Which options exist for NISQ-friendly linear response formulations?

TL;DR

This paper develops a resource-efficient quantum linear response theory tailored for near-term quantum computers, introducing new formalisms and identifying promising methods for simulating excited states in complex chemical systems.

Contribution

It introduces a truncated active-space quantum LR theory with novel operator transformations suitable for near-term hardware, and evaluates multiple formalisms to identify the most promising approaches.

Findings

Two promising qLR formalisms: 'proj LRSD' and 'all-proj LRSD'

Successful simulation of excited state potential energy curves

Identification of resource-efficient methods for NISQ devices

Abstract

Linear response (LR) theory is a powerful tool in classic quantum chemistry crucial to understanding photo-induced processes in chemistry and biology. However, performing simulations for large systems and in the case of strong electron correlation remains challenging. Quantum computers are poised to facilitate the simulation of such systems, and recently, a quantum linear response formulation (qLR) was introduced. To apply qLR to near-term quantum computers beyond a minimal basis set, we here introduce a resource-efficient qLR theory using a truncated active-space version of the multi-configurational self-consistent field LR ansatz. Therein, we investigate eight different near-term qLR formalisms that utilize novel operator transformations that allow the qLR equations to be performed on near-term hardware. Simulating excited state potential energy curves and absorption spectra for various test cases, we identify two promising candidates dubbed ``proj LRSD'' and ``all-proj LRSD''.