Explore the fourth lecture in a five-part series on building quantum machines with light, delivered by Marcelo Martinelli from IFUSP – USP, Brazil. Delve into advanced concepts of quantum optics and their applications in constructing quantum devices. Learn about the latest developments in harnessing light for quantum information processing and communication. Gain insights into the challenges and breakthroughs in creating practical quantum machines using photonic technologies. This 1-hour 17-minute talk is part of the School on Light and Cold Atoms organized by ICTP-SAIFR in March 2023, offering a comprehensive look at the intersection of quantum physics and optical science.