Probing nonclassical light fields with energetic witnesses in waveguide quantum electrodynamics

We propose an operational scenario to characterize the nature of energy exchanges between two coupled but otherwise isolated quantum systems. Defining work as the component stemming from effective unitary interactions performed by each system on one another, the remnant is stored in the correlations and generally prevents full energy extraction by local operations. Focusing on the case of a qubit coupled to the light field of a waveguide, we establish a bound relating work exchange and local energy extraction when the light statistics is coherent, and that gets violated in the presence of a quantum light pulse. Our results provide operational, energy-based witnesses to probe nonclassical resources.