Efficient quantum circuits for quantum computational chemistry

TL;DR

This paper introduces more efficient quantum circuits for simulating molecular systems using the variational quantum eigensolver, significantly reducing the number of CNOT gates needed for fermionic excitations, which enhances near-term quantum chemistry simulations.

Contribution

It presents new quantum circuits for fermionic excitations that cut CNOT gate counts by factors of 2 and 8, improving efficiency for quantum molecular simulations.

Findings

Linear reduction in CNOT gates for single excitations

Significant decrease in CNOT gates for double excitations

Enhanced feasibility of near-term quantum chemistry experiments

Abstract

Molecular quantum simulations with the variational quantum eigensolver (VQE) rely on ansatz states that approximate the molecular ground states. These ansatz states are generally defined by parametrized fermionic excitation operators and an initial reference state. Efficient ways to perform fermionic excitations are vital for the realization of the VQE on noisy intermediate-scale quantum computers. Here, we address this issue by first demonstrating circuits that perform qubit excitations, excitations that do not account for fermionic anticommutation relations. We then extend the functionality of these circuits to perform fermionic excitations. Compared to circuits constructed with the standard use of "$CNOT$ staircases", our circuits offer a linear reduction in the number of $CNOT$ gates, by a factor of $2$ and $8$ per single and double excitation, respectively. Our results reduce the requirements for near-term realizations of quantum molecular simulations.