  Energy transfer between quantum systems can either be achieved through an
effective unitary interaction or through the generation of entanglement. This
observation defines two types of energy exchange: unitary and correlation
energy. Here we propose and implement experimental protocols to access these
energy transfers in interactions between a quantum emitter and light fields.
Upon spontaneous emission, we measure the unitary energy transfer from the
emitter to the optical field and show that it never exceeds half of the total
energy and is reduced when introducing decoherence. We then study the
interference of the emitted field and a laser field at a beam splitter and show
that the energy transfers quantitatively depend on the quantum purity of the
emitted field.

Quantum Physics