**Input:** Two unentangled qubits (stored in an array of length 2) in state $\ket{+} \otimes \ket{+} = \frac{1}{2} \big( \ket{00} + \ket{01} + \ket{10} {\color{blue}+} \ket{11} \big)$.

**Goal:** Change the two-qubit state to $\frac{1}{2} \big( \ket{00} + \ket{01} + \ket{10} {\color{red}-} \ket{11} \big)$.
