Single-photon Sources Research Articles

Telecom‐Band Quantum Dots Compatible with Silicon Photonics for Photonic Quantum Applications

AbstractSilicon photonics is promising for quantum photonics applications owing to its large‐scale and high‐performance circuitry enabled by complementary‐metal‐oxide‐semiconductor fabrication processes. However, there is a lack of bright single‐photon sources (SPSs) capable of deterministic operation on Si platforms, which largely limits their applications. To this end, on‐Si integration of high‐performance solid‐state quantum emitters, such as semiconductor quantum dots (QDs), is greatly desired. In particular, it is preferable to integrate SPSs emitting at telecom wavelengths for fully leveraging the power of silicon photonics, including efficient chip‐to‐fiber coupling. In this review, recent progress and challenges in the integration of telecom QD SPSs onto silicon photonic platforms are discussed.

Single-photon emission from silicon-vacancy color centers in polycrystalline diamond membranes

Single-color centers in thin polycrystalline diamond membranes allow the platform to be used in integrated quantum photonics, hybrid quantum systems, and other complex functional materials. While single-crystal diamond membranes are still technologically challenging to fabricate as they cannot be grown on a non-diamond substrate, free-standing polycrystalline diamond membranes can be conveniently fabricated at large-scale from nanocrystalline diamond seeds on a substrate that can be selectively etched. However, their practical application for quantum photonics is so far limited by crystallographic defects, impurities, graphitic grain boundaries, small grain sizes, scattering loss, and strain. In this paper, we report on a single-photon source based on silicon-vacancy color centers in a polycrystalline diamond membrane. We discuss the spectroscopic approach and quantify the photon statistics, obtaining a g2(0) ≈ 0.04. Our findings hold promise for introducing polycrystalline diamond to quantum photonics and hybrid quantum systems.

Purcell-enhanced single photons at telecom wavelengths from a quantum dot in a photonic crystal cavity

Quantum dots are promising candidates for telecom single photon sources due to their tunable emission across the different low-loss telecommunications bands, making them compatible with existing fiber networks. Their suitability for integration into photonic structures allows for enhanced brightness through the Purcell effect, supporting efficient quantum communication technologies. Our work focuses on InAs/InP QDs created via droplet epitaxy MOVPE to operate within the telecoms C-band. We observe a short radiative lifetime of 340 ps, arising from a Purcell factor of 5, owing to integration of the QD within a low-mode-volume photonic crystal cavity. Through in-situ control of the sample temperature, we show both temperature tuning of the QD’s emission wavelength and a preserved single photon emission purity at temperatures up to 25K. These findings suggest the viability of QD-based, cryogen-free C-band single photon sources, supporting applicability in quantum communication technologies.

One (photon), two(-dimensional crystals), a lot (of potential): a quick snapshot of a rapidly evolving field

Abstract We present a concise overview of the state of affairs in the development of single-photon sources based on two-dimensional (2D) crystals, focusing in particular on transition-metal dichalcogenides and hexagonal boron nitride. We briefly discuss the current level of advancement (i) in our understanding of the microscopic origin of the quantum emitters (QEs) identified in these two material systems, and (ii) in the characterisation of the optical properties of these emitters; then, we survey the main methods developed to enable the dynamic control of the QEs’ emission energy. Finally, we summarise the main results stemming from the coupling of QEs embedded in 2D materials with photonic and plasmonic structures.

Machine learning enhanced evaluation of semiconductor quantum dots

A key challenge in quantum photonics today is the efficient and on-demand generation of high-quality single photons and entangled photon pairs. In this regard, one of the most promising types of emitters are semiconductor quantum dots, fluorescent nanostructures also described as artificial atoms. The main technological challenge in upscaling to an industrial level is the typically random spatial and spectral distribution in their growth. Furthermore, depending on the intended application, different requirements are imposed on a quantum dot, which are reflected in its spectral properties. Given that an in-depth suitability analysis is lengthy and costly, it is common practice to pre-select promising candidate quantum dots using their emission spectrum. Currently, this is done by hand. Therefore, to automate and expedite this process, in this paper, we propose a data-driven machine-learning-based method of evaluating the applicability of a semiconductor quantum dot as single photon source. For this, first, a minimally redundant, but maximally relevant feature representation for quantum dot emission spectra is derived by combining conventional spectral analysis with an autoencoding convolutional neural network. The obtained feature vector is subsequently used as input to a neural network regression model, which is specifically designed to not only return a rating score, gauging the technical suitability of a quantum dot, but also a measure of confidence for its evaluation. For training and testing, a large dataset of self-assembled InAs/GaAs semiconductor quantum dot emission spectra is used, partially labelled by a team of experts in the field. Overall, highly convincing results are achieved, as quantum dots are reliably evaluated correctly. Note, that the presented methodology can account for different spectral requirements and is applicable regardless of the underlying photonic structure, fabrication method and material composition. We therefore consider it the first step towards a fully integrated evaluation framework for quantum dots, proving the use of machine learning beneficial in the advancement of future quantum technologies.

Solid-State Color Centers for Single-Photon Generation

Single-photon sources are important for integrated photonics and quantum technologies, and can be used in quantum key distribution, quantum computing, and sensing. Color centers in the solid state are a promising candidate for the development of the next generation of single-photon sources integrated in quantum photonics devices. They are point defects in a crystal lattice that absorb and emit light at given wavelengths and can emit single photons with high efficiency. The landscape of color centers has changed abruptly in recent years, with the identification of a wider set of color centers and the emergence of new solid-state platforms for room-temperature single-photon generation. This review discusses the emerging material platforms hosting single-photon-emitting color centers, with an emphasis on their potential for the development of integrated optical circuits for quantum photonics.

Single-photon generation from a neodymium ion in optical fiber at room temperature

The realization of single-photon generation is important for implementing various quantum information technologies. The use of rare-earth ions in an optical fiber is a promising single-photon generation method due to its ability to operate at room temperature as well as the low cost involved. Neodymium ions are especially interesting because the ions are one of the most commercially affordable rare-earth materials in the current industry. The neodymium ion also has the advantage of having a rich energy level structure, which offers several possible wavelengths for emitted single photons from visible to near-telecommunication wavelengths. In this paper, we experimentally demonstrated single-photon generation using an isolated single neodymium ion in tapered silica fiber at room temperature. Our results have significant implications as a platform for low-cost wavelength-selectable single-photon sources and photonic quantum applications.

Fluorescence Enhancement in Topologically Optimized Gallium Phosphide All-Dielectric Nanoantennas.

Nanoantennas capable of large fluorescence enhancement with minimal absorption are crucial for future optical technologies from single-photon sources to biosensing. Efficient dielectric nanoantennas have been designed, however, evaluating their performance at the individual emitter level is challenging due to the complexity of combining high-resolution nanofabrication, spectroscopy and nanoscale positioning of the emitter. Here, we study the fluorescence enhancement in infinity-shaped gallium phosphide (GaP) nanoantennas based on a topologically optimized design. Using fluorescence correlation spectroscopy (FCS), we probe the nanoantennas enhancement factor and observe an average of 63-fold fluorescence brightness enhancement with a maximum of 93-fold for dye molecules in nanogaps between 20 and 50 nm. The experimentally determined fluorescence enhancement of the nanoantennas is confirmed by numerical simulations of the local density of optical states (LDOS). Furthermore, we show that beyond design optimization of dielectric nanoantennas, increased performances can be achieved via tailoring of nanoantenna fabrication.

Characterization of nanowire light-emitting diodes with InP/InAsP heterostructures emitting in telecom band

We report the growth and characterization of InP/InAsP/InP nanowires (NWs) and NW LEDs (NW-LEDs), which emit light at telecom wavelengths. InP-based NWs were grown by selective-area metal-organic vapor-phase epitaxy, and a thin InAsP layer was embedded in the NWs. The NW exhibited emission lines in their low-temperature photoluminescence spectra, suggesting the formation of quantum dots (QDs) in the NW. NW-LED operation was demonstrated at both room and low temperatures in the telecom band, but it was found that the emission wavelength range and blueshift behavior induced by current injection differed considerably between room and low temperatures. Our results suggest that an efficient path for carrier injection into the active InAsP layer should be explored for NW-QD-based single-photon sources operating via current-injection.

Scalable Bright and Pure Single Photon Sources by Droplet Epitaxy on InP Nanowire Arrays.

High-quality quantum light sources are crucial components for the implementation of practical and reliable quantum technologies. The persistent challenge, however, is the lack of scalable and deterministic single photon sources that can be synthesized reproducibly. Here, we present a combination of droplet epitaxy with selective area epitaxy to realize the deterministic growth of single quantum dots in nanowire arrays. By optimization of the single quantum dot growth and the nanowire cavity design, single emissions are effectively coupled with the dominant mode of the nanowires to realize Purcell enhancement. The resonance-enhanced quantum emitter system boasts a brightness of millions of counts per second with nanowatt excitation power, a short radiation lifetime of 350 ± 5 ps, and a high single-photon purity with g(2)(0) value of 0.05 with continuous wave above-band excitation. Finite-difference time-domain (FDTD) simulation results show that the emissions of single quantum dots are coupled into the TM01 mode of the nanowires, giving a Purcell factor ≈ 3. Our technology can be used for creating on-chip scalable single photon sources for future quantum technology applications including quantum networks, quantum computation, and quantum imaging.

Perfect single-photon sources

We introduce the gapped coherent state in the form of a single-photon source (SPS) that consists of uncorrelated photons as a background, except that we demand that no two photons can be closer in time than a time gap tG\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$t_\ ext{G}$$\\end{document}. While no obvious quantum mechanism is yet identified to produce exactly such a photon stream, a numerical simulation is easily achieved by first generating an uncorrelated (Poissonian) signal and then for each photon in the list, either adding such a time gap or removing all photons that are closer in time from each other than tG\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$t_\ ext{G}$$\\end{document}. We study the statistical properties of such a hypothetical signal, which exhibits counter-intuitive features. This provides a neat and natural connection between continuous-wave (stationary) and pulsed single-photon sources, with also a bearing on what it means for such sources to be perfect in terms of single-photon emission.

Tunable quantum dots in monolithic Fabry-Perot microcavities for high-performance single-photon sources

Cavity-enhanced single quantum dots (QDs) are the main approach towards ultra-high-performance solid-state quantum light sources for scalable photonic quantum technologies. Nevertheless, harnessing the Purcell effect requires precise spectral and spatial alignment of the QDs’ emission with the cavity mode, which is challenging for most cavities. Here we have successfully integrated miniaturized Fabry-Perot microcavities with a piezoelectric actuator, and demonstrated a bright single-photon source derived from a deterministically coupled QD within this microcavity. Leveraging the cavity-membrane structures, we have achieved large spectral tunability via strain tuning. On resonance, a high Purcell factor of ~9 is attained. The source delivers single photons with simultaneous high extraction efficiency of 0.58, high purity of 0.956(2) and high indistinguishability of 0.922(4). Together with its compact footprint, our scheme facilitates the scalable integration of indistinguishable quantum light sources on-chip, therefore removing a major barrier to the development of solid-state quantum information platforms based on QDs.

Polarized and Bright Telecom C-Band Single-Photon Source from InP-Based Quantum Dots Coupled to Elliptical Bragg Gratings.

Bright, polarized, and high-purity single-photon sources in telecom wavelengths are crucial components in long-distance quantum communication, optical quantum computation, and quantum networks. Semiconductor InAs/InP quantum dots (QDs) combined with photonic cavities provide a competitive path, leading to optimal single-photon sources in this range. Here, we demonstrate a bright and polarized single-photon source operating in the telecom C-band based on an elliptical Bragg grating (EBG) cavity. With a significant Purcell enhancement of 5.25 ± 0.05, the device achieves a polarization ratio of 0.986, a single-photon purity of g2(0) = 0.078 ± 0.016, and a single-polarized photon collection efficiency of ∼24% at the first lens (NA = 0.65) without blinking. These findings suggest that C-band QD-based single-photon sources are potential candidates for advancing quantum communication.

Experimental realization of deep sub-wavelength confinement of light in a topology-optimized InP nanocavity

We experimentally demonstrate an InP nanocavity with a mode volume of 0.26 (λ/2n)3. This is an order of magnitude smaller than the mode volumes previously demonstrated in photonic crystal point-defect cavities realized in III-V materials and four times smaller than what is often referred to as the diffraction-limited volume, Vλ= (λ/2n)3. The nanocavity is designed using topology optimization, taking into account fabrication limitations, which are pushed compared to the state-of-the-art. This work thus introduces a new class of cavities featuring extreme dielectric confinement (EDC) into the realm of III-V semiconductors, offering order-of-magnitude Purcell-enhancement of the radiative rate. EDC nanocavities may thus be employed to significantly improve the properties of nanolasers, nanoLEDs and single-photon sources, among other applications.

Topological single-photon emission from quantum emitter chains

We propose a scheme for generating highly indistinguishable single photons from an active quantum Su-Schrieffer-Heeger chain composed of a collection of noisy quantum emitters. Strikingly, the single photon emission spectrum of the active quantum chain is exceedingly narrow relative to that of a single emitter or a topologically trivial chain. Furthermore, this effect is amplified dramatically in proximity to the non-trivial-to-trivial phase transition point. Exploiting this effect, we demonstrate that the single-photon linewidth of a long topological quantum chain can be arbitrarily reduced, rendering it an ideal source of indistinguishable single photons. Finally, by analyzing the most critical parameters concerning experimental realization and providing a microscopic and quantitative analysis of our model, we take concrete examples of actual quantum emitters to establish the viability of our proposal.

Generation and characterization of polarization-entangled states using quantum dot single-photon sources

Single-photon sources based on semiconductor quantum dots find several applications in quantum information processing due to their high single-photon indistinguishability, on-demand generation, and low multiphoton emission. In this context, the generation of entangled photons represents a challenging task with a possible solution relying on the interference in probabilistic gates of identical photons emitted at different pulses from the same source. In this work, we implement this approach via a simple and compact design that generates entangled photon pairs in the polarization degree of freedom. We operate the proposed platform with single photons produced through two different pumping schemes, the resonant excited one and the longitudinal-acoustic phonon-assisted configuration. We then characterize the produced entangled two-photon states by developing a complete model taking into account relevant experimental parameters, such as the second-order correlation function, Hong–Ou–Mandel visibility, multiphoton emission and pump laser filtering. Our source shows long-term stability and high quality of the generated entangled states, thus constituting a reliable building block for optical quantum technologies.

Broadband enhancement and high extraction efficiency from single quantum dots embedded in microlenses coupled with a gold mirror

We present a broadband enhanced single photon source device of Gaussian-shaped GaAs microlens (ML) arrays coupled with a gold bottom mirror hybrid structure. The extrapolated light extraction efficiency of (60.6 ± 1.5)% at maximum (for collection numerical aperture = 0.45 and based on the an actually measured raw detection efficiency of ∼0.775%) and a broad spectral band of 890–960 nm were obtained. The ML-Au structure with embedded epitaxial quantum dots exhibits a high single-photon purity of (96.6 ± 0.7)% [g(2)(0) = 0.034 ± 0.003] under pulsed excitation with 20% saturation. Wet-etched microlenses with smooth surfaces achieve simultaneous enhancement of both XX and X exciton emissions, which is prerequisite for promising entangled photon pair generation. This work proposes the bright potential of integrated hybrid structures to enable further photon pair entanglement applications.

Single photon emission in the telecom C-band from nanowire-based quantum dots

Single photon sources operating on-demand at telecom wavelengths are required in fiber-based quantum secure communication technologies. In this work, we demonstrate single photon emission from position-controlled nanowire quantum dots emitting at λ>1530 nm. Emission in the C-band is achieved by composition engineering of an InAsxP1−x dot-in-a-rod structure. Using above-band pulsed excitation, we obtain single photon purities of g(2)(0)=0.062. These results represent an important step toward the scalable manufacture of high efficiency, high rate single photon emitters in the telecom C-band.

QUICK3$^3$ ‐ Design of a Satellite‐Based Quantum Light Source for Quantum Communication and Extended Physical Theory Tests in Space

AbstractModern quantum technologies have matured such that they can now be used in space applications, e.g., long‐distance quantum communication. Here, the design of a compact true single photon source is presented that can enhance the secure data rates in satellite‐based quantum key distribution scenarios compared to conventional laser‐based light sources. The quantum light source is a fluorescent color center in hexagonal boron nitride. The emitter is off‐resonantly excited by a diode laser and directly coupled to an integrated photonic processor that routes the photons to different experiments performed directly on‐chip: i) the characterization of the single photon source and ii) testing a fundamental postulate of quantum mechanics, namely the relation of the probability density and the wave function (known as Born's rule). The described payload is currently being integrated into a 3U CubeSat and scheduled to launch in 2024 into low Earth orbit. Therefore the feasibility of true single photon sources and reconfigurable photonic circuits in space can be evaluated. This provides a promising route toward a high‐speed quantum network.

Combination of XEOL, TR-XEOL and HB-T interferometer at the TPS 23A X-ray nanoprobe for exploring quantum materials.

In this study, a combination of X-ray excited optical luminescence (XEOL), time-resolved XEOL (TR-XEOL) and the Hanbury-Brown and Twiss (HB-T) interferometer at the Taiwan Photon Source (TPS) 23A X-ray nanoprobe beamline for exploring quantum materials is demonstrated. On the basis of the excellent spatial resolution rendered using a nano-focused beam, emission distributions of artificial micro-diamonds can be obtained by XEOL maps, and featured emission peaks of a selected local area can be obtained by XEOL spectra. The hybrid bunch mode of the TPS not only provides a sufficiently high peak power density for experiments at each beamline but also permits high-quality temporal domain (∼200 ns) measurements for investigating luminescence dynamics. From TR-XEOL measurements, the decay lifetime of micro-diamonds is determined to be approximately 16 ns. Furthermore, the XEOL spectra of artificial micro-diamonds can be investigated by the HB-T interferometer to identify properties of single-photon sources. The unprecedented strategy of combining XEOL, TR-XEOL and the HB-T interferometer at the X-ray nanoprobe beamline will open new avenues with significant characterization abilities for unraveling the emission mechanisms of single-photon sources for quantum materials.