Rovibrational energy levels of H$_2$O by quantum computing

TL;DR

This paper demonstrates the calculation of water's rovibrational energy levels using a trapped-ion quantum computer, integrating quantum algorithms with classical processing to achieve accurate results.

Contribution

It introduces a novel quantum computing approach for rovibrational spectroscopy, including deriving a qubit Hamiltonian and employing a quantum-selected configuration-interaction method.

Findings

Achieved a few cm$^{-1}$ accuracy for low-lying energy levels.

Successfully integrated quantum and classical computations for molecular spectroscopy.

Demonstrated feasibility of quantum computing for complex molecular energy calculations.

Abstract

We calculate rovibrational energy levels of H$_2$O using a trapped-ion quantum computer. We first derive the qubit form of Watson's Hamiltonian, including the rovibrational coupling terms. In a second step, we employ a variant of the quantum-selected configuration-interaction method to calculate rovibrational energy levels. A truncated form of the qubit Hamiltonian is used to generate correlated rovibrational wave functions on the quantum computer by time evolution, and a basis set is selected by sampling from the measured probability distribution. The rovibrational energy levels are obtained by constructing a Hamiltonian matrix using the selected basis set, and diagonalizing the matrix using a classical computer. We show that an accuracy of a few cm$^{-1}$ can be achieved for low-lying rovibrational energy levels.