QPE: estimate eigenphase φ of unitary U|ψ⟩ = e^{2πiφ}|ψ⟩
QPE uses n ancilla qubits to estimate the eigenphase φ of a unitary U to n-bit precision.
Step 1: Hadamard on all ancillas → superposition. Step 2: Controlled-U^(2^k) operations.
Step 3: Inverse QFT → readout. Probability peaks sharply at the closest n-bit approximation.