Circuit-Depth Reduction of Unitary-Coupled-Cluster Ansatz by Energy Sorting

TL;DR

This paper introduces an energy-sorting strategy to significantly reduce the circuit depth of the UCC ansatz in variational quantum eigensolvers, enabling more efficient quantum chemistry simulations on NISQ devices.

Contribution

The authors propose a novel energy-sorting method that pre-screens excitation operators, reducing circuit depth by up to 98% while maintaining accuracy in quantum chemistry calculations.

Findings

Achieved 50-98% reduction in operator count

Maintained accuracy of original UCCSD operator pools

Successfully applied to molecular and periodic systems

Abstract

Quantum computation represents a revolutionary approach for solving problems in quantum chemistry. However, due to the limited quantum resources in the current noisy intermediate-scale quantum (NISQ) devices, quantum algorithms for large chemical systems remains a major task. In this work, we demonstrate that the circuit depth of the unitary coupled cluster (UCC) and UCC-based ansatzes in the algorithm of variational quantum eigensolver can be significantly reduced by an energy-sorting strategy. Specifically, subsets of excitation operators are first pre-screened from the operator pool according to its contribution to the total energy. The quantum circuit ansatz is then iteratively constructed until the convergence of the final energy to a typical accuracy. For demonstration, this method has been successfully applied to molecular and periodic systems. Particularly, a reduction of 50\%$\sim$98\% in the number of operators is observed while retaining the accuracy of the origin UCCSD operator pools. This method can be straightforwardly extended to general parametric variational ansatzes.