Experimental quantum computational chemistry with optimised unitary coupled cluster ansatz

TL;DR

This paper demonstrates scalable quantum computational chemistry using an optimized unitary coupled-cluster ansatz on noisy quantum processors, achieving high-precision results and chemical accuracy for small molecules with reduced circuit depth and error mitigation.

Contribution

It introduces an improved protocol for VQE with an optimized ansatz, enabling scalable and high-precision electronic structure calculations on 12 qubits.

Findings

Achieved chemical accuracy for H2 and LiH molecules.

Scaled up VQE implementation to 12 qubits with error suppression.

Reduced circuit depth and running time significantly.

Abstract

Quantum computational chemistry has emerged as an important application of quantum computing. Hybrid quantum-classical computing methods, such as variational quantum eigensolvers (VQE), have been designed as promising solutions to quantum chemistry problems, yet challenges due to theoretical complexity and experimental imperfections hinder progress in achieving reliable and accurate results. Experimental works for solving electronic structures are consequently still restricted to nonscalable (hardware efficient) or classically simulable (Hartree-Fock) ansatz, or limited to a few qubits with large errors. The experimental realisation of scalable and high-precision quantum chemistry simulation remains elusive. Here, we address the critical challenges {associated with} solving molecular electronic structures using noisy quantum processors. Our protocol presents significant improvements in the circuit depth and running time, key metrics for chemistry simulation. Through systematic hardware enhancements and the integration of error mitigation techniques, we push forward the limit of experimental quantum computational chemistry and successfully scale up the implementation of VQE with an optimised unitary coupled-cluster ansatz to 12 qubits. We produce high-precision results of the ground-state energy for molecules with error suppression by around two orders of magnitude. We achieve chemical accuracy for H$_2$ at all bond distances and LiH at small bond distances in the experiment, even beyond the two recent concurrent works. Our work demonstrates a feasible path towards a scalable solution to electronic structure calculation, validating the key technological features and identifying future challenges for this goal.