Single Photon Emitters Research Articles

Enhancing the Emission Rate of the Inherent Silicon‐Vacancy Center Using Optimized Multipolar Moments in a Silicon Carbide Metasurface

AbstractSilicon carbide (SiC) hosts various color centers with emission lines from visible to near‐infrared, with promising applications in quantum communication. Metasurfaces made using SiC can enhance the emission rate of inherent color centers to achieve on‐demand single photon emission with a high emission rate. This is possible by engineering the amplitude and phase of the excited multipolar scattering moments in metasurfaces. The numerical simulations and analytical calculations are used to study the emission rate enhancement from an embedded single silicon vacancy () center in SiC metasurface. The optimized metasurface balances multipolar moments at the zero‐phonon line wavelength of 862 nm with 40 times field intensity confinement. The confinement provides substantial emission rate enhancement for the center, making it a bright single photon emitter at 862 nm. The significant emission enhancement offers new insights into the metasurface to achieve on‐demand single‐photon sources and efficient spin–photon interfaces.

Activated Single Photon Emitters And Enhanced Deep‐Level Emissions in Hexagonal Boron Nitride Strain Crystal

AbstractThe peculiar defect‐related photon emission processes in 2D hexagonal boron nitride (hBN) have become a topic of intense research due to their potential applications in quantum information and sensing technologies. Here, it is reported on exotic single photons and enhanced deep‐level emissions in 2D hBN strain crystal, which is fabricated by transferring multilayer hBN onto hexagonal close‐packed silica spheres on a silica substrate. Effective activation of single photon emission is realized from the defect ensembles in the multilayer hBN at positions that are in contact with the apex of the SiO2 spheres. At these points, the local tensile strain‐induced overall blue shift of the SPE ensembles is up to 12 nm. Furthermore, high spatial resolution cathodoluminescence measurements show remarkable strain‐enhanced deep‐level emissions in the multilayer hBN with the emission intensity distribution following the periodic hexagonal pattern of the strain crystal. The maximum deep‐level emission enhancement is up to 350% with an energy redshift of 6 nm. These results provide a simple on‐chip compatible method for activating and tuning the defect‐related photon emissions in multilayer hBN, demonstrating the potential of hBN strain crystal as a building block for future on‐chip quantum nanophotonic devices.

Composed Effects of Electron-Hole Exchange and Near-Field Interaction in Quantum-Dot-Confined Radiative Dipoles

Conservation of polarization is an important requirement for reliable single-photon emitters, which, in turn, are essential building blocks for light-based quantum information processing. In this work, we study the exciton-spin dynamics in a double quantum dot under the combined effects of electron-hole exchange and Förster resonance energy transfer. By means of numerical solutions of the quantum master equation, we simulate the time-dependent spin polarization for two neighboring dots. According to our results, under some conditions, the depolarization caused by the electron-hole exchange may be slowed by the near field-induced interdot energy transfer, suggesting a new mechanism to extend the exciton coherence time. This opens doors to alternative schemes for improved solid-state quantum light sources.

Photoluminescence imaging of single photon emitters within nanoscale strain profiles in monolayer WSe2

Local deformation of atomically thin van der Waals materials provides a powerful approach to create site-controlled chip-compatible single-photon emitters (SPEs). However, the microscopic mechanisms underlying the formation of such strain-induced SPEs are still not fully clear, which hinders further efforts in their deterministic integration with nanophotonic structures for developing practical on-chip sources of quantum light. Here we investigate SPEs with single-photon purity up to 98% created in monolayer WSe2 via nanoindentation. Using photoluminescence imaging in combination with atomic force microscopy, we locate single-photon emitting sites on a deep sub-wavelength spatial scale and reconstruct the details of the surrounding local strain potential. The obtained results suggest that the origin of the observed single-photon emission is likely related to strain-induced spectral shift of dark excitonic states and their hybridization with localized states of individual defects.

Control on the density and optical properties of color centers at SiO2/SiC interfaces by oxidation and annealing

Color centers in solids can serve as single photon emitters (SPEs) those are important in many quantum applications. Silicon carbide (SiC) is a promising host for color centers because of its well-established crystal growth and device technologies. Although color centers with extremely high brightness were found at the silicon dioxide (SiO2)/SiC interface, controlling their density and optical properties remains a challenge. In this study, we demonstrate control over the color centers at the SiO2/SiC interface by designing the oxidation and annealing conditions. We report that post-oxidation CO2 annealing has the ability to reduce the color centers at the interface and form well-isolated SPEs with bright emission. We also discuss the correlation between the color centers and electrically active defects.

Computational search for efficient single-photon emitters among the substitutional doping defects in two-dimensional GaSe

Single photon emitters (SPE) in the near-infrared (NIR) spectrum range are critical elements in quantum communication technology. We apply here computational methods to reveal local defects in the GaSe monolayer with properties favorable for such SPEs. Based on an educated guess, we selected twelve defects YX in GaSe, where the Y dopant substitutes the X host atom, X = Ga or Se, and Y = C, Si, Ge, N, P, or As. Our calculations show that two defects, NGa, and PGa, are dynamically unstable. For the remaining defects, we apply the linear response GW method and Bethe-Salpeter equation (BSE) to calculate the electronic structure and optical excitation spectra. Some defects are found to have two sharp peaks (occupied and unoccupied) of the density of the electronic states within the band gap. The NSe-, PSe-, and AsSe-defects have intense sharp an optical excitation peak at energies around 0.8 eV (λ = 1550 nm). Analysis of the formation of the BSE eigenstates associated with these peaks suggests that these excitations can cause a narrow zero-phonon line emission. We thus propose the NSe-, PSe-, and AsSe-defects in the GaSe monolayer as promising SPE in NIR region.

Heterogeneous Integration of Solid-State Quantum Systems with a Foundry Photonics Platform.

Diamond color centers are promising optically addressable solid-state spins that can be matter-qubits, mediate deterministic interaction between photons, and act as single photon emitters. Useful quantum computers will comprise millions of logical qubits. To become useful in constructing quantum computers, spin-photon interfaces must, therefore, become scalable and be compatible with mass-manufacturable photonics and electronics. Here, we demonstrate the heterogeneous integration of NV centers in nanodiamond with low-fluorescence silicon nitride photonics from a standard 180 nm CMOS foundry process. Nanodiamonds are positioned over predefined sites in a regular array on a waveguide in a single postprocessing step. Using an array of optical fibers, we excite NV centers selectively from an array of six integrated nanodiamond sites and collect the photoluminescence (PL) in each case into waveguide circuitry on-chip. We verify single photon emission by an on-chip Hanbury Brown and Twiss cross-correlation measurement, which is a key characterization experiment otherwise typically performed routinely with discrete optics. Our work opens up a simple and effective route to simultaneously address large arrays of individual optically active spins at scale, without requiring discrete bulk optical setups. This is enabled by the heterogeneous integration of NV center nanodiamonds with CMOS photonics.

Liquid-activated quantum emission from pristine hexagonal boron nitride for nanofluidic sensing.

Liquids confined down to the atomic scale can show radically new properties. However, only indirect and ensemble measurements operate in such extreme confinement, calling for novel optical approaches that enable direct imaging at the molecular level. Here we harness fluorescence originating from single-photon emitters at the surface of hexagonal boron nitride for molecular imaging and sensing in nanometrically confined liquids. The emission originates from the chemisorption of organic solvent molecules onto native surface defects, revealing single-molecule dynamics at the interface through the spatially correlated activation of neighbouring defects. Emitter spectra further offer a direct readout of the local dielectric properties, unveiling increasing dielectric order under nanometre-scale confinement. Liquid-activated native hexagonal boron nitride defects bridge the gap between solid-state nanophotonics and nanofluidics, opening new avenues for nanoscale sensing and optofluidics.

(Invited) Catalytic Growth of Width-Controlled Single Atomic Layer MoS2 Sub-10nm Nanoribbons

For the next generation materials for optoelectronics, quantum computing, communication and sensing, single atomic layer semiconducting nanoribbons offer additional degree of freedoms for engineering their properties. However, direct width-controlling synthesis of TMD nanoribbons, especially for the appealing width range below 30 nm, remains challenge. Therefore, developing facile methods for their direct and controllable synthesis is of central importance. We present a new growth method for single and double atomic layer of MoS2 nanoribbons with width down to sub-10 nm. Nanoribbons are grown via precipitation from the Ni-contained composite nanoparticles via vapor-liquid-solid mechanism in which the nanoparticle diameter also controls the width of nanoribbon. We also present preliminarily studies on width-dependent properties of the nanoribbons, which evidence their potential application in the quantum electronics, particularly as a single electron transistor and single photon emitter.

Two-photon interference from silicon-vacancy centers in remote nanodiamonds

Abstract The generation of indistinguishable photons is a key requirement for solid-state quantum emitters as a viable source for applications in quantum technologies. Restricting the dimensions of the solid-state host to a size well below the wavelength of light emitted by a defect-center enables efficient external optical coupling, for example, for hybrid integration into photonic devices. However, stringent restrictions on the host dimensions result in severe limitations on the spectral properties reducing the indistinguishability of emitted photons. Here, we demonstrate two-photon interference from two negatively charged silicon-vacancy centers located in remote nanodiamonds. The Hong–Ou–Mandel interference efficiency reaches 61 % with a coalescence time window of 0.35 ns. We furthermore show a high yield of pairs of silicon-vacancy centers with indistinguishable optical transitions. Therefore, our work opens new paths in hybrid quantum technology based on indistinguishable single-photon emitters in nanodiamonds.

Spin-active single photon emitters in hexagonal boron nitride from carbon-based defects

Most single photon emitters in hexagonal boron nitride have been identified as carbon substitutional defects, forming donor-acceptor systems. Unlike the most studied bulk emitters (i.e. color centers in diamond), these defects have no net spin, or have a single unpaired spin. By means of density functional calculations, we show that two non-adjacent carbon substitutional defects of the same type (i.e. CB-CB, and CN-CN), can have a triplet groundstate. In particular, one of such defects has a zero phonon line energy of 2.5 eV, and its triplet state is nearly 0.5 eV more stable than its singlet. The mechanism behind the destabilization of the singlet state is related to a larger electrostatic repulsion of a symmetric wave function in a charged lattice.

Arbitrarily structured quantum emission with a multifunctional metalens

Structuring light emission from single-photon emitters (SPEs) in multiple degrees of freedom is of great importance for quantum information processing towards higher dimensions. However, traditional control of emission from quantum light sources relies on the use of multiple bulky optical elements or nanostructured resonators with limited functionalities, constraining the potential of multi-dimensional tailoring. Here we introduce the use of an ultrathin polarisation-beam-splitting metalens for the arbitrary structuring of quantum emission at room temperature. Owing to the complete and independent polarisation and phase control at the single meta-atom level, the designed metalens enables simultaneous mapping of quantum emission from ultra-bright defects in hexagonal boron nitride and imprinting of an arbitrary wavefront onto orthogonal polarisation states of the sources. The hybrid quantum metalens enables simultaneous manipulation of multiple degrees of freedom of a quantum light source, including directionality, polarisation, and orbital angular momentum. This could unleash the full potential of solid-state SPEs for their use as high-dimensional quantum sources for advanced quantum photonic applications.

Time-Resolved Line Shapes of Single Quantum Emitters via Machine Learned Photon Correlations.

Solid-state single-photon emitters (SPEs) are quantum light sources that combine atomlike optical properties with solid-state integration and fabrication capabilities. SPEs are hindered by spectral diffusion, where the emitter's surrounding environment induces random energy fluctuations. Timescales of spectral diffusion span nanoseconds to minutes and require probing single emitters to remove ensemble averaging. Photon correlation Fourier spectroscopy (PCFS) can be used to measure time-resolved single emitter line shapes, but is hindered by poor signal-to-noise ratio in the measured correlation functions at early times due to low photon counts. Here, we develop a framework to simulate PCFS correlation functions directly from diffusing spectra that match well with experimental data for single colloidal quantum dots. We use these simulated datasets to train a deep ensemble autoencoder machine learning model that outputs accurate, noiseless, and probabilistic reconstructions of the noisy correlations. Using this model, we obtain reconstructed time-resolved single dot emission line shapes at timescales as low as 10ns, which are otherwise completely obscured by noise. This enables PCFS to extract optical coherence times on the same timescales as Hong-Ou-Mandel two-photon interference, but with the advantage of providing spectral information in addition to estimates of photon indistinguishability. Our machine learning approach is broadly applicable to different photon correlation spectroscopy techniques and SPE systems, offering an enhanced tool for probing single emitter line shapes on previously inaccessible timescales.

Culling a Self-Assembled Quantum Dot as a Single-Photon Source Using X-ray Microscopy.

Epitaxially grown self-assembled semiconductor quantum dots (QDs) with atom-like optical properties have emerged as the best choice for single-photon sources required for the development of quantum technology and quantum networks. Nondestructive selection of a single QD having desired structural, compositional, and optical characteristics is essential to obtain noise-free, fully indistinguishable single or entangled photons from single-photon emitters. Here, we show that the structural orientations and local compositional inhomogeneities within a single QD and the surrounding wet layer can be probed in a screening fashion by scanning X-ray diffraction microscopy and X-ray fluorescence with a few tens of nanometers-sized synchrotron radiation beam. The presented measurement protocol can be used to cull the best single QD from the enormous number of self-assembled dots grown simultaneously. The obtained results show that the elemental composition and resultant strain profiles of a QD are sensitive to in-plane crystallographic directions. We also observe that lattice expansion after a certain composition-limit introduces shear strain within a QD, enabling the possibility of controlled chiral-QD formation. Nanoscale chirality and compositional anisotropy, contradictory to common assumptions, need to be incorporated into existing theoretical models to predict the optical properties of single-photon sources and to further tune the epitaxial growth process of self-assembled quantum structures.

Deterministic Localization of Strain-Induced Single-Photon Emitters in Multilayer GaSe

Deterministic Localization of Strain-Induced Single-Photon Emitters in Multilayer GaSe

3D Nanoarchitectured Hexagonal Boron Nitride with Integrated Single Photon Emitters

AbstractTwo‐dimensional (2D) hexagonal boron nitride (hBN) is one of the most promising candidates to host solid‐state single photon emitters (SPEs) for various quantum technologies. However, the 2D nature with an atomic‐scale thickness leads to inevitable challenges in spectral variability caused by substrate disturbance, lattice strain heterogeneity, and defect variation. Here, three‐dimensional (3D) nanoarchitectured hBN is reported with integrated SPEs from native defects generated during high‐temperature chemical vapor deposition (CVD). The 3D hBN has a quasi‐periodic gyroid minimal surface structure and is composed of a continuous 2D hBN sheet with built‐in convex and concave curvatures that promote the formation of optically active and thermally robust native defects. The free‐standing feature of the gyroid hBN with a nearly zero mean curvature can effectively eliminate the substrate disturbance and minimize lattice strain heterogeneity. As a result, naturally occurring defects with a narrow SPE spectral distribution can be created and activated as color centers in the 3D hBN, and the density of the SPEs can be tailored by CVD temperature.

Electrically Driven Site-Controlled Single Photon Source.

Single photon sources are fundamental building blocks for quantum communication and computing technologies. In this work, we present a device geometry consisting of gold pillars embedded in a van der Waals heterostructure of graphene, hexagonal boron nitride, and tungsten diselenide. The gold pillars serve to both generate strain and inject charge carriers, allowing us to simultaneously demonstrate the positional control and electrical pumping of a single photon emitter. Moreover, increasing the thickness of the hexagonal boron nitride tunnel barriers restricts electroluminescence but enables electrical control of the emission energy of the site-controlled single photon emitters, with measured energy shifts reaching 40 meV.

Effect of environmental screening and strain on optoelectronic properties of two-dimensional quantum defects

Point defects in hexagonal boron nitride (hBN) are promising candidates as single-photon emitters (SPEs) in nanophotonics and quantum information applications. The precise control of SPEs requires in-depth understanding of their optoelectronic properties. However, how the surrounding environment of host materials, including the number of layers, substrates, and strain, influences SPEs has not been fully understood. In this work, we study the dielectric screening effect due to the number of layers and substrates, and the strain effect on the optical properties of carbon dimer and nitrogen vacancy defects in hBN from first-principles many-body perturbation theory. We report that environmental screening causes a lowering of the quasiparticle gap and exciton binding energy, leading to nearly constant optical excitation energy and exciton radiative lifetime. We explain the results with an analytical model starting from the Bethe–Salpeter equation Hamiltonian with Wannier basis. We also show that optical properties of quantum defects are largely tunable by strain with highly anisotropic response, in good agreement with experimental measurements. Our work clarifies the effect of environmental screening and strain on optoelectronic properties of quantum defects in two-dimensional insulators, facilitating future applications of SPEs and spin qubits in low-dimensional systems.

Purcell enhancement of single-photon emitters in silicon

Individual spins that are coupled to telecommunication photons offer unique promise for distributed quantum information processing once a coherent and efficient spin–photon interface can be fabricated at scale. We implement such an interface by integrating erbium dopants into a nanophotonic silicon resonator. We achieve spin-resolved excitation of individual emitters with <0.1GHz spectral diffusion linewidth. Upon resonant driving, we observe optical Rabi oscillations and single-photon emission with a 78-fold Purcell enhancement. Our results establish a promising platform for quantum networks.

Single-emitter quantum key distribution over 175 km of fibre with optimised finite key rates

Quantum key distribution with solid-state single-photon emitters is gaining traction due to their rapidly improving performance and compatibility with future quantum networks. Here we emulate a quantum key distribution scheme with quantum-dot-generated single photons frequency-converted to 1550 nm, achieving count rates of 1.6 MHz with {g}^{left(2right)}left(0right)=3.6% and asymptotic positive key rates over 175 km of telecom fibre. We show that the commonly used finite-key analysis for non-decoy state QKD drastically overestimates secure key acquisition times due to overly loose bounds on statistical fluctuations. Using the tighter multiplicative Chernoff bound to constrain the estimated finite key parameters, we reduce the required number of received signals by a factor 108. The resulting finite key rate approaches the asymptotic limit at all achievable distances in acquisition times of one hour, and at 100 km we generate finite keys at 13 kbps for one minute of acquisition. This result is an important step towards long-distance single-emitter quantum networking.