Many quantum technologies require light sources with intrinsically quantum properties, in which the photons that are emitted are correlated and entangled with each other. Such quantum light sources often are inefficient and/or too large for integration in compact devices. A promising route to solve these issues are nanophotonic setups which offer subwavelength confinement of light, which dramatically enhances light-matter interactions and reduces system sizes, opening the door to the efficient production of highly nonclassical photon states. However, the complex structure of the electromagnetic field in such systems makes it challenging to theoretically access and predict the quantum state of the photons, which is a prerequisite for the efficient design of nanophotonic quantum light sources. In research published in PRX Quantum, we introduce a new theoretical framework that overcomes this challenge. To probe the photons at the positions of interest, we include auxiliary two-level systems that act as field detectors, sensing the electric field at their respective locations. To make the problem computationally tractable, we map the complex electromagnetic field to an equivalent network of a few lossy, interacting modes. This approach enables the use of standard quantum optical tools to fully characterize the quantum correlations of the emitted photons in space, frequency, time and polarization. We apply our framework to a hybrid system consisting of quantum emitters and metallic nanoparticles and show that even in these simplified scenarios, the emitted light displays quantum features that depend nontrivially on the position at which it is detected. The presented techniques and obtained results pave the way to a deeper understanding of light-matter interactions at the nanoscale and open up possibilities for designing novel sources of nonclassical light. [Full article]
