We studied one-qubit and two-qubit gates (last chapter) before the readout mechanism (this chapter) because they are mathematically easier. In that process, we only need to describe them as the interactions between the quantized artificial atom and classical voltage signal (although variables such as the charge are promoted to operators). One example of an artificial atom is the transmon qubit which has nonuniformly separated energy levels (anharmonicity). The readout scheme in a superconductor qubit is to interrogate the qubit through an electromagnetic wave. It thus requires the study of the interaction between the electromagnetic wave and the artificial atom. In this process, even the electromagnetic wave needs to be quantized which leads to second quantization. We can no longer treat the system semi-classically (classical electromagnetic wave and quantum mechanical artificial atoms). This system resembles cavity quantum electrodynamics (c-QED). Thus it is called circuit quantum electrodynamics (circuit-QED). Since the relevant mathematics are profound, we will take electromagnetic wave quantization for granted.