Instantaneous Detection of Quantum Coherences in 2D Electronic Spectroscopy with Bright Entangled Beams.

Quantum coherences are the primary signatures of nonadiabatic dynamics in biological complexes, yet their unambiguous detection remains a challenge. Conventional time-resolved spectroscopies rely on temporal beating patterns that are inherently convoluted with population-relaxation dynamics and instrumental artifacts, often hiding the genuine coherences. We introduce a technique that exploits macroscopic spectro-temporal entanglement for the snapshot detection of electronic, vibrational, and vibronic coherences in molecules, avoiding temporal scanning. We show how to use entangled beams containing tens of thousands of photons in 2D electronic spectroscopy to detect quantum coherences at a given population time, thus enabling an instantaneous detection of quantum coherences in the molecule during its interaction with the probe pulse. Each coherence appears at a distinct point in a 2D frequency-dispersed signal due to energy anticorrelation, and the coherence period can be read directly off the spectrum. This enables an unambiguous discrimination of population and coherence dynamics, facilitating a robust detection of quantum coherences in complex molecules.