Efficient, Oracle-free Quantum Simulation of Nonadiabatic Quantum Molecular Dynamics

Nonadiabatic dynamics at conical intersections and avoided crossings govern photochemical processes in numerous interstellar and terrestrial contexts. Applications ranging from the ionization of polycyclic aromatic hydrocarbons in the interstellar medium to singlet fission in organic photovoltaics depend critically on understanding these mechanisms. Accurate simulation for these systems becomes classically intractable for systems exhibiting strong, unstructured correlations with only a few dozen vibrational modes. We present a digital quantum algorithm for the simulation of dense Hamiltonians common in vibronic and scattering dynamics. Real-Time Evolution of Quantum Wavepackets in Explicit Modularity (REQWIEM) is a product-formula based algorithm for Hamiltonian simulation, exploiting the algebraic structure of matrix elements to apply nonadiabatic couplings through uniformly controlled rotations. Classical emulation directly validates these quantum circuits against tensor network and subspace diagonalization methods. Recovery of accurate observables including spectra and ultrafast state transitions support REQWIEM as a numerically validated and nonhierarchical quantum algorithm that does not experience the curse of dimensionality for strongly correlated signals. As a linear algebra solver, REQWIEM is a promising front end for large scale quantum Krylov subspace diagonalization, with order-of-magnitude depth reduction relative to oracle-based approaches.