Calculating the ground-state energy of benzene under spatial deformations with noisy quantum computing

Abstract

In this paper we calculate the ground-state energy of benzene under spatial deformations by using the variational quantum eigensolver. The primary goal of the study is to estimate the feasibility of using quantum computing Ans\"atze on near-term devices to solve problems with a large number of orbitals in regions where classical methods are known to fail. Furthermore, by combining our advanced simulation platform with real quantum computers, we provide an analysis of how the noise, inherent to quantum computers, affects the results. At the center of our study are the hardware efficient and quantum unitary coupled-cluster (QUCC) Ans\"atze. First, we find that the hardware efficient Ansatz has the potential to outperform mean-field methods for extreme deformations of benzene. However, key problems remain at equilibrium, preventing real chemical application. Moreover, the hardware efficient Ansatz yields results that strongly depend on the initial guess of parameters (in both noisy and noiseless cases) and optimization issues have a higher impact on their convergence than noise. This is confirmed by comparison with real quantum computing demonstrations. On the other hand, the QUCC Ansatz alternative exhibits deeper circuits. Therefore, noise effects increase and are so extreme that the method never outperform mean-field theories. Our dual simulator, (8--16)-qubit QPU computations of QUCC Ansatz appears to be much more sensitive to hardware noise than shot noise, which further indicates where the noise-reduction efforts should be directed. Finally, the study shows that the QUCC method better captures the physics of the system as the QUCC method can be utilized together with the H\"uckel approximation. We discussed how going beyond this approximation sharply increases the optimization complexity of such a difficult problem.