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Report of Referee B -- LC19350/Tan
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P. K. Tan et al. report on an experiment where they use the radiation
of a bright thermal source for range finding. After reflection from a
distant (nearly 2 km) target, thermal light showed Hanbury Brown –
Twiss correlations with a beam reflected from a beam splitter next to
the source, and the distance could be inferred from the position of
the intensity correlation peak in the timestamp measurement.

The work is technically of very high quality and may even have some
practical importance. The paper is very clearly written, and the
results are presented extremely neatly.

But the novelty of this work is not of the PRL level. Range finding
has been demonstrated earlier using spontaneous parametric
down-conversion (SPDC). The paper I mean, S. Frick et al., Optics
Express 28, 37118 (2020), is surprisingly not cited by the authors;
they do mention SPDC-based range finding but the papers they cite
(Refs. 5,6) are on another subject. The authors claim that thermal
sources are more practical because they can be bright and narrowband;
here they use a diode laser below threshold. Such sources, however,
are known; for instance, the authors of A. Jechow et al. Nat.
Photonics 7, p. 973 (2013) used thermal light from a similar
below-threshold laser to pump second-harmonic generation. Therefore,
even if the experiment shows a much better quality than
above-mentioned SPDC range finding, it does not have enough novelty
for a PRL publication.

Another point that I find problematic is where the authors write that
‘quantum light sources have an advantage of being stationary, and
therefore carrying no obvious timing structure that may be subject to
manipulation or eavesdropping.’ Further, they consider their source as
an alternative to an SPDC source. However, the radiation of a thermal
source, in principle, can still be ‘subject to manipulation or
eavesdropping’, because anyone can split part of it and measure the
same things as the authors. In this sense, despite being stationary,
such a thermal source is not better than, for instance, femtosecond
pulses, which do the job of range finding even better. The advantage
of SPDC sources is exactly that the range finding can be eavesdropper
free, in contrast to thermal sources.

Similarly, the title mentions ‘Quantum Sensing’. This is wrong and
should be removed.

As the last remark: Fig.1 lists different SPDC sources, but somehow
ignores cavity-generated CW squeezed vacuum, which has a narrow
bandwidth and a correlation function close to 3. Or similar states
generated in ring resonators.

After corresponding corrections, I would suggest the paper makes a
good fit for ‘Applied Optics Letters’ or a similar journal but
certainly not for PRL.