Hi, welcome to the second lecture of this quantum optics course. In this lesson you will discover how to use the formalism of quantum optics and apply it to the case of a number state with a number of photons equal to one, that is a one-photon state. This situation is one of the simplest examples of a state of radiation whose properties cannot, absolutely not be understood without the formalism of quantum optics. In order not to delay the study of such an emblematic quantum behavior, I will use a simplified model of a one photon state involving a single mode of radiation. We leave for a later lesson, a more general description based on the multimode formalism of quantum optics. Today, you will not only discover an emblematic phenomenon of quantum optics, you will also learn the formalism to calculate signals fundamental in quantum optics. These are the photodetection signals. Let us then have an overview of this lesson. I will first recall what is the semi-classical model of optics. It will allow you to better appreciate by contrast, what is a fully quantum situation. I will then focus on single mode, one-photon state of radiation, at the heart of this lesson, and of the next one. A realistic description of a one-photon state should use a formalism of multi-mode quantized radiation which you will learn later. But it is interesting to encounter an emblematic example of a fully quantum phenomenon, even at the price of an approximation. You have to trust me. Although one could object to this over simplification. What will you learn in this lesson and the next one, conveys correctly, many fundamental features of quantum optics. To understand quantum optics you need to know an experimental method called photon counting and photon correlation. Based on the photoelectric effect introduced by Einstein in 1905 from a theoretical point of view, but technically developed only after World War II. You will learn how to express the probabilities of single and double photo detections with the quantum formalism that you have learned in lesson 1. You will first calculate the single photo detection signal for one photon state. This will allow us to define the important notion of quantum efficiency of a photo detector. You will then apply the double photo-detection signal formalism to a one-photon state to discover an emblematic quantum optics result, the absence of double detection for a single photon. In order that you fully appreciate that result, I will contrast it with what a semi-classical model would predict. At this point it will be time to conclude. You should be convinced then that quantum optics is necessary to describe the fact that the double detection signal is null for one-photon wave packet and to understand the meaning of that property, which plays a role in quantum technologies. A little bit of thinking, however, will show us that it is not the end of the story and that there should be more in the quantum formalism.