The chapter discusses quantum emitters, exploring their fundamental mechanisms, properties, and applications. Beginning with two-level systems, we introduce the concept of extinction cross-section. To capture phenomena, such as fluorescence, the discussion extends to four-level systems and spontaneous as well as stimulated emission processes, crucial for understanding laser operation. We then examine the dependence of the quantum yield on the local environment. Single-photon emission is scrutinized in terms of the second-order autocorrelation function through both steady-state and time-dependent analyses, providing a comprehensive understanding of this essential feature of quantum emitters. The chapter further addresses the generation of indistinguishable single photons, a key requirement for quantum computing and secure communication. Various types of quantum emitters are then introduced, including fluorescent molecules, semiconductor quantum dots, and color centers in diamond, each with unique properties and applications. Finally, single molecules are presented as probes for localized fields, with an in-depth look at field distributions in a laser focus and sources of strongly localized fields.

The chapter discusses the evolution of neurosurgical visualization techniques, drawing an analogy to Plato’s “Allegory of the Cave.” Traditional medical imaging provides an incomplete view of reality, similar to shadows on a cave wall. Neurosurgeons, however, can “escape the cave” by directly observing the body’s internal structures during surgery. The chapter highlights the advancements in magnification and visual augmentation tools, such as operating microscopes, endoscopes, and exoscopes. These tools have significantly improved surgical precision and outcomes. Fluorescent molecules such as fluorescein and 5-ALA enhance the surgeon’s ability to distinguish between normal and abnormal tissues. The chapter also explores the future potential of augmented reality (AR) and virtual reality (VR) in neurosurgery, which could further revolutionize surgical practices by providing enhanced visualization and planning capabilities.