Colloidal perovskite quantum dots (pQDs) are promising quantum light emitters, yet single-pQD studies have so far relied mostly on hot-injection synthesis, which requires precise temperature control and an inert atmosphere. Demonstrating high optical quality at the single-emitter level is the most stringent benchmark for milder synthesis routes, which often yield structural and surface defects. Here, we show that a modified ligand-assisted reprecipitation (LARP) approach produces ∼10 nm CsPbBr3 pQDs with state-of-the-art single-emitter optical properties. Combining amine-mediated postsynthetic size-trimming with didodecyldimethylammonium bromide (DDAB) ligands for enhanced passivation and colloidal stability, we obtain isolated pQDs with stable emission and minimal spectral diffusion at cryogenic temperatures. Microphotoluminescence resolves the bright-exciton fine structure, its optical phonon replicas, and the trion and biexciton states, while time-resolved and photon-correlation measurements reveal a short emission lifetime and high-purity single-photon emission. Modified LARP thus offers an accessible, room-temperature alternative to hot injection, with added flexibility for postsynthetic ligand engineering.

Solid-state quantum emitters (QEs) in two-dimensional semiconductors offer compact, chip-compatible sources for quantum photonics. In transition-metal dichalcogenides (TMDs), nanopillars are widely used to induce localized emission, yet the underlying confinement mechanism and the relative roles of strain versus dielectric environment remain unclear. The general problem addressed here is whether strain alone explains quantum emitter formation and placement in MoSe_2, or whether dielectric contrast at suspended-supported interfaces is also required. Here, we combine hyperspectral superlocalization of photoluminescence with co-registered AFM topography and phase to map the positions of localized states (LS) in MoSe_2 suspended on GaN pillars and correlate them with bending strain and the local dielectric context. Contrary to the common assumption of purely strain-driven activation, LS frequently occur at suspended–supported interfaces around the pillar apex and span a broad strain range without a clear threshold, while being scarce along high-strain ripples. Our data indicate that deterministic emitter positioning in Mo-based TMDs benefits from co-engineering both strain gradients and nanoscale dielectric heterogeneity, rather than strain alone. More broadly, this combined optical-mechanical characterization approach provides a general framework for mapping structure-property relationships in 2D quantum materials at the single-emitter level.

Abstract Solid-state quantum emitters (QEs) in two-dimensional semiconductors offer compact, chip-compatible sources for quantum photonics. In transition-metal dichalcogenides (TMDs), nanopillars are widely used to induce localized emission, yet the underlying confinement mechanism and the relative roles of strain versus dielectric environment remain unclear. Here we combine hyperspectral superlocalization of photoluminescence with coregistered AFM topography and phase to map localized states (LS) positions in MoSe2 on GaN nanopillars and correlate them with bending strain and the local environment. Contrary to a purely strain-driven picture, LS cluster at suspended–supported interfaces around the pillar apex and span a broad strain range with no clear threshold, while being scarce along high-strain ripples. We discuss the candidate mechanisms, including the local dielectric environment, and suggest coengineering of strain gradients and nanoscale interface heterogeneity for deterministic emitter positioning in TMDs.

Precise characterization of emitter-cavity metrics is required for developing novel cavity quantum electrodynamics platforms. For single solid-state quantum emitters, the Rabi coupling strength (g) and the homogeneous linewidth are the most challenging, the latter being often blurred by spectral diffusion. Recently, perovskite quantum dots emerged as promising quantum emitters, motivating their integration into photonic structures. We demonstrate the deterministic and reversible coupling of individual CsPbBr3 perovskite quantum dots to a tunable, high-quality factor, low mode volume open fibered microcavity at 10 K. Spatial and spectral tuning yields up to a 2-fold increase in single photon emission rate by the Purcell effect. Cavity tunability further enables the study of the spectral reshaping upon increasing the electromagnetic confinement. Combining temporal and spectral analyses allows us to assess g (up to 40 μeV) and to delineate the contributions of spectral diffusion and pure dephasing, paving the way for optimized cavity-coupled perovskite quantum dots.

Quantum emitters of single indistinguishable photons play a key role in quantum technologies. Among condensed matter systems, colloidal perovskite quantum dots have emerged as promising candidates, exhibiting high-purity single photon emission at room temperature and two-photon interference visibilities up to 0.5 at cryogenic temperatures. Achieving deterministic coupling of individual perovskite quantum dots to photonic structures is now a critical step towards harnessing cavity quantum electrodynamics (cQED) effects, such as the Purcell effect, to enhance single photon emission rate and indistinguishability. Here, we demonstrate the deterministic and reversible coupling of individual CsPbBr_3 perovskite quantum dots to a tunable, high-quality factor, low mode volume fiber-based Fabry-Pérot microcavity at 10 . By spatially and spectrally tuning the cavity mode in resonance with the quantum dot emission, we observe up to a twofold increase in single photon emission rates. We build on the original multiplet excitonic fine structure to assess the vacuum Rabi coupling strength (g ≃ 40 eV) from the shaping of the spectral profile of the emission upon increasing the electromagnetic confinement. This approach also made it possible to delineate the contributions of spectral diffusion and pure dephasing to the total linewidth of emission, paving the way to a fully optimized control of the emission properties of cavity coupled perovskite quantum dots.

Carbon nanotubes (CNTs) photophysical properties can be deeply enriched by chemical functionalisation inducing luminescent point defects. In particular, the grafting of small, light emitting organic molecules, known as organic color centers (OCC) can be exploited as infrared quantum light sources. These can provide a key resource in the context of quantum technologies such as quantum cryptography or quantum sensing. In fact, several types of OCC (dichlo benzene, oxygen,..) have been proposed to improve and to tune the optical properties of their CNTs hosts. Nevertheless, the fine electronic structure of these OCC is still poorly documented on the experimental side. In fact, luminescence mostly arises from the lowest lying state, whereas absorption spectroscopy can reveal a full series of electronic levels. Absorption spectrosopy has only been reported on ensembles and at room temperature, blurring out the fine structure. Here, we propose an original experimental approach to tackle the measurement of the absorption spectrum of a single OCC at low temperature, by means of a tunable high Q micro-cavity. This yields orders of magnitude enhanced absorption signals, due to the many round trips of light in the cavity, resulting in an effective magnification of the absorption cross-section of the OCC. states. In this framework, we use a flexible high finesse, low mode volume Fabry-Perot microcavity that allows us to explore a heterogeneous sample of dispersed CNTs and adjust both spatially and spectrally the cavity to each single emitter. We report on a very large light-matter coupling showing up as strong and sharp transmission dips in the cavity spectrum. Such broadband low intensity excitation of the system also results in a striking reduction of linewidth as compared to photoluminescence, confirming the key role of photo-induced charge fluctuations in the broadening of the luminescence of these quantum emitters. This paves the way to much improved coherence of these sources with suitable excitation schemes.

We report on the photoluminescence of pairs of organic color centers in single-wall carbon nanotubes grafted with 3,5-dichlorobenzene. Using various techniques such as intensity correlations, superlocalization microscopy, and luminescence excitation spectroscopy, we distinguish two pairs of color centers grafted on the same nanotube; the distance between the pairs is on the order of several hundreds of nanometers. In contrast, by studying the strong temporal correlations in the spectral diffusion in the framework of the photoinduced Stark effect, we can estimate the distance within each pair to be on the order of a few nanometers. Finally, the electronic population dynamics is investigated using time-resolved luminescence and saturation measurements, showing a biexponential decay with a fast overall recombination (compatible with a fast population transfer between the color centers within a pair) and a weak delayed repopulation of the traps, possibly due to the diffusion of excitons along the tube axis.

In this paper, we introduce a secure optical communication protocol that harnesses quantum correlation within entangled photon pairs. A message written by acting on one of the photons can be read by measuring exclusively the other photon of the pair. In this scheme, a bright, meaningless optical beam hides the message, rendering it inaccessible to potential eavesdroppers. Unlike traditional methods, our approach only affects unauthorized users, fundamentally limiting their access to the communication channel. We demonstrate the effectiveness of our protocol by achieving secure communication through both amplitude and phase modulation, relying on single-photon measurements, as opposed to most approaches which rely on coincidence measurements. We successfully demonstrate the resilience of the data transfer to noise up to 105 times greater than the signal, and we employ this technique for the secure transfer of an image.

The development of covalent diazonium functionalization of carbon nanotubes brings unique opportunities in terms of original quantum sources of light. In particular, it should be possible to create a linear ensemble of closely packed single-photon sources along the carbon nanotube backbone. The physical proximity of several dopants would make it possible to couple them simultaneous to a resonant cavity mode and to induce original dynamics and light emission properties. Here, we investigate experimentally the case of a series of four quantum emitters attached to the same carbon nanotube coupled to a high finesse fiber cavity. We explore their properties through PLE, polarization and super-localization measurements and their light emission properties using time-correlation techniques and photo-luminescence decay measurements. We show that two pairs show qualitatively different behaviors in terms of spectral diffusion and luminescence saturation, which we interpret using two models of electronically coupled or uncoupled two-level systems.

We report on the coupling of a reconfigurable high Q fiber micro-cavity to an organic color center grafted to a carbon nanotube for telecom wavelength emission of single photons in the Purcell regime. Using three complementary approaches we assess various figures of merit of this tunable single photon source and of the cavity quantum electrodynamical effects : the brightening of the emitter is obtained by comparison of the count rates of the very same emitter in free-space and cavity coupled regimes. We demonstrate a fiber coupled single-photon output rate up to 20 MHz at 1275~nm. Using time-resolved and saturation measurements, we determine independently the radiative quantum yield and the Purcell factor of the system with values up to 30 for the smallest mode volumes. Finally, we take advantage of the tuning capability of the cavity to measure the spectral profile of the brightness of the source which gives access to the vacuum Rabi splitting $g$ with values up to $25 \; \mu$eV.

The development of covalent diazonium functionalization of carbon nanotubes brings unique oportunities in terms of original quantum sources of light. In particular, it should be possible to create a linear ensemble of closely packed single-photon sources along the carbon nanotube backbone. The physical proximity of several dopants would make it possible to couple them simultaneous to a resonant cavity mode and to induce original dynamics and light emission properties. Here, we investigate experimentally the case of a series of four quantum emitters attached to the same carbon nanotube coupled to a high finesse fiber cavity. We explore their properties through PLE, polarization and superlocalization measurements and their light emission properties using time-correlation techniques, photo-luminescence decay measurements.

Graphene quantum dots, atomically precise nanopieces of graphene, are promising nano-objects with potential applications in various domains such as photovoltaics, quantum light emitters and bio-imaging. Despite their interesting prospects, precise reports on their photophysical properties remain scarce. Here, we report on a study of the photophysics of C96H24(C12H25) graphene quantum dots. A combination of optical studies down to the single molecule level with advanced molecular modelling demonstrates the importance of coupling to vibrations in the emission process. Optical fingerprints for H-like aggregates are identified. Our combined experimental-theoretical investigations provide a comprehensive description of the light absorption and emission properties of nanographenes, which not only represents an essential step towards precise control of sample production but also paves the way for new exciting physics focused on twisted graphenoids.

Recent years have shown an increasing number of studies dedicated to new light emitters for diverse applications such as optoelectronics, bio-imaging, and quantum technologies. In this context, graphene quantum dots (GQD) have important assets since bottom-up chemistry allows complete control of the structure, opening the way to wide customization of their electronic, optical, and spin properties [1-3]. The full benefit from these opportunities requires addressing GQD’s intrinsic photophysical properties.To do so, single molecule photoluminescence experiment is a powerfull tool [4]. Here, we highlight the influence of vibrations on GQDs’ optical properties, by comparing optical studies to extensive DFT/TDDFT calculations combined with molecular dynamics simulations. Specifically, we discussed their role in the transitions' oscillator strengths [5]. In order to get deeper in the photophysics of GQD, we investigate the spectroscopy of single GQDs at cryogenic temperatures. In particular, we show a narrowing of the emission lines at low temperature, that allows us to characterize and identify vibrational replicas that are characteristic to GQDs [6]. [1] M. G. Debije, J. Am. Chem. Soc . 2004, 126, 4641 [2] X. Yan, X. Cui, and L.-s. Li, J. Am. Chem. Soc . 2010 132, 5944 [3] A. Konishi et al , J. Am. Chem. Soc. 2010, 132, 11021 [4] S. Zhao et al , Nature Communications , 2018 , 9, 3470 [5] T. Liu et al , under review [6] T. Liu et al , in preparation

Atomically precise graphene quantum dots synthesized by bottom-up chemistry are promising versatile single emitters with potential applications for quantum photonic technologies. Toward a better understanding and control of graphene quantum dot (GQD) optical properties, we report on single-molecule spectroscopy at cryogenic temperature. We investigate the effect of temperature on the GQDs' spectral linewidth and vibronic replica, which we interpret building on density functional theory calculations. Finally, we highlight that the vibronic signatures are specific to the GQD geometry and can be used as a fingerprint for identification purposes.

Single photon emission in the near infrared was recently reported in individual carbon nanotubes, either plain or functionalized, with however a moderate quantum yield. Here we use a tunable fiber cavity to enhance light-matter interaction, using the so-called Purcell effect to boost the performances of the single photon source in view of applications in quantum telecommunications. Preferential excitation conditions building on localized excited states, both for plain and functionalized nanotubes, are considered and the performances of the sources in terms of purity, brightness and indistinguishability are discussed.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Optical properties of core-shell systems based on carbon nanotubes Lucile Orcin-Chaix, Yannick Chassagneux, Christophe Voisin, Stéphane Campidelli, Jean-Sébastien Lauret