Berlin 2024 – scientific programme

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QI: Fachverband Quanteninformation

QI 12: Poster I

QI 12.22: Poster

Tuesday, March 19, 2024, 11:00–14:30, Poster B

Exact circuit implementation of S^2-conserving fermionic UCCSD-singlet excitations — •Felix Rupprecht and Sabine Wölk — Institute for Quantum Technologies, German Aerospace Center, Ulm, Germany

Finding groundstates of chemical systems is considered to be one of the most promising tasks to be solved on quantum computers. Most of the quantum algorithms proposed for solving this problem either try to prepare the groundstate directly, e.g. via variational methods like (Adapt-)VQE, or at least require a good initial guess of a groundstate candidate (QPE).

In the context of (Adapt-)VQE it was observed [Bertels et. al, J. Chem. Theory Comput. 18, 11 (2022)] that the use of non-S^2-conserving excitations and low order trotterization leads to spin contamination, i.e. the state leaving the spin sector in which the algorithm started in, resulting in slower convergence.

We investigate S^2-conserving fermionic UCCSD-singlet excitations and observe that the space on which the excitations act may be decomposed into a direct sum of invariant subspaces. Within those subspaces we then find exact quantum circuits implementing the excitation.

We compare the S^2-conserving excitations to other excitations in terms of convergence rate and resources required when used as the excitation pool for the Adapt-VQE algorithm.

This work is part of the QuEST+ project funded by the Baden-Württemberg Ministry of Economic Affairs, Labour and Housing.