Quantum Circuits for General Multiqubit Gates

摘要

We consider a generic elementary gate sequence which is needed to implement a general quantum gate acting on n qubits — a unitary transformation with 4~n degrees of freedom. For synthesizing the gate sequence, a method based on the so-called cosine-sine matrix decomposition is presented. The result is optimal in the number of elementary one-qubit gates, 4~n, and scales more favorably than the previously reported decompositions requiring 4~n — 2~(n+1) controlled NOT gates.