Single organic molecules are a well-established platform for high-quality single-photon generation: they can emit lifetime-limited photons with high purity and indistinguishability. However, their emission is accompanied by a pronounced red-shifted phonon sideband and higher-order vibrational peaks, which reduce the fraction of photons emitted into the desired narrowband zero-phonon line. The standard approach to suppressing this unwanted emission is to integrate the emitter into a monolithic photonic nanostructure that provides Purcell enhancement. However, incorporating organic materials using clean-room techniques has proven challenging, and has in fact so far prevented their integration into monolithic microcavities altogether. As a result, efficient, narrowband organic single-photon sources for applications in quantum information processing have remained elusive despite their considerable promise. Here, we propose three monolithic microcavity designs tailored to provide sufficient Purcell enhancement for organic quantum emitters. The Purcell effect induced by these cavities preferentially enhances emission into the 0-0 zero-phonon line, increasing spectral purity, photon extraction, and shortening the excited-state lifetime, which in turn relaxes the requirements for generating indistinguishable photons. The cavities have been optimized using Bayesian optimization and the adaptive Antoulas-Anderson (AAA) algorithm for rational approximation, which offer global optimization and the efficient reconstruction of spectra from scattering simulations, respectively. These structures are compatible with both standard clean-room processing and single-molecule preparation techniques. Therefore, they offer a clear route to realize high-quality, practically monochromatic organic single-photon light sources.

Engineering the photonic environment using lattices of coupled resonators, which we refer to as lattice quantum electrodynamics (QED), provides a route to control both the spontaneous emission of individual quantum emitters and the photon-mediated interactions between them. Here we introduce an optical lattice QED platform based on individual dibenzoterrylene (DBT) molecules embedded in anthracene crystals and coupled to lattices of open optical microcavities. This hybrid architecture benefits from narrow-linewidth molecular emitters, site-resolved optical access, engineered coupled-resonator bands, and compatibility with established molecular frequency-tuning techniques. As a proof-of-principle demonstration, we observe emitter-photon bound states formed when the optical transition of a single molecule is tuned to the band gap of a Su-Schrieffer-Heeger (SSH) cavity lattice. These in-gap states display directional localization and photon emission on a single sublattice, inherited from the vacancy-induced topological edge modes of the underlying SSH lattice. Our results establish open-cavity lattices coupled to DBT molecules as a versatile architecture for engineering many-emitter quantum optical systems with controllable photon-mediated interactions.

A physics-constrained neural network is presented for predicting the optical response of metasurfaces. Our approach incorporates physical laws directly into the neural network architecture and loss function, addressing critical challenges in the modeling of metasurfaces. Unlike methods that require specialized weighting strategies or separate architectural branches to handle different data regimes and phase wrapping discontinuities, this unified approach effectively addresses phase discontinuities, energy conservation constraints, and complex gap-dependent behavior. We implement sine-cosine phase representation with Euclidean normalization as a non-trainable layer within the network, enabling the model to account for the periodic nature of phase while enforcing the mathematical constraint sin^2 ϕ+ cos^2 ϕ= 1. A Euclidean distance-based loss function in the sine-cosine space ensures a physically meaningful error metric while preventing discontinuity issues. The model achieves good, consistent performance (e.g., coefficient of determinations above 0.9) with small, imbalanced datasets of 580 and 1075 data points, compared to several thousand typically required by alternative approaches. This physics-informed approach preserves physical interpretability while reducing reliance on large datasets and could be extended to systems involving periodic or wrapped quantities.

We theoretically investigate a single fluorescent molecule as a hybrid quantum optical device, in which multiple external laser sources exert control of the vibronic states. In the high-saturation regime, a coherent interaction is established between the vibrational and electronic degrees of freedom, and molecules 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. In a nutshell, this work assesses the role of molecules as an optomechanical quantum toolbox for creating hybrid entangled states of electrons, photons, and vibrations, hence enabling frequency conversion over very different energy scales.

In the realm of fundamental quantum science and technologies, non-classical states of light, such as single-photon Fock states, are widely studied. However, current standards and metrological procedures are not optimized for low light levels. Progress in this crucial scientific domain depends on innovative metrology approaches, utilizing reliable devices based on quantum effects. A new generation of molecule-based single-photon sources is presented, combining their integration in a polymeric micro-lens with pulsed excitation schemes, thereby realizing suitable resources in quantum radiometry. The strategy enhances the efficiency of generated single photon pulses and improves stability, providing a portable source at 784.7 nm that maintains consistent performance even through a cooling and heating cycle. The calibration of a single-photon avalanche detector is demonstrated using light sources with different photon statistics, and the advantages of the single-molecule device are discussed. A relative uncertainty on the intrinsic detection efficiency well below 1% is attained, representing a new benchmark in the field. Exploiting the integration in polymeric micro-lenses, a new generation of molecule-based single-photon sources for quantum radiometry is presented. The emitters exhibit enhanced efficiency and improved stability, as their optical properties result largely unaffected by a successive cooling-heating cycle. The calibration of a single-photon avalanche detector is demonstrated and the advantages of using the single-molecule device are discussed. image

The Stark effect provides a powerful method to shift the spectra of molecules, atoms, and electronic transitions in general, becoming one of the simplest and most straightforward ways to tune the frequency of quantum emitters by means of a static electric field. At the same time, in order to reduce the emitter sensitivity to charge noise, inversion symmetric systems are typically designed, providing a stable emission frequency with a quadratic-only dependence on the applied field. However, such nonlinear behavior might be reflected in correlations between the tuning ability and unwanted spectral fluctuations. Here, we provide experimental evidence of this trend using molecular quantum emitters in the solid state cooled down to liquid helium temperatures. We finally combine the electric field generated by electrodes, which is parallel to the molecule's induced dipole, with optically excite long-lived charge states acting in the perpendicular direction. Based on the anisotropy of the molecule's polarizability, our two-dimensional control of the local electric field allows us not only to tune the emitter's frequency but also to sensibly suppress the spectral instabilities associated with field fluctuations.

Quantum key distribution (QKD) allows the distribution of cryptographic keys between multiple users in an information-theoretic secure way, exploiting quantum physics. While current QKD systems are mainly based on attenuated laser pulses, deterministic single-photon sources could give concrete advantages in terms of secret key rate (SKR) and security owing to the negligible probability of multi-photon events. Here, we introduce and demonstrate a proof-of-concept QKD system exploiting a molecule-based single-photon source operating at room temperature and emitting at 785 nm. With an estimated maximum SKR of 0.5 Mbps, our solution paves the way for room-temperature single-photon sources for quantum communication protocols.

A lens-less optical fiber designed for enhanced-fluorescence biosensor applications is presented. In order to obtain the enhanced sensor performances, two elements are essential: a planar antenna that redirects fluorescence emission into a narrow cone and an automated fiber-based optical system for multi-spot analysis. In particular, the potential early diagnosis of sepsis via C-reactive protein (CRP) detection is here demonstrated, reaching a limit of detection of 1.5 ng/mL), which is in the clinical range of interest for such biomarker. Upon the combination with other sepsis biomarkers, the presented sensor can become relevant for the early diagnosis of sepsis. These results validate the developed prototype as a simple, affordable, easy-to-operate, plug&play device with fast turnaround times, compatible with standardized micro-well arrays, and potentially suitable for POC applications with respect to the diagnosis of sepsis. It is also suitable for implementation with other biomarkers and liquid environments.

In today’s medicine, the celerity of the bio-assays analysis is crucial for the timely selection of the appropriate therapy and hence its effectiveness, especially in case of diseases characterized by the late onset of symptoms. In this paper, a lens-less fiber optics-based fluorescence sensor designed for the measurement of labeled bio-assays is presented and its potential for the early diagnosis of sepsis via C-reactive protein (CRP) detection is demonstrated. The sensor performance results from the combination of two key elements: a planar antenna that redirects fluorescence the marker emission and an automated fiber-based optical system for multi-spot analysis. First, the working principle of the device is demonstrated with a well-established antibody–antigen format (immunoglobulin IgG/anti-IgG assay), reporting more than one order of magnitude enhanced limit of detection (LOD) and limit of quantification (LOQ) for the planar antenna with respect to a standard glass substrate. The prototype is then tested against a sample mimicking a realistic case, prepared with commercially available human serum, showing a LOD and LOQ in the clinical range of interest (0.0015 μg/mL and 0.005 μg/mL, respectively) for the investigation of the sepsis biomarker CRP. These results validate the developed prototype as a simple and easy-to-operate device, compatible with standardized micro-well arrays, and potentially suitable for POC applications.

A major challenge in photonic quantum technologies is two-photon interference from distinct quantum emitters on the same chip. Here, we show and discuss recent results on Hong–Ou–Mandel interference experiments using couples of single organic molecules within few tens of microns, yielding post-selected visibilities of up to 97%. In particular, we discuss the potential interest for future realizations of measurement-device independent quantum key distribution protocols for information-theoretic secure communication.

Two-photon interference underlies the functioning of many quantum photonics devices. It also serves as the prominent tool for testing the indistinguishability of distinct photons. However, as their time-spectral profile becomes more involved, extracting relevant parameters, foremost the central frequency difference, may become difficult. In a parametric approach, these arise from the need for an exhaustive model combined with limited count statistics. Here we discuss a solution to curtail these effects on the evaluation of frequency separation relying on a semiparametric method. The time trace of the quantum interference pattern of two photons from two independent solid-state emitters is preprocessed by means of singular spectral analysis before inspecting its spectral content. This approach allows one to single out the relevant oscillations from both the envelope and the noise, without resorting to fitting. This opens the way for robust and efficient on-line monitoring of quantum emitters.

Single molecules of polyaromatic hydrocarbons (PAH) in suitable host matrices are known for emitting with high quantum efficiency in very narrow and stable zero-phonon lines (ZPL) [1]. For our experiments we used dibenzotherrylene (DBT) molecule inserted as impurity in anthracene (Ac) nanocrystals, dispersed on a gold substrate and cooled down to 3 K in a closed-cycle cryostat. DBT in Ac features a single-photon dipole-allowed transition around 784 nm. Each molecule is namely identical but environmental conditions like local trapped charges or crystal strain can shift the frequency of its transition in a range of approximately 1 nm [2]. Those static differences alongside any instantaneous interaction with the environment can degrade the ability of the emitted photons to undergo two-photon interference (TPI).

Exceptionally high secret key generation rates of 64 Mbits–1 and 115.8 Mbits–1 over a 10 km optical fibre link have been achieved, thanks to custom-built 14-pixel and 16-pixel superconducting nanowire single-photon detectors, respectively, and the use of fast quantum key distribution transmitters.

The time trace of two-photon interference from distinct molecular emitters in the solid-state is analyzed to extract their spectral difference without resorting to fitting. This opens the path towards robust on-line monitoring of quantum emitters.

Understanding heat transport is relevant to develop efficient strategies for thermal management in microelectronics for instance, as well as for fundamental science purposes. However, measuring temperatures in nanostructured environments and in cryogenic conditions remains a challenging task, that requires both high sentitivity and a non-invasive approach. Here we present a portable nanothermometer based on a molecular two-level quantum system that operates in the 3 - 30 K temperature range, with excellent temperature and spatial resolutions on the order of mK and $\mu$m, respectively. We validate the performance of this molecular thermometer on nanostructures, by estimating the thermal conductivity of a patterned silicon membrane. In addition, we demonstrate the two-dimensional temperature mapping of a patterned surface via the simultaneous spectroscopy of all thermometers deposited on a sample. These results demonstrate the potential of this molecular thermometer to explore thermal properties and related phenomena at cryogenic temperatures.

We demonstrate a quantum key distribution testbed exploiting a molecule-based single-photon source operating at room-temperature. With an estimated secret key rate of 0.5Mbps, we set the ground for room-temperature deterministic emitters for quantum communication protocols.

Single-photon sources have a variety of applications. One of these is quantum radiometry, which is reported on in this paper in the form of an overview, specifically of the current state of the art in the application of deterministic single photon sources to the calibration of single photon detectors. To optimize single-photon sources for this purpose, extensive research is currently carried out at the European National Metrology Institutes (NMIs), in collaboration with partners from universities. Single-photon sources of different types are currently under investigation, including sources based on defect centres in (nano-)diamonds, on molecules and on semiconductor quantum dots. We will present, summarise, and compare the current results obtained at European NMIs for single-photon sources in terms of photon flux, single-photon purity, and spectral power distribution as well as the results of single-photon detector calibrations carried out with this type of light sources.

Scalability and miniaturization are hallmarks of solid-state platforms for photonic quantum technologies. Still a main challenge is two-photon interference from distinct emitters on chip. This requires local tuning, integration and novel approaches to understand and tame noise processes. A promising platform is that of molecular single photon sources. Thousands of molecules with optically tuneable emission frequency can be easily isolated in solid matrices and triggered with pulsed excitation. We here discuss Hong-Ou-Mandel interference experiments using several couples of molecules within few tens of microns. Quantum interference is observed in real time, enabling the analysis of local environment effects at different time-scales.

We present Hong-Ou-Mandel (HOM) experiments with photons emitted by a single molecule of Dibenzoterrylene in an Anthracene nanocrystal under pulsed excitation, and preliminary results for photons emitted by two spatially-separated molecules on the same sample.

Quantum light sources are crucial for the future of quantum photonic technologies and, among them, single photons on-demand are key resources in quantum communications and information processing. Ideal quantum emitters providing indistinguishable photons in a clocked manner, negligible decoherence and spectral diffusion, and with potential for scalability are today still a major challenge. We report on photostable and indistinguishable single photon emission from dibenzoterrylene molecules isolated in anthracene nanocrystals (DBT:Ac NCs) at 3K. The visibility of two-photon interference is preserved even when they are separated more than thirty times the excited-state lifetime, or ten fluorescence cycles. One of the advantages of organic molecules is the low-cost mass production of nominally identical emitters, that also allow for on-chip integration. These aspects combined with high spectral stability and coherence make them promising for applications and future quantum technologies.