Efficient step-merged quantum imaginary time evolution algorithm for quantum chemistry

TL;DR

The paper introduces a resource-efficient quantum imaginary time evolution algorithm (smQITE) that achieves high accuracy in molecular ground state calculations with shallow circuits, suitable for current NISQ devices.

Contribution

It presents a novel step-merged quantum imaginary time evolution method that maintains fixed shallow circuit depth and matches VQE accuracy without complex optimization.

Findings

smQITE achieves high-accuracy molecular ground states.

The method reduces quantum resource requirements.

It is implementable on current quantum hardware.

Abstract

We develop a resource efficient step-merged quantum imaginary time evolution approach (smQITE) to solve for the ground state of a Hamiltonian on quantum computers. This heuristic method features a fixed shallow quantum circuit depth along the state evolution path. We use this algorithm to determine binding energy curves of a set of molecules, including H$_2$, H$_4$, H$_6$, LiH, HF, H$_2$O and BeH$_2$, and find highly accurate results. The required quantum resources of smQITE calculations can be further reduced by adopting the circuit form of the variational quantum eigensolver (VQE) technique, such as the unitary coupled cluster ansatz. We demonstrate that smQITE achieves a similar computational accuracy as VQE at the same fixed-circuit ansatz, without requiring a generally complicated high-dimensional non-convex optimization. Finally, smQITE calculations are carried out on Rigetti quantum processing units (QPUs), demonstrating that the approach is readily applicable on current noisy intermediate-scale quantum (NISQ) devices.