Quantum Ghost Spectroscopy Reveals Hidden Coherence in Molecular Aggregates

Abstract Ultrafast spectroscopy of complex molecules has been achieved under light–matter coupling by the use of laser pulses and single photons. Yet, the excited-state dynamics essential to photochemistry is suffering from the convolution of multiple relaxation pathways and limited resolution in nonlinear signal, which still remain challenging. To address these, we develop time-resolved quantum ghost spectroscopy (tr-QGS) with entangled light, which enables a background-free detection of excited-state emission while offering precise control over temporal resolution. We apply this approach to perylene bisimide (PBI-1) trimers by combining a quantum description of light-molecule interaction with time-dependent density matrix renormalization group (TD-DMRG) simulations. This explicitly includes five vibrational modes and nonadiabatic coupling between electronic states. Our simulations reveal that tr-QGS captures electronic coherence oscillating at 0.6 eV for ∼50 fs, a signature of nonadiabatic coupling that was obscured in time-resolved measurements. Moreover, we observe a direct transfer from electronic to vibrational coherence at 200 fs, providing real-time visualization of vibronic relaxation pathways. Photon entanglement eliminates background noise and ground-state bleaching artifacts, which plague time-resolved measurements. These results establish tr-QGS as a transformative tool for interrogating nonadiabatic dynamics in molecular aggregates, light-harvesting complexes, and photocatalysts, offering a route to reveal quantum coherence effects in chemistry with unprecedented precision.