Determining the ensemble N-representability of Reduced Density Matrices

TL;DR

This paper introduces a quantum algorithm framework to determine ensemble N-representability of reduced density matrices, enabling error correction and quantum-state reconstruction in electronic structure calculations.

Contribution

It presents a practical variational algorithm using purification and Hilbert-Schmidt distance minimization for ensemble N-representability determination.

Findings

Successfully validated on small electron systems and molecular models.

Demonstrated robustness in error correction and density-matrix refinement.

Applicable to finite-temperature quantum systems.

Abstract

The N-representability problem for reduced density matrices remains a fundamental challenge in electronic structure theory. Following our previous work that employs a unitary-evolution algorithm based on an adaptive derivative-assembled pseudo-Trotter variational quantum algorithm to probe pure-state N-representability of reduced density matrices [J. Chem. Theory Comput. 2024, 20, 9968], in this work we propose a practical framework for determining the ensemble N-representability of a p-body matrix. This is accomplished using a purification strategy consisting of embedding an ensemble state into a pure state defined on an extended Hilbert space, such that the reduced density matrices of the purified state reproduce those of the original ensemble. By iteratively applying variational unitaries to an initial purified state, the proposed algorithm minimizes the Hilbert-Schmidt distance between its p-body reduced density matrix and a specified target p-body matrix, which serves as a measure of the N-representability of the target. This methodology facilitates both error correction of defective ensemble reduced density matrices, and quantum-state reconstruction on a quantum computer, offering a route for density-matrix refinement. We validate the algorithm with numerical simulations on systems of two, three, and four electrons in both, simple models as well as molecular systems at finite temperature, demonstrating its robustness.