**Input:** A qubit in state $\ket{\psi} = \alpha \ket{0} + \beta \ket{1}$.

**Goal:** Change the state of the qubit to $\alpha \ket{1} + \beta \ket{0}$.

**Example:**

If the qubit is in state $\ket{0}$, change its state to $\ket{1}$.

If the qubit is in state $\ket{1}$, change its state to $\ket{0}$.

> Note that this operation is self-adjoint: applying it for a second time
> returns the qubit to the original state.
