Theory of analytical energy derivatives for the variational quantum eigensolver

TL;DR

This paper develops methods to compute analytical energy derivatives within the VQE framework, enabling detailed analysis of quantum systems and chemical reactions using near-term quantum computers.

Contribution

It introduces explicit low-depth quantum circuits for measuring energy derivatives, extending VQE's capabilities to analyze physical properties and reactions.

Findings

Derived formulas for energy derivatives in VQE

Designed low-depth quantum circuits for derivatives

Validated methods with numerical simulations

Abstract

The variational quantum eigensolver (VQE) and its variants, which is a method for finding eigenstates and eigenenergies of a given Hamiltonian, are appealing applications of near-term quantum computers. Although the eigenenergies are certainly important quantities which determines properties of a given system, their derivatives with respect to parameters of the system, such as positions of nuclei if we target a quantum chemistry problem, are also crucial to analyze the system. Here, we describe methods to evaluate analytical derivatives of the eigenenergy of a given Hamiltonian, including the excited state energy as well as the ground state energy, with respect to the system parameters in the framework of the VQE. We give explicit, low-depth quantum circuits which can measure essential quantities to evaluate energy derivatives, incorporating with proof-of-principle numerical simulations. This work extends the theory of the variational quantum eigensolver, by enabling it to measure more physical properties of a quantum system than before and to explore chemical reactions.