  We analyze energy exchanges between a qubit and a resonant field propagating
in a waveguide. The joint dynamics is analytically solved within a repeated
interaction model. The work received by the qubit is defined as the unitary
component of the field-induced energy change. Using the same definition for the
field, we show that both work flows compensate each other. Focusing on the
charging of a qubit battery by a pulse of light, we evidence that the work
provided by a coherent field is an upper bound for the qubit ergotropy, while
this bound can be violated by non-classical fields, e.g. a coherent
superposition of zero- and single-photon states. Our results provide
operational, energy-based witnesses to probe the non-classical nature of a
light field.
