Probing non-classical light fields with energetic witnesses in Waveguide Quantum Electro-Dynamics

TL;DR

This paper investigates energy exchanges between a qubit and a waveguide-propagating light field, demonstrating that non-classical light can violate classical energy bounds, serving as a witness to non-classicality.

Contribution

It introduces an energy-based method to identify non-classical light fields by analyzing work bounds in qubit-light interactions within waveguides.

Findings

Coherent fields set an upper bound for qubit ergotropy.

Non-classical fields can violate this energy bound.

Operational energy-based witnesses can probe non-classicality.

Abstract

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.