We live an exciting time, when the concepts at the heart of the second quantum revolution, entanglement and single quantum object control, are put at work in quantum technologies. There are dozens of startup companies involved in America, Europe, Asia, and large companies also invest significantly in quantum technologies. Basic concepts on entanglement and on single quantum objects have been clarified by quantum optics experiments and discussions about these experiments. They have prompted the emergence of proposals to use these concepts for quantum technologies. Now, these proposals are implemented in many different platforms using ions, superconducting devices, atoms, as material supports of qubits. Compared to photons which are flying qubits impossible to keep at rest, these are static qubits which can store the information. Moreover, these static qubits have a variety of mutual interactions which can be controlled in order to implement quantum gates. Quantum optics, however, remains a crucial tool. Firstly, because photons are remarkable qubits that are the privilege vector to transfer quantum information between static qubits. Moreover, many operations with ions and atoms are based upon quantum optics techniques. So it is not exaggerated to claim that quantum optics is at the heart of quantum technologies. Most of the current developments of quantum technologies aim at using entanglement and individual quantum objects control to develop groundbreaking applications. None of these technologies has yet reached a broad market, but some of them are already available on a commercial basis. I can cite, for instance, quantum cryptography, which is commercially available and can be used in domains where security is a major concern as defense, diplomacy, finance. Quantum cryptography is only one element of quantum communication which would allow quantum nodes to directly exchange quantum information using quantum teleportation. As you have learned today, in order to be deployed at large distance, quantum teleportation and quantum cryptography need reliable quantum memories with long enough storage times and good fidelity. One can reasonably expect that such memories will be developed in the near future. For the time being, there are enough applications at distances of a few kilometers to allow quantum cryptography and quantum teleportation to be developed to the point where they are user-friendly. Quantum metrology has already lead to commercial applications with atomic clocks and cold atoms matter-wave gravimeters. In their present stage, these technologies use a large number of independent quantum objects, and from this point of view, they belong to the first quantum revolution. But when an atomic wave packet or a single photon is split into components that are separated by macroscopic distances, we are beyond the usual situation of the first quantum revolution and in a sense quantum non-locality is at work. Moreover, it is possible to go beyond the standard quantum limit using squeezed states in order to increase sensitivity. It is thus fair to claim that sensors based on matter-wave interferometry definitely belong to the second quantum revolution. The same is true for nano-sensors based on single quantum objects such as NV centers, which yield extraordinary sensitivity in magnetic field measurements at the nano-scale. A big question mark must be attached to quantum computing. Universal quantum computing is a wonderful theoretical idea which has led to remarkable research efforts, still going on, to develop elementary quantum gates and associate them to produce proof of principles demonstrations. Various systems have been evaluated as quantum bits, photons, atoms, ions, molecules, superconducting devices. Remarkable success have been obtained with a few tens of entangled qubits that one can manipulate to realize elementary quantum algorithms. But because of the need for quantum error correction codes, one would need of the order of one million or more entangled qubits with an excellent fidelity to have a useful universal quantum computer. I think that nobody can predict if that goal will be reached. When I think of it, I can make the same comment than 30 years ago when I was a member of a committee that evaluated the project of a large interferometer to detect gravitational waves. No fundamental physical law forbids such interferometers to work, but the technological gap sounded amazing. Because we believed in fundamental physics, we decided anyway to approve the project, and 30 years later such interferometers do work and detect several gravitational waves per year. Based on that comparison, I would not say that building a universal quantum computer is impossible since no physical law seems to forbid it, but I would be surprised that we have such a quantum computer in the coming years. Today, quantum simulators exist, and they are close to the stage where they will provide results impossible to obtain with classical computers. This will be a true revolution, not only conceptual but also in the domain of applications, for instance, the design of new materials. Do you know, for instance, that we still do not understand superconductors with high critical temperature? Many groups try to use quantum simulators to understand what is the mechanism at work in these superconductors. This could lead to the development of better kinds of such materials, hopefully working at room temperature. The impact on society would be immense, reducing dramatically the losses in electric equipment. You've also learned today, that quantum simulators might be used as programmable quantum computers to solve difficult problems of optimization. We are only at the beginning of that new development, but there is a good hope that useful applications will exist. To describe the possibilities opened by real imperfect quantum simulators, there is a word: NISQ. Coming years will tell us whether noisy intermediate scale quantum technologies are reasonably useful. To conclude, I have no doubt that some quantum technologies will provide useful applications important when they allow us to obtain results impossible to get with standard methods. The possibility of a universal quantum computer remains an open question. But a last and not least comment can be made about the active research bearing on it by the best groups in the world. These groups make great efforts to obtain larger and larger ensembles of entangled qubits. Beyond the prospect for fantastic applications, that quest may allow us to get an answer to a fundamental conceptual question: Is the difficulty to entangle many quantum objects a purely technological problem? Or is there a fundamental limit, a maximum number of microscopic objects that can be entangled? If the answer was yes, if there was a fundamental limit, we would have found the frontier between the quantum and the classical world and the answer to the Schrodinger cat conundrum. It would allow us to understand the connection between the two worlds a problem still in front of us after a century of quantum physics. For the time being, the question is still open and working on quantum technologies might be a way to address it. This is a good point to make a break in that second course on quantum optics. In the last five lessons, you have encountered quantum states involving several photons. You have learned that entangled photons have extraordinary properties as confirmed by many experiments. Einstein discovered these properties and found them hard to swallow, and it took a long time to sophisticated theorists such as Feynman to realize their importance. Nowadays, entanglement is at the root of many of the quantum technologies developed in the framework of the second quantum revolution. Surprisingly, it was also necessary to consider quantum states with many photons to understand in the quantum optics formalism the most emblematic classical property of light, its coherence. Quasi-classical states of light provide a crystal clear answer to the question asked by many physicists trained in classical optics and discovering quantum mechanics: How to reconcile the notion of photon and the notion of coherence of a classical light beam? I was one of these physicists, and I am sure that many of you asked the same question. Quasi-classical states are the answer. I hope that you are satisfied with it. Moreover, considering quasi-classical states has allowed physicists to understand what is the standard quantum limit and how to pass that limit with other kinds of multi-photon quantum states, squeezed states of light. This is another emblematic example of modern quantum technology. You are now equipped with many important tools of quantum optics, but you still miss one, the fully quantum description of the interaction of light with matter. This is obviously a very important topics, which allows one to understand a phenomenon impossible to describe correctly in the framework of the semi-classical model. That phenomenon is spontaneous emission, the fact that an isolated atom in an excited state eventually decays to a lower state while emitting a photon. If we consider only the atom as a quantized system, theory tells us that it remains forever in an eigenstate of its Hamiltonian. In fact, spontaneous emission stems from the interaction between the quantized atom and the quantized radiation, more precisely, the vacuum of quantized radiation. That quantized vacuum has properties that depend on boundary conditions, and spontaneous emission is different if the atom is in an unlimited space or in a resonant cavity. Surprising, isn't it? These quantum wonders will be presented in future lessons of our course which will be published as soon as possible. I hope to meet you again then.