In Chap. 13, we introduced classical Lagrangian and Hamiltonian mechanics. Lagrangian is used to construct Hamiltonian, which connects classical mechanics to quantum mechanics through the promotion of conjugate variables to operators. We demonstrated this through the quantization of a simple harmonic oscillator (SHO) in Chap. 14. In Chap. 15, we applied the same methodology to quantize an LC tank but showed that it could not be used as a qubit due to the lack of anharmonicity. We, therefore, introduced the Josephson junction, which can serve as a nonlinear inductor to introduce anharmonicity, and its equations in Chap. 16. In this chapter, we will construct a circuit using a Josephson junction and a capacitor, known as a Cooper pair box, which can be used as a superconducting qubit. We will first study its property when it is fully isolated. Then, we will study a Cooper pair box capacitively connected to an external source. In this process, we will review the systematic approach to quantizing a circuit. We will go through the derivation step-by-step and understand its physics and implications.