Dear Editor,

please find enclosed our manuscript on "Time-resolved Scattering of a Single
Photon by a Single Atom", where we report on a measurement of the absorption
dynamics of heralded optical photons by a single atom in a free-space
geometry.

This, to our knowledge, is the first time the scattering dynamics is observed
for single photons with different temporal envelopes. In particular, we tested
the time symmetry of the model of spontaneous emission introduced by Weisskopf
and Wigner in 1934. Our work complements the large number of experiments in this
direction where the single photon was contained in a cavity with its discrete
mode spectrum. In our case, we do not use an optical cavity, and therefore
consider the full continuous mode spectrum of the electromagnetic field. We are
able to shape the temporal envelope of a single photon state of resonant light,
send it onto a single atom, and measure the scattering dynamics with high
temporal resolution. In agreement with a theoretical prediction, we do find that
the excitation probability of an atom is higher for an exponentially rising
single photon as compared to an exponentially decaying photon with the same
spectral distribution. We reach a maximal excitation probability of about 3%,
which is close to the theoretical maximum we expect for the spatial mode of the
single photon impinging on the single atom.

We feel that this work could be interesting to the audience of Nature Physics
because it
demonstrates, in an almost textbook-like manner, the
elementary absorption process of photons by atoms in free space.
It also contributes to the longstanding debate if heralded single photons have
the same physical action as "true" single photons - which we can confirm with
our experiment, involving a single atom as a measurement tool as compared to
photodetectors with constraints on what they are able to observe.

With Best Regards on behalf of all authors,

Christian Kurtsiefer