Using ion trap quantum computers to design composite quantum systems: Case study in vibronic energy transfer.

Computational simulation of molecular systems often require accurate modeling of vibronic interactions (coupled vibrational and electronic degrees of freedom) to be relevant towards artificial light harvesting or microscopic energy transfer. The inclusion of these interactions can exponentially increase complexity of systems of more than a few molecules. Fault-tolerant quantum computers (FTQC) could naturally represent these types of strongly correlated quantum states, and our current noisy intermediate-scale quantum devices (NISQ) can be used for proof-of-principle demonstrations. We have demonstrated multiple quantum subroutines to aid in design of molecular clusters on a digital trapped ion quantum computer. Using isolated laboratory molecular spectroscopy data, energetic and vibrational excitations are stored onto 2 qubits using variational quantum eigensolvers (VQEs). These quantum-classical algorithms yield optimized circuit representations for individual molecules, which are used in a 3-molecule dynamic quantum simulation tracking the propagation of a single electronic excitation. The inter-molecular orientation and coupling are tuned to predict the optimal configuration for fast exciton energy transfer.