  The concepts of work and heat in the quantum domain, as well as their
interconversion principles, are still an open debate. We have found theoretical
evidence that a single photon packet is capable of extracting work from a
single two-level system (TLS). More importantly, this effect is found for a
photon as it is spontaneously emitted, therefore carrying only heat away from
the emitter. This is the most elementary process in which heat is converted
into work, requiring not more than two off-resonance atoms for that purpose.
From a more practical point of view it is found that, surprisingly, work can be
extracted from a TLS in a completely passive initial state. The process is
cyclic in the sense that the TLS initial and final states are equal. The state
of the TLS remains passive throughout the interaction time with the
single-photon packet. The physical meaning of the work performed by the TLS is
found to be a dynamical change in the color of the photon during the reemission
process. All our predictions are, in principle, measurable within
state-of-the-art experiments.
