Dear Editor,

please find enclosed our manuscript entitled "Hong-Ou-Mandel interference
between triggered and heralded single photons from separate atomic systems" -
we believe this reports on an important experimental demonstration how to
connect to different physical systems for quantum information purposes on the
way towards more complicated quantum networks.

As stated before, this is mostly experimental work, where we can demonstrate
that the two photons emitted by two different physical systems can be made
indistinguishable enough to allow for an interference between them. While (to
our knowledge very few) comparable experiments have been performed before, we
do want to stress that in our case, no spectral filtering did take place,
which makes the process much more efficient, and could potentially lead to a
more scalable network of different quantum nodes.

We believe this work is an important demonstration towards making different
physical systems involved in quantum networks compatible with each other, that
shows for specific situation this not only remains an interesting concept, but
can be done with real physical systems. As this may be relevant not only to
the atomic physics community, but also interesting to a wider audience in the
physics community, we felt that Physical Review Letters may be an adequate
platform to present this work.

With Best Regards on behalf of all authors,

Christian Kurtsiefer