"The transition from quantum to classical is an essential issue in physics. At a practical level, quantum information thrives to build large quantum systems for tasks in communication or computing beyond the reach of classical devices. At the fundamental level, the question is whether there exists, in addition to environment-induced decoherence, another mechanism responsible for the disappearance of state superpositions at the macroscopic scale. Harmonic oscillators coupled to qubits are ideal to probe the limits of the quantum domain. Among various versions of this system, microwave Cavity Quantum Electrodynamics coupling Rydberg atoms to superconducting cavities has developed tools of un-matched sensitivity and precision. Building on these advances and on the development of deterministic atomic sources, DECLIC proposes to explore the dynamics of fields trapped in cavities and to study their decoherence under various perspectives. It will implement novel ways to generate non-classical states with large photon numbers stored in one cavity or non-locally split between two. DECLIC will record the gradual evolution of these states towards classicality and locality. Along this way, it will explore promising processes such as quantum random walks and collective photonic effects leading to non-classical interferometry breaking the standard quantum limit. Beyond witnessing decoherence, DECLIC will investigate ways to manipulate and control it, either by implementing feedback procedures steering the field towards targeted states, or by engineering artificial environments protecting against decoherence specific states of light. These experiments will provide invaluable clues for the understanding of other oscillator-qubit systems exploring the quantum to classical boundary."