<p>In this article, I examine how research on the foundations of quantum mechanics contributed to the development of quantum optics during the 1970s and 1980s. To this end, I draw on the scientific trajectories of two renowned physicists, John Clauser and Alain Aspect, both of whom were awarded the 2022 Nobel Prize in Physics for their experimental tests of Bell’s inequalities. These experiments, which rely on optical techniques, not only shed light on the debate between Bohr and Einstein on the interpretation of quantum mechanics, but also prompted Clauser and Aspect to conceive and carry out new experiments that directly addressed the quantum properties of light. I will explain the technical features of this shift, particularly Aspect’s source of entangled photons, which shows an instrumental continuity between his tests of Bell’s inequalities and his subsequent single-photon interference experiments.</p>