The first lesson began with a general reminder of the essential properties of measurement of a microscopic system in quantum physics. Measurement" is broadly understood to mean any experiment that extracts information from a quantum system. Whereas in classical physics, the system under study can be in a state independent of any observer and be measured without disturbance, in quantum physics a measurement is a more complex process, in which the state of the object being measured is usually modified. The result of the measurement and its effect on the system are described statistically, as theory can only determine the probability law of the process ("God plays dice"). The properties of quantum measurement lead to limitations (some measurements are incompatible with each other - cf. Heisenberg's uncertainty relations) and impossibilities (e.g. the "non-cloning" theorem for an unknown state). These characteristics, often described negatively, can be exploited positively to perform operations that are impossible in classical physics (quantum cryptography and computation).
After these general reminders, the lesson turned to the more specific problem of the quantum measurement of light. The electromagnetic field is a central system in physics. It carries most of the information we receive from the world. Understanding the measurement of light in quantum theory has always preoccupied physicists, ever since Planck (1900), Einstein (1905) and the beginnings of modern quantum mechanics (thought experiments by Bohr, Einstein and Schrödinger). The duality of light (wave and set of photons at the same time) plays a fundamental role in this theory. Quantum optics, which came into being with the laser in 1960, put the questions posed in the 1920s into a modern context. The theory of light measurement, photon counting, established by R. Glauber (1963), forms the analytical framework for all optics experiments.
