All You Need is pi: Quantum Computing with Hermitian Gates

Abstract

Universal gate sets for quantum computation, when single and two qubit operations are accessible, include both Hermitian and non-Hermitian gates. Here we utilize the fact that any single-qubit operator may be implemented as two Hermitian gates, and thus a purely Hermitian universal set is possible. This implementation can be used to prepare high fidelity single-qubit states in the presence of amplitude errors, and helps to achieve a high fidelity single-qubit gate decomposition using four Hermitian gates. An implementational convenience can be that non-identity single-qubit Hermitian gates are equivalent to $\pi$ rotations up to a global phase. We show that a gate set comprised of $\pi$ rotations about two fixed axes, along with the CNOT gate, is universal for quantum computation. Moreover, we show that two $\pi$ rotations can transform the axis of any multi-controlled unitary, a special case being a single CNOT sufficing for any controlled $\pi$ rotation. These gates simplify the process of circuit compilation in view of their Hermitian nature. We exemplify by designing efficient circuits for a variety of controlled gates, and achieving a CNOT count reduction for the four-controlled Toffoli gate in LNN-restricted qubit connectivity.