Flexibility of the factorized form of the unitary coupled cluster ansatz

TL;DR

This paper explores the flexibility of the factorized unitary coupled cluster ansatz in quantum chemistry, showing it can interpolate between different methods and still achieve high accuracy through variational optimization.

Contribution

It demonstrates that the factorized form is highly adaptable, bridging conventional configuration interaction and unitary coupled cluster, with effective approximations via variational energy minimization.

Findings

Factorized form can interpolate between CI and UCC methods.

High accuracy achieved with simpler approximations through variational minimization.

Flexibility allows for efficient quantum state preparation in electronic structure calculations.

Abstract

The factorized form of the unitary coupled cluster ansatz is a popular state preparation ansatz for electronic structure calculations of molecules on quantum computers. It often is viewed as an approximation (based on the Trotter product formula) for the conventional unitary coupled cluster operator. In this work, we show that the factorized form is quite flexible, allowing one to range from conventional configuration interaction, to conventional unitary coupled cluster, to efficient approximations that lie in between these two. The variational minimization of the energy often allows simpler factorized unitary coupled cluster approximations to achieve high accuracy, even if they do not accurately approximate the Trotter product formula. This is similar to how quantum approximate optimization algorithms can achieve high accuracy with a small number of levels.