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Producing isolated single-photon emitters in hexagonal boron nitride with predefined spin transitions is challenging. Oxygen annealing enables the controlled fabrication of narrowband quantum emitters with optically active spin for quantum applications.

Impurities and defects embedded in diamond are a promising platform for spintronics and photonics. Here, Magyar and colleagues incorporate europium defects in diamond, whose optical properties promise their use in quantum information applications.

Optical spin defects in semiconductors are crucial for applications, but it is often difficult to establish their microscopic origin. A mechanism for the spin behaviour of a family of bright emitters in hexagonal boron nitride has now been identified.

Silicon vacancy centres in diamond have been identified as potential highly efficient solid-state qubits for on-chip integration. Here, Zhouet al. demonstrate coherent control of silicon vacancy centres in nanodiamonds and observe Autler-Townes splitting, Mollow triplet and Rabi oscillations.

This Review covers state-of-the-art reconfigurable and tunable optical components and highlights the emergence of a set of materials that offer a new toolkit for tunability and control.

Defects in the crystal lattice of silicon carbide prove to be a useful room-temperature source of non-classical light.

Upconversion nanoparticles offer the potential for deep tissue biological imaging. Here, Chen et al. develop super resolution optical imaging in the near-infrared for imaging with sub-50 nm resolution through almost 100 microns of tissue.

Silicon is the material of choice for modern microelectronics, whereas diamond is a luxurious gem. Now, researchers have demonstrated that silicon impurities in diamond can generate indistinguishable single photons — a requirement for quantum photonics and computing.

Nuclear quadrupole resonance spectroscopy is used to map the properties of atomically thin hexagonal boron nitride, with the help of the nitrogen–vacancy colour centres engineered in a diamond layer placed under the 2D material.

The 2D material hBN hosts various optically addressable spin defects, promising for quantum technology applications. Here the authors report the co-existence of spin-1 and spin-1/2 defects in hBN, show their room temperature coherent control and optical readout, as well as cross-relaxation.

Previous research reported enhanced emission from spin defects in hBN by coupling to optical resonators; however, this approach has limited scalability. Here the authors use a monolithic metasurface featuring quasi bound states fabricated from hBN to enhance photoemission and optical spin-readout efficiency of defects in the same material.

Interfacing single-photon emitters (SPEs) with high-finesse cavities can prevent decoherence processes, especially at elevated temperature, but its implementation remains challenging. Here, the authors report room-temperature strong coupling of SPEs in hexagonal boron nitride with a dielectric cavity based on bound states in the continuum, showing a Rabi splitting of ~ 4 meV.

Hexagonal boron nitride (hBN) is a layered van der Waals material showing promise for nanophotonics. Here, the authors design hBN photonic crystal cavities with quality factors exceeding 2000, and further demonstrate deterministic tuning of individual cavities by minimally-invasive electron beam induced etching.

Comparison of hexagonal boron nitride samples grown with different techniques and with varying carbon-doping content provides evidence that the defects emitting single photons in the visible range are carbon related.

An ensemble of spins associated with an intrinsic defect of two-dimensional hexagonal boron nitride is shown to be optically addressable, allowing spin polarization of its triplet ground state and providing evidence of spin coherence.

Direct writing of active defects is a much sought-after step towards scalable quantum technologies. Here, the authors show a flexible approach by transferring momentum from an ion beam to thin films pre-deposited onto diamond surfaces (using films of Si, Ge, Sn or Pb) or fiber cores (using Eu).

The photophysical properties of quantum emitters in layered van der Waals materials are receiving growing attention as they could be leveraged for nanophotonics applications. Here, the authors devise a two-laser super-resolution imaging setup capable of observing highly non-linear emission from hBN emitters.

A semiconductor quantum dot that generates polarization-entangled photon pairs on demand has been realized, marking an important milestone for scalable integrated quantum photonics and information processing.

The integration of diamond waveguide arrays into an aluminium nitride photonic platform offers hope for the realization of scalable chips for quantum information processing.

Defects in materials can be used to detect magnetic fields at the nanoscale. Here the authors show that a carbon-related defect in hexagonal boron nitride acts as a robust nanoscale sensor capable of vectorial magnetic field detection.

Single-photon emission at room temperature can be achieved with hexagonal boron nitride due to electron and hole confinement in vacancy-related defects.

Hexagonal boron nitride is a common component of 2D heterostructures. Defects implanted in boron nitride crystals can be used to perform spatially resolved sensing of properties, including temperature, magnetism and current.

Quantum coherent control of single-photon-emitting defect spins have been reported in hexagonal boron nitride, revealing that spin coherence is mainly governed by coupling to a few proximal nuclei and can be prolonged by decoupling protocols.

A common direct-bandgap semiconductor has been found to host optically addressable spins, opening the door to scalable quantum sensor manufacturing.

Spin defects in two-dimensional materials potentially offer unique advantages for quantum sensing in terms of sensitivity and functionality. Here, the authors demonstrate the use of spin defects in hexagonal boron nitride as sensors of magnetic field, temperature and pressure, and show that their performance is comparable or exceeds that of existing platforms.

Optically active spins in solid-state materials hold promise for future quantum technologies. Here, the authors demonstrate optically detected magnetic resonance at room temperature for single defects in a two-dimensional material, hexagonal boron nitride.

Defects in hexagonal boron nitride exhibit room-temperature quantum emission, but their unknown structural origin challenges their technological utility. A combination of optical and electron microscopy helps to distinguish at least four classes of defects and correlate them with local strain.

Inhomogeneous spectral distribution and multi-photon emission are currently hindering the use of defects in layered hBN as reliable single photon emitters. Here, the authors demonstrate strain-controlled wavelength tuning and increased single photon purity through suitable material processing.

This Review reports the recent progress in utilizing van der Waals layered materials in various nanophotonics applications and provides an overview of their future developments in hybrid and tunable nanophotonics, 3D photonic structures, optical trapping, polariton devices and van der Waals integrated nanophotonic circuits.

The first online-only meeting in photonics, held on January 13th 2020, was a resounding success, with 1100 researchers participating remotely to discuss the latest advances in photonics. Here, the organizers share their tips and advice on how to organize an online conference.

This Review summarizes recent progress of single-photon emitters based on defects in solids and highlights new research directions. The photophysical properties of single-photon emitters and efforts towards scalable system integration are also discussed.

The image depicts a schematic illustration of a van der Waals heterostructure used to electrically activate quantum emitters in hexagonal boron nitride.

Diamond colour centres have applications in quantum sensing, quantum communication and other important technologies. Bradac et al. survey the progress made in using group IV defect centres, which are anticipated to have practical advantages over the more commonly-used nitrogen vacancy centres.

Hexagonal boron nitride (hBN) is highly sought after for mid-IR nanophotonics, nonlinear and quantum optics, and as an efficient UV emitter. This Review surveys its fundamental physical properties, applications and synthesis.

This Review highlights the role of transition metal dichalcogenides, hexagonal boron nitride and stacked heterostructures in applications in quantum communication, computation, sensing and single-photon detection.

Photonics is one of the key platforms for emerging quantum technologies, but its full potential can only be harnessed by exploiting miniaturization via on-chip integration. This Roadmap charts new directions and discusses the challenges associated with the hybrid integration of a variety of materials, devices and components.