What Is a Quantum Gate?

To do quantum computing, we need devices that will operate on and change the quantum state associated with a qubit (the physical object). In quantum mechanics, those devices are called operators. However, in analogy to the language used to describe the manipulation of bits in classical computing, we will call the corresponding quantum devices quantum gates. For example, we might think about a system with two qubits as input to some device, which then produces some output. A generic quantum gate is shown in the following figure: