**Input:** Two entangled qubits in Bell state $\ket{\Phi^{+}} = \frac{1}{\sqrt{2}} \big(\ket{00} + \ket{11}\big)$.

**Goal:**  Change the two-qubit state, without adding a global phase, to $\ket{\Psi^{-}} = \frac{1}{\sqrt{2}} \big(\ket{01} - \ket{10}\big)$.