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Single-photon emitters serve as building blocks for many emerging concepts in quantum photonics. The recent identification of bright, tunable and stable emitters in hexagonal boron nitride (hBN) has opened the door to quantum platforms operating across the infrared to ultraviolet spectrum. Although it is widely acknowledged that defects are responsible for single-photon emitters in hBN, crucial details regarding their origin, electronic levels and orbital involvement remain unknown. Here we employ a combination of resonant inelastic X-ray scattering and photoluminescence spectroscopy in defective hBN, unveiling an elementary excitation at 285 meV that gives rise to a plethora of harmonics correlated with single-photon emitters. We discuss the importance of N π* anti-bonding orbitals in shaping the electronic states of the emitters. The discovery of elementary excitations in hBN provides fundamental insights into quantum emission in low-dimensional materials, paving the way for future investigations in other platforms. 

Due to the enhanced nature of the interactions of light with quantum excitations, topological polaritonic (TP) systems form a unique platform that offers on‐chip control over half‐light, half‐matter excitations via synthetic degrees of freedom. Among other polaritonic platforms, van der Waals materials (vdW) have recently attracted significant interest due to the relative simplicity of their integration into topological photonic structures. Several TP insulators based on vdW materials have been demonstrated; however, they rely on hybrid structures with nanopatterned dielectric substrates, which limit the strength of light‐matter interactions. Here, a monolithic all‐vdW TP insulator based on bulk crystals of transition metal dichalcogenide WS₂is designed and experimentally realized. Due to their high refractive index and the presence of exciton modes, these nanomaterials prove to be excellent platforms for TPs, offering both excellent confinement and strong light‐matter interactions in monolithic structures. The emergence of TP boundary modes is confirmed by Fourier and real‐space imaging, and a dramatic reduction in dissipation is observed at cryogenic temperatures. The proposed monolithic all‐vdW topological insulators, which are characterized by extreme confinement of optical fields and moderate losses, can serve as an alternative to silicon photonics‐based systems in the quest for the development of polaritonic quantum technologies. 

Abstract The growing field of quantum information technology requires propagation of information over long distances with efficient readout mechanisms. Excitonic quantum fluids have emerged as a powerful platform for this task due to their straightforward electro-optical conversion. In two-dimensional transition metal dichalcogenides, the coupling between spin and valley provides exciting opportunities for harnessing, manipulating, and storing bits of information. However, the large inhomogeneity of single layers cannot be overcome by the properties of bright excitons, hindering spin-valley transport. Nonetheless, the rich band structure supports dark excitonic states with strong binding energy and longer lifetime, ideally suited for long-range transport. Here we show that dark excitons can diffuse over several micrometers and prove that this repulsion-driven propagation is robust across non-uniform samples. The long-range propagation of dark states with an optical readout mediated by chiral phonons provides a new concept of excitonic devices for applications in both classical and quantum information technology.