Hi. Welcome to a new lesson of our quantum optics course. In the last lesson, you have learned many properties of quasi-classical states of radiation which play an important role in quantum optics. But we restricted that first approach to the case of a single mode. When it comes to describing real situations, the only example of a single mode quasi-classical state is the case of the beam emitted by a quasi-ideal single mode laser. All other types of light emitted by classical sources involve many different frequencies, directions of propagation, phases, polarizations, and describing such a radiation demands the use of the formalism of multimode quasi-classical states. This is what you are going to learn today. But you may ask the question: why is it important to describe classical light in the quantum optics formalism, although we know that all its properties are perfectly well-described with classical electromagnetic waves? Is it a purely academic reason? Well, firstly, the academic reason is not insignificant. Knowing how the various levels of description of a system, here light, are linked together is an important task of physics. It is a means to improve our basic understanding of the world. But there is another more practical reason to describe classical light in the context of quantum optics. It will allow you to clearly understand in the quantum context what are the ultimate limits of classical optics measurements and it will be the pathway towards understanding how to surpass these limits in the quantum optics framework. So whatever your motivations, it is a good idea to learn the content of this lesson. To start with, I will first introduce the quantum optics formalism for multimode states of light. In order to assimilate it without problem, it may be a good idea to have a look at the lesson four of quantum optics one. In section two, you will learn how to apply the general formalism to the case of light described by quasi-classical states in different modes. An important result is that the total number of photons is not perfectly determined and is described as a random Poisson distribution. An interesting case of multimode quasi-classical states is a quasi-classical wave packet. As in the case of one-photon wave packets introduced in lesson five of quantum optics one, you will find that for a wave packet, you do not need to introduce an arbitrary volume of quantization, a notion that makes many of you uncomfortable. Do not be embarrassed, it took me a long time to feel somewhat comfortable with it. One should not make the common mistake that quasi-classical states have classical properties only when they are made of many photons. A good way not to fall into that pitfall is to study a wave packet with an average number of photons much less than one. Believe it or not, you will find that it has a very classical behavior and this has been shown in an experiment involving a beam-splitter. Historically, an important step in the understanding of quantum optics was the quantum description of the beat note signal that can be observed when two laser beams are mixed and sent to a photo-detector. This is an opportunity to introduce the heterodyne detection method, a fundamental tool in modern quantum optics as you will discover in future lessons. You will also learn how shot noise affects the ultimate sensitivity and you will become able to answer the important question: does the heterodyne method provides a better sensitivity than direct detection? In this section, I will introduce a case of multimode radiation with various components mutually incoherent. This is a very important concept in classical optics since it allows one to describe light produced by classical sources such as incandescent lamps, stars, discharge lamps, light-emitting diodes, you name them. Here, statistics play an important role and it will be an opportunity to better understand the similarities and differences between classical and quantum statistical average. It will be enough for today. This lesson is a necessary warm-up to enter again the domain of fully quantum light with properties impossible to describe with the formalism of quasi-classical states of light. This is what we will do in the next lessons, and if you want to fully appreciate the breakthrough associated with quantum light, you must assimilate the lesson of today.