Modeling Singlet Fission on a Quantum Computer

Wedemonstrate a practical application of quantum computingbyusing it to investigate the linear H-4 molecule as a simplemodel for singlet fission. We use the Peeters-Devreese-Soldatov energyfunctional to calculate the necessary energetics based on the momentsof the Hamiltonian estimated on the quantum computer. To reduce thenumber of required measurements, we use several independent strategies:1) reduction of the size of the relevant Hilbert space by taperingoff qubits; 2) measurement optimization via rotations to eigenbasesshared by groups of qubit-wise commuting Pauli strings; and 3) parallelexecution of multiple state preparation and measurement operationsusing all 20 qubits available on the Quantinuum H1-1 quantum hardware.Our results meet the energetic requirements for singlet fission, arein excellent agreement with exact transition energies (for the chosenone-particle basis), and outperform classical methods considered computationallyfeasible for singlet fission candidates.