Solid-state Single-photon Sources Research Articles

Quantum emitters in aluminum nitride induced by heavy ion irradiation

The integration of solid-state single-photon sources with foundry-compatible photonic platforms is crucial for practical and scalable quantum photonic applications. This study explores aluminum nitride (AlN) as a material with properties highly suitable for integrated on-chip photonics and the ability to host defect-center related single-photon emitters. We have conducted a comprehensive analysis of the creation of single-photon emitters in AlN, utilizing heavy ion irradiation and thermal annealing techniques. Subsequently, we have performed a detailed analysis of their photophysical properties. Guided by theoretical predictions, we assessed the potential of Zirconium (Zr) ions to create optically addressable spin defects and employed Krypton (Kr) ions as an alternative to target lattice defects without inducing chemical doping effects. With a 532 nm excitation wavelength, we found that single-photon emitters induced by ion irradiation were primarily associated with vacancy-type defects in the AlN lattice for both Zr and Kr ions. The density of these emitters increased with ion fluence, and there was an optimal value that resulted in a high density of emitters with low AlN background fluorescence. Under a shorter excitation wavelength of 405 nm, Zr-irradiated AlN exhibited isolated point-like emitters with fluorescence in the spectral range theoretically predicted for spin-defects. However, similar defects emitting in the same spectral range were also observed in AlN irradiated with Kr ions as well as in as-grown AlN with intrinsic defects. This result is supportive of the earlier theoretical predictions, but at the same time highlights the difficulties in identifying the sought-after quantum emitters with interesting properties related to the incorporation of Zr ions into the AlN lattice by fluorescence alone. The results of this study largely contribute to the field of creating quantum emitters in AlN by ion irradiation and direct future studies emphasizing the need for spatially localized Zr implantation and testing for specific spin properties.

Acoustic wave-based single photon shifter for solid-state sources

Controlling the frequency of nonclassical light is essential for the implementation of quantum computation, communication, and the integration of various quantum systems. However, there is a practical absence of easy-to-integrate frequency-shift devices for solid-state single-photon sources. Here, we propose an integrated single-photon frequency shifter that utilizes acousto-optic modulation. The device is composed of two interdigital transducers (IDTs) for generating acoustic waves on a lithium niobate on insulator (LNOI) platform, along with a silicon waveguide that is periodically positioned at the nodes of the acoustic wave to enhance the interaction length. We achieved a low half-wavelength voltage length product Vπ×L of 0.18 V cm. With a driving frequency of 129.7 MHz and a driving voltage of 10 V, a frequency shift of up to ± 405 GHz is realized with near-unity conversion efficiency. Our findings illustrate the feasibility of deterministic on-chip quantum spectral control, which is pivotal for constructing hybrid quantum networks.

Engineering the Impact of Phonon Dephasing on the Coherence of a WSe_{2} Single-Photon Source via Cavity Quantum Electrodynamics.

Emitter dephasing is one of the key issues in the performance of solid-state single-photon sources. Among the various sources of dephasing, acoustic phonons play a central role in adding decoherence to the single-photon emission. Here, we demonstrate that it is possible to tune and engineer the coherence of photons emitted from a single WSe_{2} monolayer quantum dot via selectively coupling it to a spectral cavity resonance. We utilize an open cavity to demonstrate spectral enhancement, leveling, and suppression of the highly asymmetric phonon sideband, finding excellent agreement with a microscopic description of the exciton-phonon dephasing in a truly two-dimensional system. Moreover, the impact of cavity tuning on the dephasing is directly assessed via optical interferometry, which points out the capability to utilize light-matter coupling to steer and design dephasing and coherence of quantum emitters in atomically thin crystals.

Deterministic storage and retrieval of telecom light from a quantum dot single-photon source interfaced with an atomic quantum memory.

A hybrid interface of solid-state single-photon sources and atomic quantum memories is a long sought-after goal in photonic quantum technologies. Here, we demonstrate deterministic storage and retrieval of light from a semiconductor quantum dot in an atomic ensemble quantum memory at telecommunications wavelengths. We store single photons from an indium arsenide quantum dot in a high-bandwidth rubidium vapor-based quantum memory, with a total internal memory efficiency of (12.9 ± 0.4)%. The signal-to-noise ratio of the retrieved light field is 18.2 ± 0.6, limited only by detector dark counts.

Room-temperature strong coupling in a single-photon emitter-metasurface system

Solid state single-photon sources with high brightness and long coherence time are promising qubit candidates for modern quantum technology. To prevent decoherence processes and preserve the integrity of the qubits, decoupling the emitters from their surrounding environment is essential. To this end, interfacing single photon emitters (SPEs) with high-finesse cavities is required, especially in the strong coupling regime, when the interaction between emitters can be mediated by cavity fields. However, achieving strong coupling at elevated temperatures is challenging due to competing incoherent processes. Here, we address this long-standing problem by using a quantum system, which comprises a class of SPEs in hexagonal boron nitride and a dielectric cavity based on bound states in the continuum (BIC). We experimentally demonstrate, at room temperature, strong coupling of the system with a large Rabi splitting of ~4 meV thanks to the combination of the narrow linewidth and large oscillator strength of the emitters and the efficient photon trapping of the BIC cavity. Our findings unveil opportunities to advance the fundamental understanding of quantum dynamical system in strong coupling regime and to realise scalable quantum devices capable of operating at room temperature.

Monolayer-Based Single-Photon Source in a Liquid-Helium-Free Open Cavity Featuring 65% Brightness and Quantum Coherence.

Solid-state single-photon sources are central building blocks in quantum information processing. Atomically thin crystals have emerged as sources of nonclassical light; however, they perform below the state-of-the-art devices based on volume crystals. Here, we implement a bright single-photon source based on an atomically thin sheet of WSe2 coupled to a tunable optical cavity in a liquid-helium-free cryostat without the further need for active stabilization. Its performance is characterized by high single-photon purity (g(2)(0) = 4.7 ± 0.7%) and record-high, first-lens brightness of linearly polarized photons of 65 ± 4%, representing a decisive step toward real-world quantum applications. The high performance of our devices allows us to observe two-photon interference in a Hong-Ou-Mandel experiment with 2% visibility limited by the emitter coherence time and setup resolution. Our results thus demonstrate that the combination of the unique properties of two-dimensional materials and versatile open cavities emerges as an inspiring avenue for novel quantum optoelectronic devices.

Plasmonic Nanodiamonds.

While nitrogen-vacancy (NV) centers in diamonds have emerged as promising solid-state quantum emitters for sensing applications, the tantalizing possibility of coupling them with photonic or broadband plasmonic nanostructures to create ultrasensitive biolabels has not been fully realized. Indeed, it remains technologically challenging to create free-standing hybrid diamond-based imaging nanoprobes with enhanced brightness and high temporal resolution. Herein, we leverage the bottom-up DNA self-assembly to develop hybrid free-standing plasmonic nanodiamonds, which feature a closed plasmonic nanocavity completely encapsulating a single nanodiamond. Correlated single nanoparticle spectroscopical characterizations suggest that the plasmonic nanodiamond displays dramatically and simultaneously enhanced brightness and emission rate. We believe that they hold huge potential to serve as a stable solid-state single-photon source and could serve as a versatile platform to study nontrivial quantum effects in biological systems with enhanced spatial and temporal resolution.

High-speed thin-film lithium niobate quantum processor driven by a solid-state quantum emitter.

Scalable photonic quantum computing architectures pose stringent requirements on photonic processing devices. The needs for low-loss high-speed reconfigurable circuits and near-deterministic resource state generators are some of the most challenging requirements. Here, we develop an integrated photonic platform based on thin-film lithium niobate and interface it with deterministic solid-state single-photon sources based on quantum dots in nanophotonic waveguides. The generated photons are processed with low-loss circuits programmable at speeds of several gigahertz. We realize a variety of key photonic quantum information processing functionalities with the high-speed circuits, including on-chip quantum interference, photon demultiplexing, and reprogrammability of a four-mode universal photonic circuit. These results show a promising path forward for scalable photonic quantum technologies by merging integrated photonics with solid-state deterministic photon sources in a heterogeneous approach to scaling up.

Efficient Single-Photon Sources Based on Chlorine-Doped ZnSe Nanopillars with Growth Controlled Emission Energy.

Isolated impurity states in epitaxially grown semiconductor systems possess important radiative features such as distinct wavelength emission with a very short radiative lifetime and low inhomogeneous broadening, which make them promising for the generation of indistinguishable single photons. In this study, we investigate chlorine-doped ZnSe/ZnMgSe quantum well (QW) nanopillar (NP) structures as a highly efficient solid-state single-photon source operating at cryogenic temperatures. We show that single photons are generated due to the radiative recombination of excitons bound to neutral Cl atoms in ZnSe QW and the energy of the emitted photon can be tuned from about 2.85 down to 2.82 eV with ZnSe well width increase from 2.7 to 4.7 nm. Following the developed advanced technology, we fabricate NPs with a diameter of about 250 nm using a combination of dry and wet-chemical etching of epitaxially grown ZnSe/ZnMgSe QW structures. The remaining resist mask serves as a spherical- or cylindrical-shaped solid immersion lens on top of NPs and leads to the emission intensity enhancement by up to an order of magnitude in comparison to the pillars without any lenses. NPs with spherical-shaped lenses show the highest emission intensity values. The clear photon-antibunching effect is confirmed by the measured value of the second-order correlation function at a zero time delay of 0.14. The developed single-photon sources are suitable for integration into scalable photonic circuits.

Tailoring solid-state single-photon sources with stimulated emissions.

The coherent interaction of electromagnetic fields with solid-state two-level systems can yield deterministic quantum light sources for photonic quantum technologies. To date, the performance of semiconductor single-photon sources based on three-level systems is limited mainly due to a lack of high photon indistinguishability. Here we tailor the cavity-enhanced spontaneous emission from a ladder-type three-level system in a single epitaxial quantum dot through stimulated emission. After populating the biexciton (XX) of the quantum dot through two-photon resonant excitation, we use another laser pulse to selectively depopulate the XX state into an exciton (X) state with a predefined polarization. The stimulated XX-X emission modifies the X decay dynamics and improves the characteristics of a polarized single-photon source, such as a source brightness of 0.030(2), a single-photon purity of 0.998(1) and an indistinguishability of 0.926(4). Our method can be readily applied to existing quantum dot single-photon sources and expands the capabilities of three-level systems for advanced quantum photonic functionalities.

Narrow-linewidth diamond single-photon sources prepared via femtosecond laser

Nitrogen-vacancy (NV) color centers in diamonds with narrow optical linewidths are commonly used as solid-state single-photon sources that emit indistinguishable photons. However, NV color centers prepared using ion implantation typically have large optical linewidths of over 100 MHz. Herein, we used the femtosecond laser direct writing (FsLDW) technique to prepare single NV color centers in diamond with a narrow optical linewidth of 13.05 ± 0.2 MHz and a long decoherence time of 445 ± 27.6 μs. In addition, the density functional theory was adopted to establish calculation models and illustrate why single NV color centers prepared by FsLDW have narrower linewidths compared to those of the ion implantation method. Thus, this study provides an effective reference for the preparation of narrow-linewidth single-color centers in diamonds and other wide-gap crystals.

Transferable room-temperature single-photon emitters in hexagonal boron nitride grown by molecular beam epitaxy

The solid-state single-photon source is the core of applications such as quantum cryptography, quantum sensing, and quantum computing. Recently, the point defects in two-dimensional (2D) hexagonal boron nitride (h-BN) have become excellent candidates for next-generation single-photon sources due to their chemical and physical stability and ultra-high brightness at room temperature. The 2D layered structure of h-BN allows the single-photon emitters (SPEs) in it to have high photon extraction efficiency and be integrated into photonic circuits easily. However, most of the SPEs found in h-BN flakes are present at the edges or wrinkles. Here, we report on the room-temperature SPEs in h-BN film grown by molecular beam epitaxy followed by a high-temperature post-annealing process and their deterministic transfer. Using the all-dry viscoelastic stamping method, the h-BN film grown on the Al2O3 substrate can be transferred to other substrates. The transferred SPEs are discretely distributed among the continuous h-BN flakes, and the SPE density is as high as ∼0.17 μm−2. After identification, the determined SPE can be deterministically transferred to other structures by the all-dry transfer method. The deterministic transfer of SPEs distributed on the h-BN flakes promises the potential to integrate SPEs into many quantum technology applications.

High-purity single photons obtained with moderate-NA optics from SiV center in nanodiamonds on a bullseye antenna

Coherent exchange of single photons is at the heart of applied quantum optics. The negatively-charged silicon vacancy center in diamond is among most promising sources for coherent single photons. Its large Debye–Waller factor, short lifetime and extraordinary spectral stability is unique in the field of solid-state single photon sources. However, the excitation and detection of individual centers requires high numerical aperture (NA) optics which, combined with the need for cryogenic temperatures, puts technical overhead on experimental realizations. Here, we investigate a hybrid quantum photonics platform based on silicon-vacancy center in nanodiamonds and metallic bullseye antenna to realize a coherent single-photon resource that operates efficiently down to low NA optics with an inherent resistance to misalignment.

Magneto-optical spectra of the split nickel-vacancy defect in diamond

Nickel is a common impurity in high-pressure high-temperature diamond and may contaminate chemical vapor deposited diamond used for high-power electronics or quantum technology applications. Magneto-optical fingerprints of nickel have been known since decades, however, no consensus has been reached about the microscopic origins of nickel-related electron paramagnetic resonance, photoluminescence, and optically detected magnetic resonance spectra. The unknown nickel-related defect structures in diamond make it difficult to control them or harness them for a given application. As a consequence, nickel is considered as an impurity in diamond that should be avoided or its concentration should be minimized. Recent advances in the development of ab initio magneto-optical spectroscopy have significantly increased its accuracy and predictive power that can be employed for identification and in-depth characterization of paramagnetic color centers in diamond. In this study, we extend the accuracy of the ab initio magneto-optical spectroscopy tools towards self-consistent calculation of second-order spin-orbit coupling for paramagnetic color centers in solids. We apply the full arsenal of the ab initio magneto-optical spectroscopy tools to characterize the split nickel-vacancy defect in diamond which is one of the most stable nickel-related defect configurations. As a result, electron paramagnetic resonance and optical centers are positively identified in various charge states of the nickel-vacancy defect in diamond. In particular, the 1.40-eV optical center and the NIRIM-2 electron paramagnetic resonance center are identified as the single negative charge state of the split nickel-vacancy center. The defect possesses $S=\frac{1}{2}$ spin state with an orbital doublet ground state. We find that the coherence time of the ground-state spin is about 0.1 ms at cryogenic temperatures which can be optically initialized and readout by a $\mathrm{\ensuremath{\Lambda}}$-scheme protocol. Since the defect has inversion symmetry the optical signal is insensitive to the stray electric fields, which is an advantage for creating indistinguishable solid-state single-photon sources. We predict that the negatively charged nickel-vacancy defect has similar optical properties to those of the well-known silicon-vacancy defect in diamond but is superior in terms of electron spin coherence times. Our study resolves a few decades controversy about the nickel-related spectroscopy centers in diamond and turns nickel from an impurity to a resource in quantum technology applications.

Quantum Light Emission from Coupled Defect States in DNA-Functionalized Carbon Nanotubes.

Solid-state single-photon sources are essential building blocks for quantum photonics and quantum information technologies. This study demonstrates promising single-photon emission from quantum defects generated in single-wall carbon nanotubes (SWCNTs) by covalent reaction with guanine nucleotides in their single-stranded DNA coatings. Low-temperature photoluminescence spectroscopy and photon-correlation measurements on individual guanine-functionalized SWCNTs (GF-SWCNTs) indicate that multiple, closely spaced guanine defect sites within a single ssDNA strand collectively form an exciton trapping potential that supports a localized quantum state capable of room-temperature single-photon emission. In addition, exciton traps from adjacent ssDNA strands are weakly coupled to give cross-correlations between their separate photon emissions. Theoretical modeling identifies coupling mechanism as a capture of band-edge excitons. Because the spatial pattern of nanotube functionalization sites can be readily controlled by selecting ssDNA base sequences, GF-SWCNTs should become a versatile family of quantum light emitters with engineered properties.

(Invited) Quantum Light Emission from Coupled Defect-States in DNA-Functionalized Carbon Nanotubes

Solid-state single-photon sources are the essential building block for quantum photonics and quantum information technologies. The report here illustrates an advanced perspective of implanting coupled fluorescent quantum defects in single-wall carbon nanotubes (SWCNTs) through chemical reactions between the nanotube sidewall and the guanine nucleotides in single-stranded DNA (ssDNA) that coats the nanotube. Through low-temperature photoluminescence spectroscopy and photon correlation experiments, we have revealed that multiple guanine defects within a single ssDNA strand couple together to form a cumulative deep trapping potential supporting a localized quantum state that is capable of room-temperature single-photon emission. Helical wrapping of multiple ssDNA strands in single nanotubes allows a series of exciton localization sites to be created along the length of the SWCNTs. Such unique exciton trapping potential landscape gives rise to previously inaccessible intrinsic emission characteristics including coupling of multiple spatially separated quantum emitters. Our joint experimental and computational studies together reveal quantum emission properties from multiple spatially patterned covalent defects in SWCNTs, identify capture of the band-edge exciton as the underlying cause of the cross-correlated photon emission and open a distinctive prospect for tailoring chemically altered nanotubes as quantum light emitters.

Which plasmonic nano-antenna is best for free-space out-coupling of single photon emission from a hyperbolic metamaterial resonator?

Recently, hyperbolic metamaterials (HMMs) have shown large emission-rate/Purcell enhancement for emitters coupled to them. However, due to the large momentum ($ k $) mismatch between the high-k hyperbolic modes of HMMs and free-space modes, the far-field out-coupling of the emission is limited and requires an antenna. In this work, we present an in-depth theoretical study of the performance of some commonly known plasmonic antennas-cylindrical, cuboid, crossed, and bow-tie-when coupled to a HMM. Of all these antennas, the cylindrical antenna was observed to be the best for out-coupling to the high-$ k $ HMM modes with the Purcell factor and collection efficiencies reaching 1000 and 0.5, respectively. The hyperbolic HMM modes are observed to be efficiently coupled to the resonance modes of the cylindrical antenna, with the antenna modes getting effectively out-coupled into free space. These values are expected to result in two to three orders of fluorescence enhancement from a solid-state single photon source. For other antennas, the greater momentum mismatch between the hyperbolic HMM modes and the antenna modes resulted in relatively much weaker free-space out-coupling.

Bright solid-state sources for single photons with orbital angular momentum.

Photons that have a helical phase front, that is, twisted photons, can carry a discrete, in principle, unlimited, but quantized amount of orbital angular momentum (OAM). Hence, twisted single photons constitute a high-dimensional quantum system with information-processing abilities beyond those of two-level single-photon qubits. To date, the generation of single photons carrying OAM has relied on a non-linear process in bulk crystals, for example, spontaneous parametric down-conversion, which limits both the efficiency and the scalability of the source. Here, we present a bright solid-state source of single photons in an OAM superposition state with a single-photon purity of g(2)(0) = 0.115(1) and a collection efficiency of 23(4)%. The mode purity of the single-photon OAM states is further examined via projection measurements. Future developments of integrated quantum photonic devices with pure OAM states as an additional degree of freedom may enable high-dimensional quantum information processing.

Hyperbolic metamaterial-based metal–dielectric resonator-antenna designs for GHz photon collection rates from wide-range solid-state single-photon sources

Solid-state single-photon sources (SPS) based on quantum dots as well as color centers in diamonds and silicon-carbide have promise for application in emerging quantum technologies. Many of these technologies, however, demand photon rates in the GHz range, thereby hindering the use of these SPS, for which the maximum observed count rates are limited to a few tens of MHz. Here we first study the performance of hyperbolic metamaterial-based 5-layered metal–dielectric resonator antenna structures with metallic as well as hybrid metal–dielectric antennas in the wavelength range of 600 to 1000 nm. The performance of these resonator-antenna structures was analyzed for the Purcell enhancement, quantum efficiency (QE), collection efficiency (CE), and normalized collected photon counts (NCPC). The hybrid metal–dielectric antenna helps in providing the directivity to the dipole emission, thereby significantly improving the collection efficiency. We then present the novel design of a 5-layered metal–dielectric pillar resonator. This resonator structure with a metallic cylindrical antenna over the top showed significantly large fluorescence enhancement values. The Purcell factor was observed to reach close to 1600 at 680 nm corresponding to the central peak of the nitrogen vacancy center spectrum. The NCPC value reached close to 550 at 680 nm. The maximum CE from the structure was observed to be around 60%, with the maximum QE reaching close to 80%. With the above performance, the detected photon count rates for a solid-state SPS is expected to be well into the GHz range. Our designs show a state-of-the-art improvement in the antenna performance for SPS with properties very close to a practical SPS.

Multiphoton Graph States from a Solid-State Single-Photon Source

This work was supported by the National Natural Science Foundation of China (Grants No. 11575174, No. 11674308, No. 11704424, and No. 11774326), the Chinese Academy of Sciences, and the National Key Research and Development Program of China.