A new class of ultrafast laser experiments will track how pairs of electrons remain quantum-mechanically linked inside molecules during photochemical reactions. The project addresses a fundamental gap: current techniques cannot directly observe how quantum correlations—specifically, entanglement and spin coherence—control the outcome of light-driven reactions in solution. This matters because reactions such as photosynthesis and photocatalysis produce products in non-random ratios that classical theory cannot explain, suggesting quantum information phenomena are at work but have never been seen directly. The team will build two-dimensional femtosecond spectrometers operating at deep-ultraviolet wavelengths, a technical feat that has so far proved exceptionally challenging. If successful, the experiments will reveal how specific quantum spin states bias a reaction toward one product over another, and whether certain entangled spin states act as more potent redox agents in photocatalysis. This is fundamental science with no immediate practical application. However, understanding how quantum coherence directs chemical reactions could eventually inspire new photochemical syntheses, more efficient photocatalysts, or even the use of photoexcited molecules as quantum sensors—outcomes that would build on the same kind of foundational insight that previously turned quantum mechanics into lasers and MRI.
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The applications of Quantum Information Science (QIS) are rapidly expanding beyond quantum computing, with their phenomena being proposed as ripe for exploration within the field of Chemistry, and how they may revolutionize our understanding of existing chemical reactions and lay the foundations for new syntheses. Important photochemical reactions, key to all life on Earth, have spin-selective photoproduct quantum yields that cannot be explained using a classical theoretical framework. QIS phenomena are implicated, but direct experimental evidence remains elusive. New ultrafast experimental techniques will be innovated to overcome this significant obstacle and unravel: how the quantum mechanical nature of photoproducts remains correlated with the initial photoprepared species; electronic coherences and correlations between photoproducts; how the potency of different entangled photoproduct spin states controls the outcome of chemical reactions in solution. Photoinduced ligand-ligand charge-transfer and photoionization reactions– critical processes in photocatalysis and protein damage, respectively– both initially form charge-transfer products comprised of a spin-correlated ion-radical pair (SC-IRP). Each ion has an unpaired electron that can take two different configurations, e.g. two different spin states. As these are binary states, and spin-coherence is long-lived, each radical ion can be considered as a qubit. Spatial confinement of the two ions will inevitably lead to spin-exchange and generate entanglement within the ion-radical pair. This key quantum property will dictate subsequent reverse reactions to regenerate the parent species, biasing either as singlet or triplet pathways. In systems we have studied, we have shown that this occurs with non-statistical yields, necessitating quantum correlations between precursor and products. Uncovering these key details requires an optical readout of the dynamics to go beyond the current state-of-the-art magnetic resonance methods, requiring the innovation of new two-dimensional femtosecond experimental techniques. Observation of these QIS in Chemistry phenomena will both utilize existing capabilities and require new technical innovations to develop novel ultrafast 2D experiments. These will extend capabilities to deep-ultraviolet (DUV) wavelengths with ultrabroad bandwidth and ultrashort time resolution, which has hitherto proven exceptionally challenging. This will yield world-class 2D electronic and 2D electronic-vibrational spectroscopy (2DES and 2DEV, respectively) experiments with DUV capabilities necessary for studying one set of exemplar SC-IRP system. These advances will only be possible by utilizing the expertise of both Bristol and USC ultrafast spectroscopy groups and their long-established collaboration. 2DEV studies will identify unique infrared spectroscopic signatures of triplet and singlet SC-IRP species in key chromophores of aromatic amino acids. It will reveal their formation timescales, and correlations with the initial electronic excitation in species where unique optical transient signatures are often hard to discern. Further, such experiments will determine the spin-dependent spatial delocalization of excited state wavefunctions. 2DES spectroscopy will investigate electronic coherences in aromatic amino acids and photosensitizers, and how spin correlations are transferred into, and persist, between photoionized products. Such data will reveal the degree of singlet-triplet mixing and the energy gaps between the two states. Critically, these experiments will also reveal whether specific quantum spin-states are more potent redox agents in a novel photocatalytic system. This project will unravel the frontier QIS phenomena underpinning photoinduced directionality over product branching. It will enhance international collaboration by exchanging ideas, advanced technical expertise and personnel between the US and UK. These studies will inspire discovery of novel photochemical syntheses and potentially enable the use of photoexcited states as quantum sensors.
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