. This, however, does not make sensing
with thermal light ‘quantum’. This method does not use any quantum
feature of light. In contrast, range finding with photon pairs is
quantum because photon pairs are an example of nonclassical light. I
therefore insist that the authors remove the words ‘quantum sensing’
from the title.

2. I brought to the authors’ attention a paper (S. Frick et al.,
Optics Express 28, 37118 (2020)), where range finding with SPDC has
been proposed and implemented. To the best of my knowledge, this is
the first work on quantum range finding. The authors still do not cite
this work, claiming that the brightness of the SPDC source there, as
well as the precision of range finding, was low. I believe they should
cite that paper, as the pioneering experiment on SPDC-based range
finding. Or, if they know an earlier experiment on SPDC-based range
finding, they should cite that one. The technical imperfections of any
proof-of-principle experiment should not be a reason for ignoring it.

3. The authors cite works 5,6 as examples of SPDC-based range finding.
But both these papers are on quantum illumination – i.e., on
discovering the presence of an object against thermal noise – but not
on finding the distance to the object. I mentioned it first time, but
the authors ignored it.

I believe the paper should be published but only after the
above-mentioned corrections are made.