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

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