On the Directivity of Radiating Quantum Electromagnetic Systems

We explore the spatial directivity of radiating quantum source systems, which are defined as any generic source capable of producing photon emission and directing it to specific regions in space. We present a comprehensive definition of quantum directivity, inspired by both classical antenna theory and photon detection theory. Through an in-depth conceptual and mathematical analysis, we identify and address several critical challenges associated with characterizing the directive properties of a general quantum source system. Our approach essentially presents a computational model that relies solely on the density operator of the radiation field as input.