Francesca Fassioli

Excitons in monolayer transition metal dichalcogenides (1L-TMDs) exhibit a Rydberg-like series of bound states, including optically dark levels. Using a three-pulse scheme, we directly track the dynamics of the 2p dark exciton in 1L-WS_{2} through the Autler-Townes splitting of the bright 1s state. The splitting, induced by a midinfrared field resonant with the 1s-2p transition, reveals distinct 1s and 2p responses to photoexcitation. This approach provides new insight into many-body effects and offers a versatile experimental framework to probe driven excitonic dynamics in quantum materials.

Producing and maintaining molecular entanglement at room temperature and detecting multipartite entanglement features of macroscopic molecular systems remain key challenges for understanding intermolecular quantum effects in chemistry. Here, we study the quantum Fisher information, as a multipartite entanglement witness. We generalize the entanglement witness functional related to quantum Fisher information regarding nonidentical local response operators. We demonstrate its effectiveness for intermolecular entanglement in ultrastrong light-matter coupling in cavity quantum electrodynamics, including near the superradiant phase transition. Additionally, we establish a connection between quantum Fisher information and the dipole correlator, indicating that entanglement could be detected through absorption spectroscopy. Our work proposes a general protocol to detect intermolecular entanglement in chemical systems at room temperature.

The intraband exciton dynamics of molecular aggregates is a crucial initial step to determine the possibly coherent nature of energy transfer and its implications for the ensuing interband relaxation pathways in strongly coupled excitonic systems. In this work, we fully characterize the intraband dynamics in linear J-aggregates of porphyrins, good model systems for multichromophoric assemblies in biological antenna complexes. Using different 2D electronic spectroscopy schemes together with Raman spectroscopy and theoretical modeling, we provide a full characterization of the inner structure of the main one-exciton band of the porphyrin aggregates. We find that the redistribution of population within the band occurs with a characteristic time of 280 fs and dominates the modulation of an electronic coherence. While we do not find that the coupling to vibrations significantly affects the dynamics of excitonic coherence, our results suggest that exciton fluctuations are nevertheless highly correlated.

Photosynthesis begins with light harvesting, where specialized pigment–protein complexes transform sunlight into electronic excitations delivered to reaction centres to initiate charge separation. There is evidence that quantum coherence between electronic excited states plays a role in energy transfer. In this review, we discuss how quantum coherence manifests in photosynthetic light harvesting and its implications. We begin by examining the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of quantum coherence in light harvesting. We then discuss recent results concerning the possibility that quantum coherence between electronically excited states of donors and acceptors may give rise to a quantum coherent evolution of excitations, modifying the traditional incoherent picture of energy transfer. Key to this (partially) coherent energy transfer appears to be the structure of the environment, in particular the participation of non-equilibrium vibrational modes. We discuss the open questions and controversies regarding quantum coherent energy transfer and how these can be addressed using new experimental techniques.

The mounting evidence of recent years regarding long-lived coherent dynamics of electronic excitations in several light-harvesting antenna proteins suggests the possibility of realizing and exploiting light-initiated quantum dynamics in synthetic molecular devices based on electronic energy transfer. Inspired by the field of molecular logic, we focus this discussion on the prospect of using quantum coherence to control the direction of energy flow in a molecular circuit. As a prototype system we consider a circuit consisting of three chromophores that deliver energy to two trap chromophores. Our aim is to control to which trap the energy is more likely to be delivered. This is achieved by switching one of the circuit chromophores ON and OFF from the system, such that the direction of energy flow substantially changes from the ON and OFF states of the circuit. We find that quantum coherence can allow a significant ability to direct energy transfer in the circuit. However, when realistic levels of noise are added, quantum coherence only slightly improves the ability to direct electronic energy in comparison to a classical hopping mechanism.

Recent two-dimensional electronic spectroscopy (2DES) experiments have reported evidence of coherent dynamics of electronic excitations in several light-harvesting antennae. However, 2DES uses ultrafast coherent laser pulses as an excitation source; therefore, there is a current debate on whether coherent excitation dynamics is present under natural sunlight – incoherent – illumination conditions. In this letter, we show that even if incoherent light excites an electronic state with no initial quantum superpositions among excitonic states, energy transfer can proceed quantum coherently if nonequilibrium dynamics of the phonon environment takes place. Such nonequilibrium behavior manifests itself in non-Markovian evolution of electronic excitations and is typical of many photosynthetic systems. We therefore argue that light-harvesting antennae have mechanisms that could support coherent evolution under incoherent illumination.

Over billions of years, evolutionary pressure has facilitated the development of sophisticated and diverse light-harvesting systems in photosynthetic organisms, enabling them to adapt to a variety of habitats and light conditions. Today, driven by the need for cheap and efficient solar power, we turn to photosynthetic organisms and their light-active supramolecular assemblies for bio-inspiration. By studying natural systems, we can learn more about the basic principles behind the fine-tuned functionalities of light capture and energy transfer on the molecular level. In this review, we use ideas from evolutionary ecology and quantum mechanics to elucidate the parameters that underpin the efficient and robust light-harvesting machinery of natural light-harvesting systems. The family of antenna proteins of cryptophyte algae serves as an example to illustrate the evolutionary diversification process on a structural and consequently a photophysical level. Two-dimensional electronic spectroscopy experiments reveal the existence of coherence among vibronic levels in the initial response of light-harvesting proteins to femtosecond optical excitation. We discuss what implications quantum transport processes might have for light harvesting.

We review various methods for measuring delocalization in light-harvesting complexes. Direct relations between inverse participation ratios (IPRs) and entanglement measures are derived. The B850 ring from the LH2 complex in Rhodopseudomonas acidophila is studied. By analysing electronic energy transfer dynamics in the B850 ring using different metrics for quantifying excitonic delocalization, we conclude that measures of entanglement are far more robust (in terms of time scale, temperature and level of decoherence) than IPRs, and are therefore more appropriate for the purpose of studying the time evolution of coherence in a system.