We explore quantum correlations in polariton systems with simultaneous strong exciton-phonon and photon-phonon coupling in a single cavity. This enables effective optomechanical interactions between light-matter excitonic and vibrational states, leading to entanglement generation robust at room temperature. We showcase the potential for deterministic quantum state preparation, with applications in sensing and quantum molecular control.

Besides emerging as excellent single photon emitters, organic molecules trapped in solid-state matrices are also promising candidates to be a new fertile and versatile complete quantum optics platform. Unlike trapped ions or neutral atoms in optical tweezers, these quantum emitters face the complexity of existing in a structured solid-state environment, which strongly affects their behavior. Particularly striking is the effect of the matrix’s phonons and intrinsic molecular vibrations, which are known to produce strong decoherence and dephasing in the optical emission properties. I will show in this talk that these features are not necessarily detrimental but can actually be a resource that brings technical advantages or even the possibility to address new physical phenomena. In particular, I will show that a single fluorescent molecule can be seen as a hybrid quantum optical device, in which multiple external laser sources exert control of the vibronic states. In this way, the molecule can simulate several cavity QED models, whereby a specific vibrational mode plays the role of the cavity mode. Focusing on the specific example where the system is turned into an analogue simulator of the quantum Rabi model, the steady state exhibits vibrational bi-modality resulting in a statistical mixture of highly non-classical vibronic cat states. Applying our paradigm to molecules with prominent spatial asymmetry and combining an optical excitation with a THz(IR) driving, the system can be turned into a single photon transducer. Two possible implementations are discussed based on the coupling to a subwavelength THz patch antenna or a resonant metamaterial.

Polaritons, hybrid light-matter states, arise from strong coupling between confined electromagnetic modes and quantum emitters, enabling quantum-level control of optical and material properties [1]. While typically studied in single-mode cavities with Lorentzian spectral densities (Fig. 1(a)), advances in nanophotonics have introduced complex cavities (e.g., plasmonic or hybrid metallodielectric [2,3]) featuring spectral structures such as Fano resonances and sharp dips (Fig. 1(b)), which go beyond simple Lorentzian descriptions. We focus on spectral dips associated with antiresonance states, which couple to quantum emitters with complex-valued strengths and lead to nontrivial polaritonic behavior. We explore the formation of polaritons with imaginary Rabi splitting in such systems and propose a realistic setup where these effects can emerge from the interaction with multiple emitters. These results broaden the understanding of polariton formation in structured photonic environments and suggest new ways to control light-matter interactions for applications in quantum optics and polaritonic chemistry.

Figure: The coupling of a two-level emitter to (a) a single-mode cavity with a Lorentzian spectral density ($J(\omega)$) and (b) a hybrid cavity with an intricate $J(\omega)$. In (a), the emitter is on-resonance with the peak of the Lorentzian in $J(\omega)$, whereas in (b), it is on-resonance with the dip, corresponding to an antiresonance state.

Low-dimensional nanostructures such as graphene flakes, acetylene chains, and topological model systems like SSH chains can sustain tightly confined polaritons and act as tunable optical nanoantennas [1,2]. When brought into close proximity with quantum emitters such as adatoms, these systems enter a hybrid-coupling regime, where both optical enhancement and electron tunnelling effects emerge. This interplay enables not only strong light–matter interaction but also back-action of the emitter on the nanoantenna’s optical response via electronic channels. In this talk, I will present our recent results on such hybrid plasmon–emitter systems, with nanoantennas realized in various quasi-one- and two-dimensional platforms. We identify two distinct regimes of interaction depending on the emitter’s position and show how the coexistence of optical and electronic couplings modifies key phenomena such as coherent coupling and Purcell enhancement [3-5]. To better understand the nature of the excitations in these systems, we propose an energy-based plasmonicity index (EPI), which captures the energy–coherence character of the response and complements existing criteria for classifying plasmonic behavior at the nanoscale [6]. All simulations were performed using our open-source software GRANAD, a Python-based platform designed to model optoelectronic dynamics in low-dimensional materials using time-domain solutions of the reduced density matrix within the tight-binding formalism. GRANAD provides access to absorption spectra, charge distributions, induced-field dynamics, plasmonic features, and more [7,8].

Figure: Artistic impression of graphene nanoantennas.

Polaritonic chemistry in Fabric-Perot cavities is a many body phenomena that is driven by collective effects. In the last years, many types of effects have been identified, ranging from collective nuclear motion, to collective effects in the electronic structure. In this contribution, I will summarize our findings from the last few years and discuss our work on cavity Born-Oppenheimer Hartree-Fock and the higher excited states of the Tavis-Cummings model.

We have derived an analytical formulation of static polarizabilities [1] within linear-response theory for molecules under strong coupling using the cavity Born–Oppenheimer Hartree–Fock ansatz. This ab-initio method consistently describes vibrational strong coupling and electron–photon interactions even for ensembles of molecules. For different types of molecular ensembles, we observed local changes in the polarizabilities and dipole moments that are induced by collective strong coupling. Furthermore, we used the polarizabilities to calculate vibro-polaritonic Raman spectra in the harmonic approximation. This allows us to comprehensively compare the effect of vibrational strong coupling on IR and Raman spectra on an equal footing.

Polaritonic chemistry in the electronic strong coupling regime are typically interpreted in terms of the Jaynes-Cummings or Tavis-Cummings models under the assumption that the molecular ensemble is only excited by a single photon. In such a model, two polariton states compete with an overwhelming number of dark states, inhibiting polaritonic reactions entropically. We analyze the higher excitation manifolds of the Tavis-Cummings model along with a three-level system that resembles photochemical reactions [2]. We demonstrate that allowing for more than a single excitation makes the reaction of the involved polaritons entropically more favorable.

Experiments suggest that placing molecules inside optical cavities can alter their photochemistry. Although these changes have been attributed to strong coupling of the optical excitations of the molecules to the confined light modes of the cavity modes and their hybridization into exciton-polaritons, there is as yet no consensus on the underlying mechanisms. Here, we demonstrate how strong coupling can be exploited to increase the yield of a photochemical process. Using a photoacid that undergoes ultra-fast excited-state proton transfer (ESIPT) as an example, we show by means of atomistic computer simulations that these molecules can enhance their photo-reactivity by borrowing intensity from bright polaritonic states that are formed when a second bright molecular species strongly couples to the cavity. In spite of unavoidable cavity losses that limit polariton lifetimes, the resulting increase in absorption can significantly improve the external quantum efficiency of the ESIPT process as compared to the bare photoacids without cavity. We furthermore demonstrate that this enhancement is robust against the inverse scaling of polaritonic effects with the number of molecules that makes many such effects disappear in the thermodynamic limit. By demonstrating how strong coupling can be leveraged for enhancing photochemical reactions, our results provide a compelling case for polaritonic chemistry and may inspire the development of polariton-enhanced optoelectronics.

Molecular systems in nanophotonic environments exhibit rich optical behaviors, especially when one goes beyond idealized models of dipolar transitions and homogeneous broadening. In this talk I will present two recent studies that explore different frontiers of molecular light-matter coupling. In the first part of the talk I will discuss our recent work extending the Kubo-Anderson stochastic theory of molecular spectral line shapes to the case of polaritons formed in the collective strong coupling regime [1]. I will discuss both the fast and slow limits of the emitter’s frequency modulations, as well as the intermediate regime, and show how the interplay between the characteristic timescales of the cavity and the molecular disorder is expressed in the observed polaritons lineshapes. In the second part of the talk I will turn to plasmonic nanocavities and show how these can activate optically dark transitions in beta-carotene via strong field gradients [2]. I will discuss the interplay between the Franck-Condon quadrupole and the Herzberg-Teller dipole contributions in governing the absorption and photoluminescence characteristics of this dark transition. Together, these studies illustrate how both disorder and multipolar light-matter interactions can be leveraged to reveal and control spectral complexity in molecular systems.

Intramolecular energy transfer driven by near-field effects plays an important role in applications ranging from biophysics and chemistry to nano-optics and quantum communications. Advances in strong light–matter coupling in molecular systems have opened new possibilities to control energy transfer. In particular, long-distance energy transfer between molecules has been reported as the result of their mutual coupling to cavity photon modes, and the formation of hybrid polariton states. In addition to strong coupling to light, molecular systems also show strong interactions between electronic and vibrational modes. The latter can act as a reservoir for energy to facilitate off-resonant transitions, and thus energy relaxation between polaritonic states at different energies. However, the non-Markovian nature of those modes makes it challenging to accurately simulate these effects. Here we capture them via process tensor matrix product operator (PT-MPO) methods, to describe exactly the vibrational environment of the molecules combined with a mean-field treatment of the light–matter interaction. In particular, we study the emission dynamics of a system consisting of two spatially separated layers of different species of molecules coupled to a common photon mode, and show that the strength of coupling to the vibrational bath plays a crucial role in governing the dynamics of the energy of the emitted light; at strong vibrational coupling this dynamics shows strongly non-Markovian effects, eventually leading to polaron formation. Our results shed light on polaritonic long-range energy transfer, and provide further understanding of the role of vibrational modes of relevance to the growing field of molecular polaritonics.

Photons lie at the heart of both foundational quantum science and next-generation quantum technologies, owing to their resilience against environmental decoherence and their unique role as carriers of quantum information. While advances in classical photonics have empowered remarkable control over photons, the absence of strong photon-photon interactions remains a central obstacle to realizing scalable quantum systems. A promising frontier in addressing this challenge is many-body quantum optics—an emerging paradigm that enables the exploration of strongly correlated quantum states of light and paves the way for quantum simulation, nonlinear quantum photonics, and complex information processing. In this talk, first, I will present our recent efforts to establish solid-state platforms that enable and harness photon-photon interactions through excitonic intermediaries. We focus on cuprous oxide (Cu₂O), a material that uniquely hosts excitonic Rydberg states characterized by large principal quantum numbers and dramatically enhanced interaction strengths. I will share the first spectroscopic observations of Rydberg excitons in synthetic Cu₂O thin films and describe our bottom-up approach to assembling 2D arrays of these excitons, laying the groundwork for solid-state quantum simulators of lattice models. I will also highlight our progress in integrating these highly interacting excitonic states with silicon nitride photonic circuits, representing a critical step toward scalable Rydberg photonics. Building on this foundation, the second part of the talk explores a complementary solid-state platform based on a novel class of GaAs quantum dots (QDs). By engineering properties of individual emitters as well as coherent interactions between multiple QDs, we propose a novel approach to realize and study collective quantum effects, followed by the controlled generation of non-classical light, emergent many-body correlations, and access to synthetic quantum phases—further expanding the landscape of many-body quantum optics in semiconductor systems. Together, these advances reflect a unified vision: to develop practical, scalable, and highly interactive photonic systems that leverage solid-state quantum materials and atomic vapors to unlock the full potential of quantum technologies.

Placing a molecule in a cavity modifies its local photonic environment. It boosts light-matter interaction, which can be used to improve the performance of single-photon sources [1] or to perform nonlinear spectroscopy on individual molecules [2,3]. I will show that self-assembled plasmonic nanocavities allow ultra-broadband photonic engineering and illustrate this opportunity with two experiments. First, a single commercial fluorophore is precisely positioned inside a plasmonic dimer with the help of DNA origami and experiences a giant Purcell factor (~10^5) and Lamb shift (>10 meV). Together with the presence of a near-infrared plasmonic mode, this extreme photonic environment completely reshapes the emission spectrum into the near-infrared, where molecules are poor emitters. [4] Second, a nanocavity supporting colocalized mid-infrared and visible resonances is realized using the “nanoparticle-on-slit” concept [5,6] and is used for vibrational sum- and difference frequency spectroscopy on a small ensemble of molecules. Compared to previous results [5], I will show data covering an extended spectral range from 900 to 1650 cm-1, giving access to the full IR vibrational fingerprint of the molecules. I will conclude with some prospects of using this system in the single-molecule regime for physical chemistry and quantum optics. [7]

Figure: Fluorescence reshaping in a “nanodimer-on-mirror” cavity due to the combination of the Purcell effect and Lamb shift and the presence of a detuned near-infrared resonance. Adapted from Ref. [4]

The coupling of molecular transitions to plasmonic resonances is of broad interest as it can strongly affect the molecular electronic structure, thus resulting in modified energy landscape and electron-transfer pathways [1]. Here, we study the interaction of molecular photoswitches with plasmonic nanoantennas, which display two spectrally separated orthogonal localized surface plasmon resonances (LSPRs). The nanoantenna dimensions are designed such that the molecular transition is resonant with the long axis LSPR, whereas it is detuned from the short axis LSPR, resulting in either strong or weak coupling interactions, respectively. Ultrafast pump-probe spectroscopy measurements reveal a sub-ps collapse of polaritons to pure molecular transition triggered by femtosecond-pulse excitation. To interpret the results, we rely on a full quantum description based on the extension of the Tavis-Cummings Hamiltonian [2] and we show that the response of the system is governed by intramolecular dynamics, occurring one order of magnitude faster with respect to the uncoupled excited molecule relaxation to the ground state [3].

Experiments have demonstrated that collective strong coupling between molecular vibrations and a Fabry-Pérot cavity mode can significantly change molecular properties, such as thermal reaction rates and equilibrium constants. However, despite the plethora of theoretical studies, the mechanism behind this change is still not well-understood. In order to make progress, we take a step back and lift some of the approximations that nearly all reactivity studies are based on, namely the single mode approximation and the neglect of Coulombic intermolecular interactions. By restraining ourselves to statistical properties, such as the equilibrium constant, we can cast the problem into the framework of discretised path integrals. This allows us to circumvent the explicit simulation of the continuum of electromagnetic modes; instead, we capture their effect in an influence functional. With this method we can calculate how adding a pair of mirrors changes the equilibrium constant of a small ensemble of molecules represented by double-well potentials. Based on what experimental features this approach can and cannot capture, we hope to shed light on which approximations are valid, and spur a systematic analysis of which thus-far neglected effects could play an important role in changing chemical reactivity.

In this talk, I will present our recent advancements in the controlled coupling of single molecules to optical microcavities. First, I will discuss a tunable Fabry-Perot microcavity operating in the strong coupling regime of cavity quantum electrodynamics (CQED), where the Purcell factor is significantly enhanced, effectively transforming the molecule into a two-level quantum system. Our experiments show a remarkable 99% extinction of a laser beam, indicating that the molecule-cavity system behaves as an almost perfect photon scatterer. In the second part, I will present our work on integrated microcavities, where two molecules are coupled via the modes of a microdisk.

Weak light-matter coupling conditions allow to generate bright single photon emission from solid-state sources. In this talk we will present results on quantum dots in monolayers of WSe2, and defects in hexagonal Boron Nitride, that are coupled to a tunable Fabry-Pérot microcavity, rendering a bright single photon emission. The nanoscale control of light-matter interactions allows to create new optoelectronic devices for a wide span of nonlinear and quantum photonic applications. A key part of this endeavour is the exploration of novel materials, which reveal unique properties that can be tailored to specific applications. Regarding the implementation of solid-state single photon sources, relevant emergent materials for quantum light emission are quantum dots in transition metal dichalcogenides monolayers, perovskite quantum dots, or also defects in hexagonal boron nitride nanocrystals. [1] An optimal single-photon source should satisfy three essential criteria: (1) the emission of photons “on demand”, (2) emit one photon at a time, and (3) high degree of temporal coherence and uniformity in the properties of emitted photons (spectrum, polarization, spatial mode). Achieving this ideal quantum performance relies on using a resonant cavity to enhance the emitter’s spontaneous decay rate via the Purcell effect. In the first part of this talk, we will present recent advancements in generating single photons from atomically thin WSe2 monolayers. Local stress within these monolayers creates a confining potential that traps single excitons, resulting in single-photon emission. By embedding these quantum dots in a cryogenic Fabry-Pérot optical cavity, we achieve record emission-efficiency levels. [2] We also study the filtering cavity effects on the temporal coherence of the emitted single photons via Michelson interferometry. [3] Initial quantum communication tests with these sources further demonstrate their viability for single-photon applications. [4] In the latter part of the talk, I will discuss our experimental progress with an alternative material platform: single defects in hexagonal boron nitride. This emitter, operable at ambient conditions and integrated with a Fabry-Pérot cavity, offers a promising option for cryogenic-free quantum optical applications, including free-space quantum key distribution.

Strong light-matter interactions are at the heart of many electromagnetic phenomena and play a key role in the rapidly evolving field of nanophotonics. In this talk, I will present several systems that support polaritons—hybrid quasiparticles arising from the strong coupling between photons and excitations in matter—and discuss their promise for future applications. I will begin by highlighting recent advances in transition metal dichalcogenides (TMDs), focusing on their one-dimensional edges and the rich physics they host [1–2]. A central theme of the talk will be the concept of self-hybridization, wherein both the photonic and material components of a polaritonic system originate from the same nanostructured material [1–4]. We have recently realized such systems in TMD nanostructures and levitated water droplets, demonstrating both electronic and vibrational strong coupling [1–5]. Water droplets are particularly intriguing due to their abundance in natural environments such as mists, fog and clouds. I will also present our findings on the spontaneous formation of Fabry-Pérot resonators in aqueous solutions of gold nanoflakes [6–8]. This phenomenon arises from a delicate interplay between attractive Casimir-Lifshitz and repulsive electrostatic forces, leading to a self-assembled optical cavity. These results not only point to novel pathways in self-assembled polaritonic systems but also offer a new perspective on the unification of Casimir and strong-coupling physics.

Single organic molecules embedded in the solid state have emerged as powerful platforms for quantum optics, thanks to their coherent optical transitions mediated by isolated electronic levels [1]. However, these systems are not limited to electronic degrees of freedom but also include nuclear and spin levels, making them inherently hybrid quantum systems. This could open new possibilities for applications in molecular quantum technologies, including quantum memories, spin-photon interfaces, and optomechanics [2]. Additionally, leveraging the vast chemical space of molecules offers significant opportunities to tailor and optimize these systems.

In this talk, I present single organic molecules as hybrid quantum systems, with a particular focus on their vibrational dynamics and potential implications for quantum technologies. I discuss the challenges associated with modeling vibrational interactions arising from long-range molecular forces. To address the computational bottlenecks, I demonstrate how Machine Learning Interatomic Potentials (MLIPs) can be used to model these dynamics [3]. By evaluating the accuracy, reliability, and extrapolation capability of MLIPs for vibrational modeling, I show their potential to enable scalable simulations of host-guest systems.

A promising route toward building quantum light–matter interfaces with rare-earth ions (REIs) lies in the nano-fabrication of crystalline host materials that preserve the quantum properties of REIs while allowing integration into nanophotonic devices. In this context, molecular chemistry offers a highly attractive approach, providing unparalleled flexibility in material composition, precise structural control, and excellent integration potential. In this presentation, we review a series of europium-based molecular crystals that exhibit record-narrow optical linewidths and long spin coherence times [1]. We discuss their prospects as a novel material platform for photonic quantum technologies.

Figure: Molecular structure of europium (Eu3+) complexes showing narrow optical homogeneous line widths and long nuclear spin relaxation times. The narrow optical transition occurs between the 5D0 and 7F0 electronic levels. The nuclear spin structure of the two stable isotopes – 151Eu and 153Eu – composed of three doubly degenerated levels in the ground and excited states is also displayed.

In the context of the development of new molecular quantum emitters, nanographenes have great assets. Indeed, their synthesis by bottom-up chemistry allows a total control on their structure, which opens the way to wide customization of their optical properties [1–3]. In particular, we have shown that nanographenes are efficient single photon emitters at room temperature with tunable energy and transition dipole amplitude [4-7]. The next step would be to reach the emission of indistiguishable photons at low temperature. In this talk, I will in particular report on a study of the photophysics of nanographenes embedded in original molecular crystals. I will describe the synthesis and characterization of these original molecular crystals and the protocol to embed the GQDs. Finally, I will show photoluminescence studies of GQDs down to the single nanographene both at room and low temperature [8].

Exciton polaritons are hybrid quasi-particles resulting from the strong coupling between photons and excitons in semiconductors, offering exciting possibilities for advanced optoelectronic and quantum technologies that operate at room temperature. Recently, two-dimensional (2D) semiconductors, such as transition metal dichalcogenides (TMDs) and hybrid organic-inorganic perovskites, have attracted significant attention due to their unique optical and electronic properties, including high excitonic binding energies [1] and strong quantum confinement. These features make them ideal platforms for exploring strong light-matter interactions and exciton-polariton phenomena up to ambient conditions. In this work, we investigate methods to maximize the cooperativity and nonlinearities of exciton-polaritons in 2D materials at room temperature [2]. We present two innovative approaches using TMDs: first, coupling excitons to a topologically protected bound state in the continuum (BIC) within a Bloch surface wave structure to enhance light-matter interaction while minimizing photonic losses; second, integrating suspended TMD monolayers within resonant microcavities to further reduce losses and amplify nonlinear polaritonic effects, achieving record interaction constants. Additionally, we explore the potential of other 2D materials, including perovskite microwires, to facilitate long-range polariton propagation and develop proof-of-concept devices like polaritonic switches operating efficiently at room temperature. Our findings demonstrate significant progress in controlling and enhancing exciton-polariton interactions in 2D semiconductors, paving the way for cost-effective, high-performance polaritonic devices that leverage strong coupling and nonlinear phenomena at ambient conditions.