Resource Estimates for Excited-State Calculations of Diarylethenes on Fault-Tolerant Photonic Quantum Computers

We estimate computational resources for computing excited-state energies of benchmark photochromic molecules of diarylethenes (DAEs) by using quantum phase estimation on photonic devices. The number of T gates, which determines the calculation time, and logical qubits for a simulation are estimated, considering the overhead of fault-tolerant computers. Three DAE molecules of increasing size are examined with active space sizes specified. Quantum resource estimation is conducted via Hamiltonian truncation within an active space of all valence π electrons in all valence bonding π and their antibonding π* orbitals. For small and medium molecules, complete active space configuration interaction generates reference energies and trial initial states, while for the large molecule, a trial state with perfect overlap with the exact excited state is assumed due to computational constraints. Notably, with 1.02 × 10⁶ resource state generators, computation for the largest molecule with an active space of 22 electrons and 22 orbitals takes 7 h and 54 min. These results provide insights into the computational resources necessary for computing excited states on quantum hardware.