npj Quantum Information (Jun 2023)

Orbital-optimized pair-correlated electron simulations on trapped-ion quantum computers

  • Luning Zhao,
  • Joshua Goings,
  • Kyujin Shin,
  • Woomin Kyoung,
  • Johanna I. Fuks,
  • June-Koo Kevin Rhee,
  • Young Min Rhee,
  • Kenneth Wright,
  • Jason Nguyen,
  • Jungsang Kim,
  • Sonika Johri

DOI
https://doi.org/10.1038/s41534-023-00730-8
Journal volume & issue
Vol. 9, no. 1
pp. 1 – 9

Abstract

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Abstract Variational quantum eigensolvers (VQE) are among the most promising approaches for solving electronic structure problems on near-term quantum computers. A critical challenge for VQE in practice is that one needs to strike a balance between the expressivity of the VQE ansatz versus the number of quantum gates required to implement the ansatz, given the reality of noisy quantum operations on near-term quantum computers. In this work, we consider an orbital-optimized pair-correlated approximation to the unitary coupled cluster with singles and doubles (uCCSD) ansatz and report a highly efficient quantum circuit implementation for trapped-ion architectures. We show that orbital optimization can recover significant additional electron correlation energy without sacrificing efficiency through measurements of low-order reduced density matrices (RDMs). In the dissociation of small molecules, the method gives qualitatively accurate predictions in the strongly-correlated regime when running on noise-free quantum simulators. On IonQ’s Harmony and Aria trapped-ion quantum computers, we run end-to-end VQE algorithms with up to 12 qubits and 72 variational parameters—the largest full VQE simulation with a correlated wave function on quantum hardware. We find that even without error mitigation techniques, the predicted relative energies across different molecular geometries are in excellent agreement with noise-free simulators.