Dear Editor,

we like to thank the referee for the clear and well-structured comments, and
the positive reception of our manuscript in general, but challenge the
assessment in the report of (un)suitability for publication in Physical Review
Letters. To make our position perhaps clearer, we modified the manuscript in a
few locations.

Specifically, we disagree on the conclusion on the interest and impact:

On Interest: 
The criticism here is that the manuscript is of interest primarily to those
working in interfacing atoms and photons. There are many groups working on
interfacing photons with other physical systems such as quantum dots, single
molecules, and even superconducting qubits etc. that rely on a very similar
level structure. Therefore, our method for generating single photons with
"time reversed", exponentially rising envelope will be of interest to these
communities that are significantly distinct from the atomic physics community.
In our revised version, we have highlighted this interest of other communities
in the introduction and the conclusion of the manuscript along with relevant
references (see detailed changes A, B, C, E below). Loosely related to
experiments on 
ghost imaging that got re-popularized very recently, and certainly of interest
to this community, we felt that the most concise connection to this, again
different community, would be a more historical work on remote dispersion
cancellation in a parametric down conversion scenario. We therefor have to
disagree with the conclusion that this work is of limited interest to a small
community only.
One may even argue that the remote manipulation of a photon envelope is a
non-obvious trick that may be aesthetically pleasing to an even wider
audience.

On Impact:
The main idea of the paper is to show how to obtain a single photon with a
rising exponential envelope resonant to the ground level. As outlined above
and clarified in the new version of the manuscript, the method presented can
be applied to a number of physical systems, and therefore can have high impact
on generation of such exponentially rising photons not only for interaction
with single atoms but for other physical systems as well. An actual
experiment of absorption by a single atom will be an important experiment we
are considering to carry out, but extends the single photon envelope shaping
concept and experimental demonstration we present in this manuscript. As a
first indication, our Figure 4 clearly shows how a rising exponential photon
improves the absorption. However, this is not the main point of this
manuscript.

On Innovation:
Though there is a wide interest in studying and using the time reversal
symmetry in experiments, most schemes until today use either attenuated lasers
pulses temporally shaped using modulators. The single photon shaping techniques
successfully demonstrated in cavity-QED experiments using a modulation of
Raman beams is significantly different from our scheme.  Other than that, we
are not aware of much work on generating the "time-reversed" single photons
other than by direct modulation techniques cited in our manuscript.  
We think the innovation here is to show that one can obtain these single
photons by combining two well known techniques. While one might take a
position that this should work in principle, we like to stress that we present
an actual experiment, and not a proposal. To our knowledge this combination of
important existing has not been shown before in an experiment. 

We now address the more technical comments of the referee:

1. The temporal shape due to spatial mode overlap does depend on the detuning
of the cavity. When the photon is far off resonant to the cavity,  a large fraction of it gets reflected off the first mirror without coupling to the cavity mode. Therefore the spatial mode mismatch with the cavity does not have a significant effect on the temporal shape of the photon. 
On the other hand, the coupling to the cavity mode is strong when the photon
is resonant to the cavity and the spatial mode mismatch has a greater effect
on the temporal shape. 

2. In figure 2, the top plot is not an exact time reversal of the bottom
plot. This is because the top plot is measured by tuning the cavity 120 MHz
from the photon frequency. Even at this detuning (\delta > 4 \gamma), there is
a small off-resonant coupling of the photon to the cavity. This is now
indicated in the text discussing the figure 2 in the revised manuscript.

3. "Off-resonant coupling of the incident signal photon to the cavity leads to
the residual coincidences at times t_i − t_s < 0." We would expect an exact
time reversal if the measurement is performed in the absence of the cavity. 

4. We thank the referee for pointing out the recent work by Reiserer et al. We
have now cited this paper in introduction.


A summary of the changes addressing both the technical and the interest/impact
concerns is given below:

(A) The word "atomic" is removed from the abstract. The sentence is changed
from  "We use the photon pairs generated from a time-ordered atomic cascade
decay" to "We use the photon pairs generated from a time-ordered cascade
decay". We did this because the technique is actually not limited to atomic
cascades.

(B) The following lines are added in the first introductory paragraph
referring to other physical systems, giving reference to work in other
communities:  "This advantage also  applies to interacting single photons with
other systems such as quantum dot [Volz:2012, Johne:2011], single molecules
[Rezus:2012] and superconducting circuits [Wenner:2014]."

(C) Further down, we expanded the end of the third paragraph with the
following: "This concept is not limited to atoms, but can be equally applied
to other physical system with a cascade level structure to obtain such photons
[Santori:2002, Dousse:2010, Akopian:2006]. A related idea has been used in the
past for non-local dispersion cancellation [Strekalov:1995]. 

(D) A sentence is added to the paragraph discussing Figure 2. "The residual
coincidences at times $t_i-t_s < 0$ comes from the off-resonant coupling of
the incident signal photons to the cavity."

(E) The conclusion of the paper now ends with "As this time reversal technique
can be used with photon pairs from other sources with time-ordered emission,
as found e.g. in molecules and quantum dots, it completes the toolbox
necessary to interconnect stationary qubits in a complex quantum information
processing scenario."

(F) The following new references are included:
  [4] A. Reiserer, N. Kalb, G. Rempe, and S. Ritter, Nature 508, 237 (2014).
  [11] T. Volz, A. Reinhard, M. Winger, A. Badolato, K. J. Hennessy, E. L. Hu,
  and A. Imamoglu,  Nat Photon 6,605 (2012).
  [12] R. Johne and A. Fiore, Phys. Rev. A 84, 053850 (2011).
  [13] Y. L. A. Rezus, S. G. Walt, R. Lettow, A. Renn, G. Zumofen,
  S. Goetzinger, and V. Sandoghdar, Phys. Rev. Lett. 108, 093601 (2012).
  [14] J. Wenner, Y. Yin, Y. Chen, R. Barends, B. Chiaro, E. Jeffrey,
  J. Kelly, A. Megrant, J. Y. Mutus, C. Neill, et al., Phys. Rev. Lett. 112, 210501 (2014).
  [19] C. Santori, D. Fattal, M. Pelton, G. S. Solomon, and Y. Yamamoto, Phys. Rev. B 66, 045308 (2002).
  [20] A. Dousse, J. Suffczynski, A. Beveratos, O. Krebs, A. Lemaitre, I. Sagnes, J. Bloch,
 P. Voisin, and P. Senellart, Nature 466, 217 (2010).
  [21] N. Akopian, N. H. Lindner, E. Poem, Y. Berlatzky, J. Avron, D. Gershoni, B. D. Gerardot, 
and P. M. Petroff, Phys. Rev. Lett. 96, 130501 (2006).
  [22] D. V. Strekalov, A. V. Sergienko, D. N. Klyshko, and Y. H. Shih, Phys. Rev. Lett. 74, 3600 (1995).
  [25] G. S. Agarwal and S. D. Gupta, Phys. Rev. A 49, 3954 (1994).

We therefore would like the Editor to reconsider the decision on suitability
for publication in Physical Review Letters.

With Best Regards on behalf of all authors,

Christian Kurtsiefer