Coherent state sampling for molecular vibronic spectra

Electronic spectroscopy is today among the most powerful methods for studying chemical systems, and state-resolved (line-assigned) simulations are essential for attributing spectral features to specific vibrational excitations. The standard time-independent sum-over-states (SOS) route provides line assignments; while effective with truncation and prescreening, it can become computationally demanding as the number of modes increases. Vibronic boson sampling (VBS) offers an event-based alternative, but scaling photonic implementations remains challenging and the required vibronic inputs can be demanding for larger systems. Here we introduce coherent-state sampling (CSS), a sampling scheme for the Linear Coupling Model (LCM), where vibrational modes are independent. This enables a photonic architecture based on a single reconfigurable optical channel with essentially fixed hardware complexity; increasing system size is handled by additional measurement iterations rather than additional optical modes. Using pentacene as a test case, we demonstrate an end-to-end workflow—electronic-structure input, sampling strategy, hardware validation, and comparison to experiment—showing that both a laptop implementation and a superconducting-detector photonic implementation reproduce reference spectra with high fidelity and yield band shapes in good agreement with experiment. While not targeting quantum advantage, CSS provides a practically scalable, state-resolved tool for LCM vibronic spectroscopy and a controlled displacement-dominated complement to more general VBS schemes.