Unitary Selective Coupled-Cluster Method

TL;DR

This paper introduces a selective iterative approach to constructing a more efficient unitary coupled-cluster ansatz for quantum molecular simulations, reducing energy errors without extra quantum measurements.

Contribution

It presents a novel method for building a tailored UCC ansatz iteratively using a selection process based on Hamiltonian elements and excitation amplitudes.

Findings

Systematically reduces energy error with larger ansatz.

Does not require additional quantum measurements to improve the ansatz.

Applicable to test molecules showing improved efficiency.

Abstract

Simulating molecules using the Variational Quantum Eigensolver method is one of the promising applications for NISQ-era quantum computers. Designing an efficient ansatz to represent the electronic wave function is crucial in such simulations. Standard unitary coupled-cluster with singles and doubles (UCCSD) ansatz tends to have a large number of insignificant terms that do not lower the energy of the system. In this work, we present a unitary selective coupled-cluster method, a way to construct a unitary coupled-cluster ansatz iteratively using a selection procedure with excitations up to fourth order. This approach uses the electronic Hamiltonian matrix elements and the amplitudes for excitations already present in the ansatz to find the important excitations of higher order and to add them to the ansatz. The important feature of the method is that it systematically reduces the energy error with increasing ansatz size for a set of test molecules. The main advantage of the proposed method is that the effort to increase the ansatz does not require any additional measurements on a quantum computer.