7.6 Professor Hadamard, meet Professor Pauli

Other than mapping the state of a qubit to a Bloch sphere and so looking at it in a different way, what can you do with a qubit? In this section we look at the operations, also called gates, you can apply to a single qubit. Later we expand our exploration to gates that have multiple qubits as inputs and outputs. In chapter 9

Here, for example, is a circuit with one qubit initialized to |0⟩ that performs one operation, X, and then measures the qubit.

Gates are always reversible, but some other operations are not. This comes from quantum mechanics and gates corresponding to unitary transformations. Measurement is irreversible, and so is the |0⟩ reset operation described in subsection 7.6.13

When I include the measurement operation in a circuit in the next chapter and beyond, it looks like the graphic below.

Measurement returns a |0⟩ or...