In Chap. 15, we showed that a quantized LC tank could not be used as a qubit because the separation of adjacent eigenenergies is constant (e.g., \(E_{10}=E_{21}\) ). To have a working qubit that has its states confined in the two-dimensional Hilbert space, we need to have nonuniform separations. This can be achieved by using a nonlinear inductor to introduce anharmonicity. A Josephson junction is a very versatile non-linear inductor. A Josephson junction is just a metal/insulator/metal stack, and it only functions as intended when its metal regions become superconducting. Moreover, as demonstrated in Example 15.3, an ambient at milli-kelvin is required to distinguish the states in an LC tank. At this temperature, metals on a quantum chip usually become superconducting. Therefore, in this chapter, we will introduce the superconductor and Josephson junction.