Optimal quantum circuit synthesis from controlled-unitary gates

Using a geometric approach, we derive the minimum number of applications needed for an arbitrary controlled-unitary gate to construct a universal quantum circuit. An analytic construction procedure is presented and shown to be either optimal or close to optimal. This result can be extended to improve the efficiency of universal quantum circuit construction from any entangling gate. In addition, for both the controlled-NOT (CNOT) and double-CNOT gates, we develop simple analytic ways to construct universal quantum circuits with three applications, which is the least possible for these gates.