We have demonstrated how to build a quantum computer using spin qubits and superconducting qubits. Particularly, we showed the mechanisms to meet four of the five DiVincenzo’s criteria (Sect. 1.3), namely, well-characterized qubits, efficient initialization, efficient readout, and the availability of a “universal set” of quantum gates in both types of qubits. We also have made two assumptions. Firstly, they have long enough coherence times (the fifth DiVincenzo’s criterion). This is an active research topic. We will discuss their definitions, measurements, and equations in Chap. 25. The second is that we assume we are able to generate the required signals to initialize, manipulate, and read out qubits. These involve high-speed electronics and microwave circuits. In this chapter, we will study a typical superconducting qubit quantum computer and follow the propagation paths of the readout and manipulation microwave pulses to understand how microwave circuits play an important role in quantum computers. Although a superconducting quantum computer is used as an example, most of the theory and components are applicable directly to other types of quantum computers.