We have seen some interesting effects with single qubits and single qubit gates, but the true power of quantum computing is realised through the interactions between qubits. In this section we will introduce multiple qubit gates and explore the interesting behaviours of multi-qubit systems.
Typically, the gates that can be directly implemented in hardware will act only on one or two qubits. In our circuits, we may want to use complex gates that act on a great number of qubits. Fortunately, this will not be a problem. With the one and two qubit gates given to us by the hardware, it is possible to build any other gate.
In this chapter we will first introduce the most basic multi-qubit gates, as well as the mathematics used to describe and analyse them. Then we'll show how to prove that these gates can be used to create any possible quantum algorithm. The chapter then concludes by looking at small-scale uses of quantum gates. For example, we see how to build three-qubit gates like the Toffoli from single- and two-qubit operations.