lice and Bob, who just manipulate classical bits at a bottom network-layer level. The main argumentation in favor of PKC is the impossibility of Eve's capturing the secret key without Alice's and Bob's awareness, and this is true as long as Eve's attack only concerns the quantum channel over which the key is exchanged. It is assumed that Alice's and Bob's computers and local networks cannot be approached by Eve and be physically tapped, including by wireless means. This is the main assumption to keep in mind while discovering the extraordinary elegance of QKD and accepting the "absoluteness" of its security. In the forthcoming sections, we shall consider several QKD approaches, referred to as QKD protocols. Since QKD is about key exchange, the quantum channel must be a communication channel involving physical distance. Thus, its implementation requires one to use light and its quantum constituents, photons. It is important, therefore, to describe beforehand the properties of electromagnetic waves and the associated quantum measurements, which are both at the root of QKD. In the description, I shall also develop the necessary parallels with QIT and qubit and operator formalism. Electromagnetic waves, polarization states, photons, and quantum measurements In this section, we review the basic physical properties of light, and how these properties can be conceptually, and quite nicely, related to that of quantum states. The elementary particles of light, or equivalently, light energy quanta, are called photons. The particle-like characteristics of light only appear at relatively low light powers, where the number of quanta approaches the order of unity. 15 At macroscopic scales, with large numbers of quanta, such photon granularity vanishes, and light appears to the physical observer in the form of an electromagnetic (EM) wave. The EM wave is 15 More specifically, the average number of photons (n) emitted per second (units of photon/s) by a light source of power P (units of W) at electromagnetic frequency v (units of