Extension of the Trotterized Unitary Coupled Cluster to Triple Excitations

TL;DR

This paper extends the Trotterized Unitary Coupled Cluster method by adding triple excitations, significantly improving accuracy for quantum molecular simulations and enabling larger molecule analysis.

Contribution

Introduction of UCCSDT with spin and orbital symmetries, enhancing accuracy and efficiency over UCCSD in quantum chemistry simulations.

Findings

UCCSDT improves accuracy by at least two orders of magnitude over UCCSD.

UCCSDT reaches chemical accuracy in numerical tests.

UCCSDT is competitive with classical CCSD(T) method.

Abstract

The Trotterized Unitary Coupled Cluster Single and Double (UCCSD) ansatz has recently attracted interest due to its use in Variation Quantum Eigensolver (VQE) molecular simulations on quantum computers. However, when the size of molecules increases, UCCSD becomes less interesting as it cannot achieve sufficient accuracy. Including higher-order excitations is therefore mandatory to recover the UCC's missing correlation effects. In this Letter, we extend the Trotterized UCC approach via the addition of (true) Triple T excitations introducing UCCSDT. We also include both spin and orbital symmetries. Indeed, in practice, these later help to reduce unnecessarily circuit excitations and thus accelerate the optimization process enabling to tackle larger molecules. Our initial numerical tests (12-14 qubits) show that UCCSDT improves the overall accuracy by at least two-orders of magnitudes with respect to standard UCCSD. Overall, the UCCSDT ansatz is shown to reach chemical accuracy and to be competitive with the CCSD(T) gold-standard classical method of quantum chemistry.