# Why does a quantum circuit consist of simple quantum gates which act on at most a fixed number of qubits?

While reading the Quantum algorithm Wikipedia entry, I noticed that the sentence "A quantum circuit consists of simple quantum gates which act on at most a fixed number of qubits", was annotated with a "why?".

• That doesn't look like a good definition anyway. I don't see any particular reason to use the terms "simple" (without defining it) and "at most". – Sanchayan Dutta May 21 at 17:34

• @Chris I'd argue that quantum circuits aren't supposed to encapsulate classical logic, by definition. A quantum circuit is literally defined as a quadruplet $C=(V, I, O, G)$ where $V$ is the set of all qubits (including ancilla, input, and output), $I$ stands for the set of input qubits and $O$ stands for the set of output qubits. $G$ is a sequence of unitary transformations acting on a "bounded" number of qubits. This answer is kinda like saying "quantum circuits aren't useful for what they're not meant for in the first place"; isn't that obvious? I don't quite see the point here. – Sanchayan Dutta May 21 at 18:28