High-performance semiconductor quantum-dot single-photon sources

Controlling Quantum-Confined Stark Effect in Coupled II-VI Quantum Dots by Interface Engineering

Electric-field tuning of excitonic states in confined systems via the quantum-confined Stark effect (QCSE) provides a flexible way for electro-optic modulation with great efficiency. In epitaxial and colloidal quantum dots (QDs), the interdot coupling allows additional degrees of freedom for optical switches. Here QCSE is explored in artificial molecules formed by two coupled QDs of main group II-VI elements. Compared with a single QD, the QCSE is remarkably enhanced in QD molecules and highly tunable by the interdot coupling strength as well as by homo- and hetero-dimerization of QDs. In addition, the strong coupling between QDs can retard charge separation under an external electric field and even bring the electron and hole states from two QDs into a resonance, thereby boosting the fluorescence emission in QD molecules. These mechanistic understandings provide vital guidelines for fine manipulation of electron, spin, and exciton in coupled QDs and their assemblies for tunable optoelectronics, photonics, and quantum information applications.

Opportunities and Challenges of Solid-State Quantum Nonlinear Optics

Nonlinear interactions between photons are fundamentally weak as the photons do not interact directly with each other, and any interaction is mediated by matter. This has motivated researchers over many decades to search for strongly nonlinear materials (by controlling electronic properties) and optical resonators with strong spatial and temporal confinement of light. An extreme form of nonlinear optics is quantum nonlinear optics, where we can realize nonlinear interaction between single photons. Such quantum nonlinear optics is at the heart of any photonic quantum information system including analog quantum simulation and fault-tolerant quantum computing. While engineering light-matter interactions can effectively create photon-photon interactions, the required photon number to observe any nonlinearity are normally very high, where any quantum-mechanical signature disappears. However, with emerging low-dimensional materials and engineered photonic resonators, the photon number can be reduced to reach the quantum nonlinear optical regime. In this review paper, we discuss different mechanisms exploited in solid-state platforms to attain quantum nonlinear optics. We review emerging materials and optical resonator architectures with different dimensionalities. We also present future research directions and open problems in this field.

Deterministic resonance fluorescence improvement of single quantum dots by optimized surface passivation

The degradation caused by surface states restricts the performance of near-surface semiconductor quantum dots (QDs). Here, we demonstrate optimized passivation techniques to improve the resonance fluorescence (RF) with dot-to-dot comparisons. These optimized techniques, for the first time, reduce the linewidth and noise level of existing pulsed-RF signals, as well as revive pulsed-RF signals which originally are vanishing. The improvements are confirmed to originate from reduced surface state density and electric field after passivation, through optical and surface science characterizations. Our study promotes applications of the passivation techniques in thin-film quantum devices, paving the way for the further development of optimal QD-based quantum light sources.

Stimulated downconversion of single-photon emission in a quantum dot placed in a target-frequency microcavity

We propose efficient frequency-tunable single-photon sources using stimulated frequency downconversion occurring directly in the quantum dot (QD) inside a microcavity. This microcavity should be designed not for the exciton frequency in the QD but for the target frequency that is set by the difference between the exciton resonance and the stimulating laser energies. This frequency downconversion arises from the second-order nonlinear interaction of an exciton (bright or dark) and a light-hole exciton in the stimulating laser field. We present an analytical model for such a process and evaluate its efficiency for a widely sought-after single-photon source for the C-band telecommunications.

Room-Temperature Efficient Single-Photon Generation from CdSe/ZnS Nanoplatelets

In the search for materials for quantum information science applications, colloidal semiconductor nanoplatelets (NPLs) have emerged as a highly promising class of materials due to their interesting optical properties, such as narrow emission line widths and fast photoluminescence (PL) lifetimes at room temperature. So far, only a few works focused on the quantum properties of their emission; however, NPLs, with their atomic-scale thickness and one-dimensional quantum confinement, are promising candidates for single-photon sources. Here, we demonstrate room-temperature single-photon emission from core/shell CdSe/ZnS NPLs, which feature an 8 × 20 nm2 surface area and 1 nm shell. The limited surface area ensures effective Auger nonradiative recombination, resulting in highly efficient single-photon generation with values of photon purity as low as g(2)(0) = 0.04. The observed long-period blinking and bleaching typical of such thin shells can be easily reduced by increasing the shell thickness. This work establishes NPLs as single-photon sources that are very well suited for integration into quantum photonic systems.

High-purity and stable single-photon emission in bilayer WSe2 via phonon-assisted excitation

The excitation scheme is essential for single-photon sources, as it governs exciton preparation, decay dynamics, and the spectral diffusion of emitted photons. While phonon-assisted excitation has shown promise in other quantum emitter platforms, its proper implementation and systematic comparison with alternative excitation schemes have not yet been demonstrated in transition metal dichalcogenide (TMD) quantum emitters. Here, we investigate the impact of various optical excitation strategies on the single-photon emission properties of bilayer WSe2 quantum emitters. Based on our theoretical predictions for the exciton preparation fidelity, we compare the excitation via the longitudinal acoustic and breathing phonon modes to conventional above-band and near-resonance excitations. Under acoustic phonon-assisted excitation, we achieve narrow single-photon emission with a reduced spectral diffusion of 0.0129 nm, a 1.8-fold improvement over above-band excitation. Additionally, excitation through breathing-phonon mode yields a high purity of 0.947 ± 0.079  and reduces the decay time by over an order of magnitude, reaching (1.33 ± 0.04) ns. Our comprehensive study demonstrates the crucial role of phonon-assisted excitation in optimizing the performance of WSe2-based quantum emitters, providing valuable insights for the development of single-photon sources for quantum photonics applications.

Single photon generation from quantum dots: recent advances, challenges and future directions

A single photon source (SPS) is a device designed to emit photons, one at a time, enabling precise control over quantum states, unlike classical light sources that produce interfering streams. This capability is crucial for secure communication protocols such as quantum key distribution and for quantum networks, where single photons act as carriers of quantum information. Recent advancements have led to various SPS technologies, including quantum dots (QDs), atom-like emitters, and color centers in diamonds. Among these, QDs, semiconductor nanocrystals, have gained significant attention due to their unique optical and electronic properties derived from quantum confinement effects. They offer size-dependent tuning of emission wavelengths, high photoluminescence efficiency, and discrete energy levels, making them ideal for single photon applications while exhibiting scalability and low background noise. This review provides a comprehensive overview of recent advancements in quantum dot-based SPSs operating at room temperature, highlighting their optical properties, essential performance metrics, and the latest developments in single photon generation. It also discusses strategies to mitigate blinking and improve photon statistics through techniques such as plasmonic nanocavity and ligand exchange. The review concludes by outlining the challenges faced in the field and discussing potential solutions.

Van der Waals quantum dots on layered hexagonal boron nitride

Semiconductor quantum dots (QD) promise unique electronic, optical, and chemical properties, which can be exquisitely tuned by controlling the composition, size, and morphology. Semiconductor QDs have been synthesized primarily via two approaches, namely, epitaxial growth and wet-chemical synthesis. However, the properties of epitaxial QDs (eQDs) are susceptible to wetting layer formation and substrate dislocations, while colloidal QDs (cQDs) face fluorescence intermittency issues. Here, we report on the synthesis of a class of QDs that can overcome the fundamental limitations of eQDs and cQDs. By exploiting the sp2 bonding of layered hexagonal boron nitride (hBN), we show that GaN QDs can be epitaxially grown through a weak van der Waals (vdW) interaction without two-dimensional wetting layer formation. The photoluminescence intensity of GaN van der Waals quantum dots (vQDs) is more than six times stronger than that of conventional GaN eQDs and no optical blinking was observed from vQDs. We show that the interadatom bond strength is about one order of magnitude stronger compared with that between the adatoms and the hBN substrate. This work shows that vQDs have unique properties that are difficult to achieve using existing QDs synthesis methods and thus can potentially enable new classes of high-performance optoelectronic and quantum devices.

A hybrid single quantum dot coupled cavity on a CMOS-compatible SiC photonic chip for Purcell-enhanced deterministic single-photon emission

The ability to control nonclassical light emission from a single quantum emitter by an integrated cavity may unleash new perspectives for integrated photonic quantum applications. However, coupling a single quantum emitter to cavity within photonic circuitry towards creation of the Purcell-enhanced single-photon emission is elusive due to the complexity of integrating active devices in low-loss photonic circuits. Here we demonstrate a hybrid micro-ring resonator (HMRR) coupled with self-assembled quantum dots (QDs) for cavity-enhanced deterministic single-photon emission. The HMRR cavity supports whispering-gallery modes with quality factors up to 7.8×103. By further introducing a micro-heater, we show that the photon emission of QDs can be locally and dynamically tuned over one free spectral ranges of the HMRR ( ~ 4 nm). This allows precise tuning of individual QDs in resonance with the cavity modes, thereby enhancing single-photon emission with a Purcell factor of about 4.9. Our results on the hybrid integrated cavities coupled with two-level quantum emitters emerge as promising devices for chip-based scalable photonic quantum applications.

Shutter-Synchronized Molecular Beam Epitaxy for Wafer-Scale Homogeneous GaAs and Telecom Wavelength Quantum Emitter Growth

Quantum dot (QD)-based single-photon emitter devices today are based on self-assembled random position nucleated QDs emitting at random wavelengths. Deterministic QD growth in position and emitter wavelength would be highly appreciated for industry-scale high-yield device manufacturing from wafers. Local droplet etching during molecular beam epitaxy is an all in situ method that allows excellent density control and predetermines the nucleation site of quantum dots. This method can produce strain-free GaAs QDs with excellent photonic and spin properties. Here, we focus on the emitter wavelength homogeneity. By wafer rotation-synchronized shutter opening time and adapted growth parameters, we grow QDs with a narrow peak emission wavelength homogeneity with no more than 1.2 nm shifts on a 45 mm diameter area and a narrow inhomogeneous ensemble broadening of only 2 nm at 4 K. The emission wavelength of these strain-free GaAs QDs is <800 nm, attractive for quantum optics experiments and quantum memory applications. We can use a similar random local droplet nucleation, nanohole drilling, and now, InAs infilling to produce QDs emitting in the telecommunication optical fiber transparency window around 1.3 µm, the so-called O-band. For this approach, we demonstrate good wavelength homogeneity and excellent density homogeneity beyond the possibilities of standard Stranski-Krastanov self-assembly. We discuss our methodology, structural and optical properties, and limitations set by our current setup capabilities.

One-Step Epitaxy of Transition Metal Nitride Nanopillar Arrays with an Oxidation-Tunable Hyperbolic Dispersion

The controllable synthesis of epitaxial nanopillar arrays is fundamentally important to the development of advanced electrical and optical devices. However, this fascinating growth method has rarely been applied to the bottom-up synthesis of plasmonic nanostructure arrays (PNAs) with many broad, important, and promising applications in optical sensing, nonlinear optics, surface-enhanced spectroscopies, photothermal conversion, photochemistry, etc. Here, a one-step epitaxial approach to single-crystalline Nb0.25Ti0.75N (NbTiN) nanopillar arrays based on the layer plus island growth mode is demonstrated by strain engineering. Strikingly, the epitaxial NbTiN nanopillar arrays with high density (>1012/cm2) and sub-10 nm lateral sizes show a plasmonic hyperbolic dispersion in visible to near-infrared ranges and polarization-dependent light absorption behaviors, which can be well-simulated by theoretical modeling. Moreover, the nanovoids between NbTiN nanopillars enable an oxidation-tunable hyperbolic dispersion, which is hardly achieved in noble metal PNAs. This study paves a novel, easy, and scalable route to single-crystalline PNAs with oxidation-tunable plasmonic hyperbolic dispersions, which is beneficial to the development of low-cost and tunable electrical, optical, and electrocatalytic devices based on the PNAs.

Tunable Quantum Confinement in Individual Nanoscale Quantum Dots via Interfacial Engineering

Introducing quantum confinement has shown promise to enable control of charge carriers. Although recent advances make it possible to realize confinement from semiclassical regime to quantum regime, achieving control of electronic potentials in individual nanoscale quantum dots (QDs) has remained challenging. Here, we demonstrate the ability to tune quantum confined states in individual nanoscale graphene QDs, which are realized by inserting nanoscale monolayer WSe2 islands in graphene/WSe2 heterostructures via interfacial engineering. Our experiment indicates that scanning tunneling microscope (STM) tip pulses can trigger a local phase transition in the interfacial nanoscale WSe2 islands, which, in turn, enables us to tune discrete quantum states in individual graphene QDs. By using a STM tip, we can also generate one-dimensional (1D) position-tunable domain boundaries in the WSe2 islands. The 1D boundary introduces atomically wide electrostatic barriers that bifurcate quasibound states into two regions in the graphene QD, changing the QD from a relativistic artificial atom to a relativistic artificial molecule.

Local Strain Engineering of Two-Dimensional Transition Metal Dichalcogenides Towards Quantum Emitters

Two-dimensional transition metal dichalcogenides (2D TMDCs) have received considerable attention in local strain engineering due to their extraordinary mechanical flexibility, electonic structure, and optical properties. The strain-induced out-of-plane deformations in 2D TMDCs lead to diverse excitonic behaviors and versatile modulations in optical properties, paving the way for the development of advanced quantum technologies, flexible optoelectronic materials, and straintronic devices. Research on local strain engineering on 2D TMDCs has been delved into fabrication techniques, electronic state variations, and quantum optical applications. This review begins by summarizing the state-of-the-art methods for introducing local strain into 2D TMDCs, followed by an exploration of the impact of local strain engineering on optical properties. The intriguing phenomena resulting from local strain, such as exciton funnelling and anti-funnelling, are also discussed. We then shift the focus to the application of locally strained 2D TMDCs as quantum emitters, with various strategies outlined for modulating the properties of TMDC-based quantum emitters. Finally, we discuss the remaining questions in this field and provide an outlook on the future of local strain engineering on 2D TMDCs.

Semiconductor nanowire heterodimensional structures toward advanced optoelectronic devices

Semiconductor nanowires are considered as one of the most promising candidates for next-generation devices due to their unique quasi-one-dimensional structures and novel physical properties. In recent years, advanced heterostructures have been developed by combining nanowires with low-dimensional structures such as quantum wells, quantum dots, and two-dimensional materials. Those heterodimensional structures overcome the limitations of homogeneous nanowires and show great potential in high-performance nano-optoelectronic devices. In this review, we summarize and discuss recent advances in fabrication, properties and applications of nanowire heterodimensional structures. Major heterodimensional structures including nanowire/quantum well, nanowire/quantum dot, and nanowire/2D-material are studied. Representative optoelectronic devices including lasers, single photon sources, light emitting diodes, photodetectors, and solar cells are introduced in detail. Related prospects and challenges are also discussed.

Broad Range Tuning of InAs Quantum Dot Emission for Nanophotonic Devices in the Telecommunication Bands

InAs semiconductor quantum dots (QDs) emitting in the near-infrared are promising platforms for on-demand single-photon sources and spin-photon interfaces. However, the realization of quantum-photonic nanodevices emitting in the telecom windows with similar performance remains an open challenge. In particular, nanophotonic devices incorporating quantum light emitting diodes in the telecom C-band based on GaAs substrates are still lacking due to the relaxation of the lattice constant along the InGaAs graded layer which makes the implementation of electrically contacted devices challenging. Here, we report an optimized heterostructure design for QDs emitting in the telecom O- and C-bands grown by means of molecular beam epitaxy. The InAs QDs are embedded in mostly relaxed InGaAs matrices with fixed indium content grown on top of compositionally graded InGaAs buffers. Reciprocal space maps of the indium profiles and optical absorption spectra are used to optimize In0.22Ga0.78As and In0.30Ga0.70As matrices, accounting for the chosen indium grading profile. This approach results in a tunable QD photoluminescence (PL) emission from 1200 up to 1600 nm. Power and polarization dependent micro-PL measurements performed at 4 K reveal exciton-biexciton complexes from quantum dots emitting in the telecom O- and C-bands. The presented study establishes a flexible platform that can be an essential component for advanced photonic devices based on InAs/GaAs that serve as building blocks for future quantum networks.

The Photophysics of Perovskite Emitters: from Ensemble to Single Particle

Halide perovskite emitters are a groundbreaking class of optoelectronic materials possessing remarkable photophysical properties for diverse applications. In perovskite light emitting devices, they have achieved external quantum efficiencies exceeding 28%, showcasing their potential for next-generation solid-state lighting and ultra high definition displays. Furthermore, the demonstration of room temperature continuous-wave perovskite lasing underscores their potential for integrated optoelectronics. Of late, perovskite emitters are also found to exhibit desirable single-photon emission characteristics as well as superfluorescence or superradiance phenomena for quantum optics. With progressive advances in synthesis, surface engineering, and encapsulation, halide perovskite emitters are poised to become key components in quantum optical technologies. Understanding the underpinning photophysical mechanisms is crucial for engineering these novel emergent quantum materials. This review aims to provide a condensed overview of the current state of halide perovskite emitter research covering both established and fledging applications, distill the underlying mechanisms, and offer insights into future directions for this rapidly evolving field.

Enhanced Control of Single-Molecule Emission Frequency and Spectral Diffusion

The Stark effect provides a powerful method to shift the spectra of molecules, atoms, and electronic transitions in general, becoming one of the simplest and most straightforward ways to tune the frequency of quantum emitters by means of a static electric field. At the same time, in order to reduce the emitter sensitivity to charge noise, inversion symmetric systems are typically designed, providing a stable emission frequency with a quadratic-only dependence on the applied field. However, such nonlinear behavior might be reflected in correlations between the tuning ability and unwanted spectral fluctuations. Here, we provide experimental evidence of this trend using molecular quantum emitters in the solid state cooled down to liquid helium temperatures. We finally combine the electric field generated by electrodes, which is parallel to the molecule's induced dipole, with optically excite long-lived charge states acting in the perpendicular direction. Based on the anisotropy of the molecule's polarizability, our two-dimensional control of the local electric field allows us not only to tune the emitter's frequency but also to sensibly suppress the spectral instabilities associated with field fluctuations.

Open whispering gallery mode resonators

There are some issues with traditional whispering gallery mode (WGM) resonators such as poor light extraction and a dense mode spectrum. In this paper, we introduce a solution to these limitations by proposing open WGM (OWGM) resonators that effectively reduce the mode density and enable directional radiation through a connected waveguide at the expense of some lowering in Q-factor. Numerical simulations of two-dimensional metallic and dielectric disk resonators with holes reveal a significant increase in intermode distance. The study also extends to three-dimensional dielectric OWGM resonators, demonstrating the formation of sparse spectra suitable for photonic applications. Additionally, the design of a cylindrical Bragg microresonator connected to a single-mode fiber via an optimized topology-based connector achieves near-unity transmission and efficient coupling. This approach enhances the development of new photonic devices, addressing the limitations of traditional high Q-factor WGM resonators and offering potential advancements in laser technology and optical communications.

High-Throughput Spectroscopy of Geometry-Tunable Arrays of Axial InGaAs Nanowire Heterostructures with Twin-Induced Carrier Confinement

Predicting the optical properties of large-scale ensembles of luminescent nanowire arrays that host active quantum heterostructures is of paramount interest for on-chip integrated photonic and quantum photonic devices. However, this has remained challenging due to the vast geometrical parameter space and variations at the single object level. Here, we demonstrate high-throughput spectroscopy on 16800 individual InGaAs quantum heterostructures grown by site-selective epitaxy on silicon, with varying geometrical parameters to assess uniformity/yield in luminescence efficiency, and emission energy trends. The luminescence uniformity/yield enhances significantly at prepatterned array mask opening diameters (d0) greater than 50 nm. Additionally, the emission energy exhibits anomalous behavior with respect to d0, which is notably attributed to rotational twinning within the InGaAs region, inducing significant energy shifts due to quantum confinement effects. These findings provide useful insights for mapping and optimizing the interdependencies between geometrical parameters and electronic/optical properties of widely tunable sets of quantum nanowire heterostructures.

Site-controlled growth of In(Ga)As/GaAs quantum dots on patterned substrate

In(Ga)As quantum dot (QD) with uniform size and controlled sites have great potential in optical communications and quantum computing. In this review, we focus on the site-controlled preparation of In(Ga)As quantum dot arrays based on patterned substrates, including the improvements made by the researchers to enhance the quantum dot site-control capability and optical quality. Based on the current research on site-controlled In(Ga)As QDs, it has been possible to grow uniformly ordered In(Ga)As QD arrays, in which the size, morphology, and nucleus location of each quantum dot can be precisely controlled. In addition, the study of deoxidation treatment of patterned substrates has led to the performance enhancement of the prepared QD arrays. Finally, we propose that the future development of site-controlled In(Ga)As QD arrays lies in improving the optical quality and tuning their emission wavelength to the telecommunication band.

High-dimensional spin-orbital single-photon sources

Hybrid integration of solid-state quantum emitters (QEs) into nanophotonic structures opens enticing perspectives for exploiting multiple degrees of freedom of single-photon sources for on-chip quantum photonic applications. However, the state-of-the-art single-photon sources are mostly limited to two-level states or scalar vortex beams. Direct generation of high-dimensional structured single photons remains challenging, being still in its infancy. Here, we propose a general strategy to design highly entangled high-dimensional spin-orbital single-photon sources by taking full advantage of the spatial freedom to design QE-coupled composite (i.e., Moiré/multipart) metasurfaces. We demonstrate the generation of arbitrary vectorial spin-orbital photon emission in high-dimensional Hilbert spaces, mapping the generated states on hybrid-order Bloch spheres. We further realize single-photon sources of high-dimensional spin-orbital quantum emission and experimentally verify the entanglement of high-dimensional superposition states with high fidelity. We believe that the results obtained facilitate further progress in integrated solutions for the deployment of next-generation high-capacity quantum information technologies.

Heralded Generation of Correlated Photon Pairs from CdS/CdSe/CdS Quantum Shells

Quantum information processing demands efficient quantum light sources (QLS) capable of producing high-fidelity single photons or entangled photon pairs. Single epitaxial quantum dots (QDs) have long been proven to be efficient sources of deterministic single photons; however, their production via molecular-beam epitaxy presents scalability challenges. Conversely, colloidal semiconductor QDs offer scalable solution processing and tunable photoluminescence, but suffer from broader linewidths and unstable emissions. This leads to spectrally inseparable emission from exciton (X) and biexciton (XX) states, complicating the production of single photons and triggered photon pairs. Here, we demonstrate that colloidal semiconductor quantum shells (QSs) achieve significant spectral separation (∼75-80 meV) and long temporal stability of X and XX emissive states, enabling the observation of exciton-biexciton bunching in colloidal QDs. Our low-temperature single-particle measurements show cascaded XX-X emission of single photon pairs for over 200 s, with minimal overlap between X and XX features. The X-XX distinguishability allows for an in-depth theoretical characterization of cross-correlation strength, placing it in perspective with photon pairs of epitaxial counterparts. These findings highlight a strong potential of semiconductor quantum shells for applications in quantum information processing.

A single-photon emitter coupled to a phononic-crystal resonator in the resolved-sideband regime

A promising route towards the deterministic creation and annihilation of single-phonons is to couple a single-photon emitter to a mechanical resonator. The challenge lies in reaching the resolved-sideband regime with a large coupling rate and a high mechanical quality factor. We achieve this by coupling self-assembled InAs quantum dots to a small mode-volume phononic-crystal resonator with mechanical frequency Ωm/2π = 1.466 GHz and quality factor Qm = 2.1 × 103. Thanks to the high coupling rate of gep/2π = 2.9 MHz, and by exploiting a matching condition between the effective Rabi and mechanical frequencies, we observe the interaction between the two systems via correlations in the emitted photons. Our results represent a major step towards quantum control of the mechanical resonator via a single-photon emitter.

Near-critical Stranski-Krastanov growth of InAs/InP quantum dots

This work shows how to control the surface density and size of InAs/InP quantum dots over a wide range by tailoring the conditions of Stranski-Krastanov growth. We demonstrate that in the near-critical growth regime, the density of quantum dots can be tuned between 10 7 and 10 10   cm - 2 . Furthermore, employing both experimental and modeling approaches, we show that the size (and therefore the emission wavelength) of InAs nanoislands on InP can be controlled independently from their surface density. Finally, we demonstrate that our growth method gives low-density ensembles with well-isolated QD-originated emission lines in the telecom C-band.

Information processing at the speed of light

In recent years, quantum computing has made significant strides, particularly in light-based technology. The introduction of quantum photonic chips has ushered in an era marked by scalability, stability, and cost-effectiveness, paving the way for innovative possibilities within compact footprints. This article provides a comprehensive exploration of photonic quantum computing, covering key aspects such as encoding information in photons, the merits of photonic qubits, and essential photonic device components including light squeezers, quantum light sources, interferometers, photodetectors, and waveguides. The article also examines photonic quantum communication and internet, and its implications for secure systems, detailing implementations such as quantum key distribution and long-distance communication. Emerging trends in quantum communication and essential reconfigurable elements for advancing photonic quantum internet are discussed. The review further navigates the path towards establishing scalable and fault-tolerant photonic quantum computers, highlighting quantum computational advantages achieved using photons. Additionally, the discussion extends to programmable photonic circuits, integrated photonics and transformative applications. Lastly, the review addresses prospects, implications, and challenges in photonic quantum computing, offering valuable insights into current advancements and promising future directions in this technology.

Unveiling the 3D Morphology of Epitaxial GaAs/AlGaAs Quantum Dots

Strain-free GaAs/AlGaAs semiconductor quantum dots (QDs) grown by droplet etching and nanohole infilling (DENI) are highly promising candidates for the on-demand generation of indistinguishable and entangled photon sources. The spectroscopic fingerprint and quantum optical properties of QDs are significantly influenced by their morphology. The effects of nanohole geometry and infilled material on the exciton binding energies and fine structure splitting are well-understood. However, a comprehensive understanding of GaAs/AlGaAs QD morphology remains elusive. To address this, we employ high-resolution scanning transmission electron microscopy (STEM) and reverse engineering through selective chemical etching and atomic force microscopy (AFM). Cross-sectional STEM of uncapped QDs reveals an inverted conical nanohole with Al-rich sidewalls and defect-free interfaces. Subsequent selective chemical etching and AFM measurements further reveal asymmetries in element distribution. This study enhances the understanding of DENI QD morphology and provides a fundamental three-dimensional structural model for simulating and optimizing their optoelectronic properties.

Robust consistent single quantum dot strong coupling in plasmonic nanocavities

Strong coupling between a single quantum emitter and an optical cavity (at rate Ω) accesses fundamental quantum optics and provides an essential building block for photonic quantum technologies. However, the minimum mode volume of conventional dielectric cavities restricts their operation to cryogenic temperature for strong coupling. Here we harness surface self-assembly to make deterministic strong coupling at room temperature using CdSe/CdS quantum dots (QDs) in nanoparticle-on-mirror (NPoM) plasmonic nanocavities. We achieve a fabrication yield of ~70% for single QD strong coupling by optimizing their size and nano-assembly. A clear and reliable Rabi splitting is observed both in the scattering of each nanocavity and their photoluminescence, which are however not equal. Integrating these quantum elements with electrical pumping allows demonstration of strong coupling in their electroluminescence. This advance provides a straightforward way to achieve practical quantum devices at room temperature, and opens up exploration of their nonlinear, electrical, and quantum correlation properties.

Plasmon Mediated Single Photon Emission from a Nanocrystal Ensemble

Quantum photonic devices require robust sources of single photons to perform basic computational and communication protocols. Thus, developing scalable, integrable, and efficient quantum light sources has become crucial for the realization of quantum photonic devices. Single quantum dots are promising sources of quantum light due to their tunable emission wavelength. Here, we show the emergence of quantum-emitter-like antibunched emission behavior when multiple quantum dots are located in the vicinity of plasmonic particles. To evaluate the robustness of this phenomenon, we consider both monometallic and bimetallic particles. We find that the photoluminescence intensity of the plasmon coupled quantum dots fits well to a single sublinear power law exponent that is distinct from the behavior of CQD aggregates. Significantly, we find that plasmon coupling results in reduced flickering, thus enabling the realization of a more stable and reliable single photon source. Possible roles of emergent excitonic interactions in the coupled system are discussed.

Solution-phase sample-averaged single-particle spectroscopy of quantum emitters with femtosecond resolution

The development of many quantum optical technologies depends on the availability of single quantum emitters with near-perfect coherence. Systematic improvement is limited by a lack of understanding of the microscopic energy flow at the single-emitter level and ultrafast timescales. Here we utilize a combination of fluorescence correlation spectroscopy and ultrafast spectroscopy to capture the sample-averaged dynamics of defects with single-particle sensitivity. We employ this approach to study heterogeneous emitters in two-dimensional hexagonal boron nitride. From milliseconds to nanoseconds, the translational, shelving, rotational and antibunching features are disentangled in time, which quantifies the normalized two-photon emission quantum yield. Leveraging the femtosecond resolution of this technique, we visualize electron-phonon coupling and discover the acceleration of polaronic formation on multi-electron excitation. Corroborated with theory, this translates to the photon fidelity characterization of cascaded emission efficiency and decoherence time. Our work provides a framework for ultrafast spectroscopy in heterogeneous emitters, opening new avenues of extreme-scale characterization for quantum applications.

Coherent phenomena and dynamics of lead halide perovskite nanocrystals for quantum information technologies

Solution-processed colloidal nanocrystals of lead halide perovskites have been intensively investigated in recent years in the context of optoelectronic devices, during which time their quantum properties have also begun to attract attention. Their unmatched ease of synthetic tunability and unique structural, optical and electronic properties, in conjunction with the confinement of carriers in three dimensions, have motivated studies on observing and controlling coherent light-matter interaction in these materials for quantum information technologies. This Review outlines the recent efforts and achievements in this direction. Particularly notable examples are the observation of coherent single-photon emission, evidence for superfluorescence and the realization of room-temperature coherent spin manipulation for ensemble samples, which have not been achieved for prototypical colloidal CdSe nanocrystals that have been under investigation for decades. This Review aims to highlight these results, point out the challenges ahead towards realistic applications and bring together the efforts of multidisciplinary communities in this nascent field.

A source of entangled photons based on a cavity-enhanced and strain-tuned GaAs quantum dot

A quantum-light source that delivers photons with a high brightness and a high degree of entanglement is fundamental for the development of efficient entanglement-based quantum-key distribution systems. Among all possible candidates, epitaxial quantum dots are currently emerging as one of the brightest sources of highly entangled photons. However, the optimization of both brightness and entanglement currently requires different technologies that are difficult to combine in a scalable manner. In this work, we overcome this challenge by developing a novel device consisting of a quantum dot embedded in a circular Bragg resonator, in turn, integrated onto a micromachined piezoelectric actuator. The resonator engineers the light-matter interaction to empower extraction efficiencies up to 0.69(4). Simultaneously, the actuator manipulates strain fields that tune the quantum dot for the generation of entangled photons with corrected fidelities to a maximally entangled state up to 0.96(1). This hybrid technology has the potential to overcome the limitations of the key rates that plague QD-based entangled sources for entanglement-based quantum key distribution and entanglement-based quantum networks.

Supplementary Information

The online version contains supplementary material available at 10.1186/s43593-024-00072-8.

Tunable quantum emitters on large-scale foundry silicon photonics

Controlling large-scale many-body quantum systems at the level of single photons and single atomic systems is a central goal in quantum information science and technology. Intensive research and development has propelled foundry-based silicon-on-insulator photonic integrated circuits to a leading platform for large-scale optical control with individual mode programmability. However, integrating atomic quantum systems with single-emitter tunability remains an open challenge. Here, we overcome this barrier through the hybrid integration of multiple InAs/InP microchiplets containing high-brightness infrared semiconductor quantum dot single photon emitters into advanced silicon-on-insulator photonic integrated circuits fabricated in a 300 mm foundry process. With this platform, we achieve single-photon emission via resonance fluorescence and scalable emission wavelength tunability. The combined control of photonic and quantum systems opens the door to programmable quantum information processors manufactured in leading semiconductor foundries.

Continuous and deterministic all-photonic cluster state of indistinguishable photons

Cluster states are key resources for measurement-based quantum information processing. Photonic cluster and graph states, in particular, play indispensable roles in quantum network and quantum metrology. We demonstrate a semiconductor quantum dot based device in which the confined hole spin acts as a needle in a quantum knitting machine producing continuously and deterministically at sub-Gigahertz repetition rate single indistinguishable photons which are all polarization entangled to each other and to the spin in a one dimensional cluster state. By projecting two nonadjacent photons onto circular polarization bases we disentangle the spin from the photons emitted in between. This way we demonstrate a novel way for producing deterministic and continuous all-photonic cluster states. We use polarization tomography on four sequentially detected photons to demonstrate and to directly quantify the robustness of the cluster's entanglement and the determinism in its photon generation.

Reconfigurable quantum photonic circuits based on quantum dots

Quantum photonic integrated circuits, composed of linear-optical elements, offer an efficient way for encoding and processing quantum information on-chip. At their core, these circuits rely on reconfigurable phase shifters, typically constructed from classical components such as thermo- or electro-optical materials, while quantum solid-state emitters such as quantum dots are limited to acting as single-photon sources. Here, we demonstrate the potential of quantum dots as reconfigurable phase shifters. We use numerical models based on established literature parameters to show that circuits utilizing these emitters enable high-fidelity operation and are scalable. Despite the inherent imperfections associated with quantum dots, such as imperfect coupling, dephasing, or spectral diffusion, we show that circuits based on these emitters may be optimized such that these do not significantly impact the unitary infidelity. Specifically, they do not increase the infidelity by more than 0.001 in circuits with up to 10 modes, compared to those affected only by standard nanophotonic losses and routing errors. For example, we achieve fidelities of 0.9998 in quantum-dot-based circuits enacting controlled-phase and - not gates without any redundancies. These findings demonstrate the feasibility of quantum emitter-driven quantum information processing and pave the way for cryogenically-compatible, fast, and low-loss reconfigurable quantum photonic circuits.

High-quality on-chip entangled photon source with broad tunable range based on coupling compensation

This paper introduces a tunable and high-quality photon source that utilizes evanescent-wave coupling phase matching. By adjusting the coupling gap, the signal light can be tuned from 1307 nm to 1493.9 nm, and the idler light can be tuned from 1612.8 nm to 1907 nm. Throughout the entire tuning range, the purity of the photon pairs remains above 92%. In specific tuning ranges (signal photons from 1307 nm to 1421.7 nm and idler photons from 1706.4 nm to 1907 nm), the purity exceeds 99% and the full width at half-maximum of the generated photon spectra is less than 1.85 nm. The photon source is designed using a silicon-organic hybrid waveguide, which effectively minimizes the impact of two-photon absorption and achieves a 15 dB enhancement in four-wave mixing conversion efficiency compared to a strip waveguide. This design may promote efficient and precise generation of high-quality photons at desired frequencies, offering promising potential for various applications in quantum technologies.

Sub-Diffraction Correlation of Quantum Emitters and Local Strain Fields in Strain-Engineered WSe2 Monolayers

Strain-engineering in atomically thin metal dichalcogenides is a useful method for realizing single-photon emitters (SPEs) for quantum technologies. Correlating SPE position with local strain topography is challenging due to localization inaccuracies from the diffraction limit. Currently, SPEs are assumed to be positioned at the highest strained location and are typically identified by randomly screening narrow-linewidth emitters, of which only a few are spectrally pure. In this work, hyperspectral quantum emitter localization microscopy is used to locate 33 SPEs in nanoparticle-strained WSe2 monolayers with sub-diffraction-limit resolution (≈30 nm) and correlate their positions with the underlying strain field via image registration. In this system, spectrally pure emitters are not concentrated at the highest strain location due to spectral contamination; instead, isolable SPEs are distributed away from points of peak strain with an average displacement of 240 nm. These observations point toward a need for a change in the design rules for strain-engineered SPEs and constitute a key step toward realizing next-generation quantum optical architectures.

Engineering dynamical photon blockade with Liouville exceptional points

We investigate the dynamical blockade in a nonlinear cavity and demonstrate the connection between the correlation function g(2)(t) and system parameters in the entire nonlinear region. Utilizing the Liouville exceptional points (LEPs) and quantum dynamics, a near-perfect single-photon blockade (1PB) can be achieved. By fine-tuning system parameters to approach the second-order LEP (LEP2), we improved single-photon statistics in both weak and strong nonlinearity regimes, including a significant reduction of g(2)(t) and a pronounced increase in the single-photon occupation number. In the strong nonlinearity region, the maximum photon population may correspond to stronger antibunching effect. Simultaneously, the time window and period of blockade can be controlled by selecting detuning based on the LEP2. Furthermore, the 1PB exhibits robustness against parameter fluctuations, and this feature can be generalized to systems for implementing single-photon sources with nonharmonic energy levels.

Engineering quantum dots for improved single photon emission statistics

High fidelity single photon sources are required for the implementation of quantum information processing and communications protocols. Although colloidal quantum dots (CQDs) are single photon sources, their efficacy is limited by their tendency to show finite multiphoton emission at higher excitation powers. Here, we show that wave function engineering of CQDs enables the realization of emitters with significantly improved single photon emission performance. We study the ZnS/CdSe/CdS system. It is shown that this system offers significantly improved probabilities of single photon emission. While conventional CQDs such as CdSe/CdS exhibit a g2(0) > 0.5 ± 0.02 at ⟨N⟩ = 2.17, ZnS/CdSe/CdS show a greatly improved g2(0) ≈ 0.04 ± 0.01. Improved single photon emission performance encourages the use of colloidal materials as quantum light sources in emerging quantum devices.

Research Progress of Single-Photon Emitters Based on Two-Dimensional Materials

From quantum communications to quantum computing, single-photon emitters (SPEs) are essential components of numerous quantum technologies. Two-dimensional (2D) materials have especially been found to be highly attractive for the research into nanoscale light-matter interactions. In particular, localized photonic states at their surfaces have attracted great attention due to their enormous potential applications in quantum optics. Recently, SPEs have been achieved in various 2D materials, while the challenges still remain. This paper reviews the recent research progress on these SPEs based on various 2D materials, such as transition metal dichalcogenides (TMDs), hexagonal boron nitride (hBN), and twisted-angle 2D materials. Additionally, we summarized the strategies to create, position, enhance, and tune the emission wavelength of these emitters by introducing external fields into these 2D system. For example, pronounced enhancement of the SPEs' properties can be achieved by coupling with external fields, such as the plasmonic field, and by locating in optical microcavities. Finally, this paper also discusses current challenges and offers perspectives that could further stimulate scientific research in this field. These emitters, due to their unique physical properties and integration potential, are highly appealing for applications in quantum information and communication, as well as other physical and technological fields.

Zincblende InAsxP1-x/InP Quantum Dot Nanowires for Telecom Wavelength Emission

InAsxP1-x quantum dots (QDs) in InP nanowires (NWs) have been realized as a platform for emission at telecom wavelengths. These QDs are typically grown in NWs with the wurtzite crystal phase, but in this case, ultrathin diameters are required to achieve defect-free heterostructures, making the structures less robust. In this work, we demonstrate the growth of pure zincblende InAsxP1-x QDs in InP NWs, which enabled an increase in NW diameters to about 45 nm, achieved by employing Au-assisted vapor liquid solid growth in a chemical beam epitaxy system. We studied the growth of InP/InAsxP1-x heterostructures with different compositions to control the straight growth along the ⟨100⟩ direction and to tune the emission wavelength. Interestingly, we found that the growth mechanism for pure InAs QDs is different compared to that for InAsxP1-x alloy QDs. This allowed us to optimize different growth protocols to achieve straight growth of the final QD NWs. We successfully obtained the growth of InAsxP1-x QDs with a composition in the range of x = 0.24-1.00. By means of microphotoluminescence measurements, we demonstrate the tunability of the emission in dependence of the InAsxP1-x QD composition and morphology, remarkably observing an emission at the telecom O-band for a 10 nm thick QD with 80% of As content.

Cavity-enhanced photon indistinguishability at room temperature and telecom wavelengths

Indistinguishable single photons in the telecom-bandwidth of optical fibers are indispensable for long-distance quantum communication. Solid-state single photon emitters have achieved excellent performance in key benchmarks, however, the demonstration of indistinguishability at room-temperature remains a major challenge. Here, we report room-temperature photon indistinguishability at telecom wavelengths from individual nanotube defects in a fiber-based microcavity operated in the regime of incoherent good cavity-coupling. The efficiency of the coupled system outperforms spectral or temporal filtering, and the photon indistinguishability is increased by more than two orders of magnitude compared to the free-space limit. Our results highlight a promising strategy to attain optimized non-classical light sources.

Programmable multiphoton quantum interference in a single spatial mode

The interference of nonclassical states of light enables quantum-enhanced applications reaching from metrology to computation. Most commonly, the polarization or spatial location of single photons are used as addressable degrees of freedom for turning these applications into praxis. However, the scale-up for the processing of a large number of photons of these architectures is very resource-demanding due to the rapidly increasing number of components, such as optical elements, photon sources, and detectors. Here, we demonstrate a resource-efficient architecture for multiphoton processing based on time-bin encoding in a single spatial mode. We use an efficient quantum dot single-photon source and a fast programmable time-bin interferometer to observe the interference of up to eight photons in 16 modes, all recorded only with one detector, thus considerably reducing the physical overhead previously needed for achieving equivalent tasks. Our results can form the basis for a future universal photonics quantum processor operating in a single spatial mode.

Tempering Multichannel Photon Emission from Emitter-Coupled Holographic Metasurfaces

On-chip manipulation of photon emission from quantum emitters (QEs) is crucial for quantum nanophotonics and advanced optical applications. At the same time, the general design strategy is still elusive, especially for fully exploring the degrees of freedom of multiple channels. Here, the vectorial scattering holography (VSH) approach developed recently for flexibly designing QE-coupled metasurfaces is extended to tempering the strength of QE emission into a particular channel. The VSH power is demonstrated by designing, fabricating, and optically characterizing on-chip QE sources emitted into six differently oriented propagation channels, each representing the entangled state of orthogonal circular polarizations with different topological charges and characterized with a specific relative strength. We postulate that the demonstration of tempered multichannel photon emission from QE-coupled metasurfaces significantly broadens the possibilities provided by the holographic metasurface platform, especially those relevant for high-dimensional quantum information processing.

Control of Quantized Spontaneous Emission from Single GaAs Quantum Dots Embedded in Huygens' Metasurfaces

Advancements in photonic quantum information systems (QIS) have driven the development of high-brightness, on-demand, and indistinguishable semiconductor epitaxial quantum dots (QDs) as single photon sources. Strain-free, monodisperse, and spatially sparse local-droplet-etched (LDE) QDs have recently been demonstrated as a superior alternative to traditional Stranski-Krastanov QDs. However, integration of LDE QDs into nanophotonic architectures with the ability to scale to many interacting QDs is yet to be demonstrated. We present a potential solution by embedding isolated LDE GaAs QDs within an Al0.4Ga0.6As Huygens' metasurface with spectrally overlapping fundamental electric and magnetic dipolar resonances. We demonstrate for the first time a position- and size-independent, 1 order of magnitude increase in the collection efficiency and emission lifetime control for single-photon emission from LDE QDs embedded within the Huygens' metasurfaces. Our results represent a significant step toward leveraging the advantages of LDE QDs within nanophotonic architectures to meet the scalability demands of photonic QIS.

Circular photonic crystal grating design for charge-tunable quantum light sources in the telecom C-band

Efficient generation of entangled photon pairs at telecom wavelengths is a key ingredient for long-range quantum networks. While embedding semiconductor quantum dots into hybrid circular Bragg gratings has proven effective, it conflicts with p-i-n diode heterostructures which offer superior coherence. We propose and analyze hybrid circular photonic crystal gratings, incorporating air holes to facilitate charge carrier transport without compromising optical properties. Through numerical simulations, a broad cavity mode with a Purcell factor of 23 enhancing both exciton and biexciton transitions, and exceptional collection efficiency of 92.4% into an objective with numerical aperture of 0.7 are achieved. Furthermore, our design demonstrates direct coupling efficiency over 90.5% into a single-mode fiber over the entire telecom C-band. The hybrid circular photonic crystal grating thereby emerges as a promising solution for the efficient generation of highly coherent, polarization-entangled photon pairs.

Heralded Three-Photon Entanglement from a Single-Photon Source on a Photonic Chip

In the quest to build general-purpose photonic quantum computers, fusion-based quantum computation has risen to prominence as a promising strategy. This model allows a ballistic construction of large cluster states which are universal for quantum computation, in a scalable and loss-tolerant way without feed forward, by fusing many small n-photon entangled resource states. However, a key obstacle to this architecture lies in efficiently generating the required essential resource states on photonic chips. One such critical seed state that has not yet been achieved is the heralded three-photon Greenberger-Horne-Zeilinger (3-GHZ) state. Here, we address this elementary resource gap, by reporting the first experimental realization of a heralded 3-GHZ state. Our implementation employs a low-loss and fully programmable photonic chip that manipulates six indistinguishable single photons of wavelengths in the telecommunication regime. Conditional on the heralding detection, we obtain the desired 3-GHZ state with a fidelity 0.573±0.024. Our Letter marks an important step for the future fault-tolerant photonic quantum computing, leading to the acceleration of building a large-scale optical quantum computer.

Engineering Quantum Light Sources with Flat Optics

Quantum light sources are essential building blocks for many quantum technologies, enabling secure communication, powerful computing, and precise sensing and imaging. Recent advancements have witnessed a significant shift toward the utilization of "flat" optics with thickness at subwavelength scales for the development of quantum light sources. This approach offers notable advantages over conventional bulky counterparts, including compactness, scalability, and improved efficiency, along with added functionalities. This review focuses on the recent advances in leveraging flat optics to generate quantum light sources. Specifically, the generation of entangled photon pairs through spontaneous parametric down-conversion in nonlinear metasurfaces, and single photon emission from quantum emitters including quantum dots and color centers in 3D and 2D materials are explored. The review covers theoretical principles, fabrication techniques, and properties of these sources, with particular emphasis on the enhanced generation and engineering of quantum light sources using optical resonances supported by nanostructures. The diverse application range of these sources is discussed and the current challenges and perspectives in the field are highlighted.

Polarization-Selective Enhancement of Telecom Wavelength Quantum Dot Transitions in an Elliptical Bullseye Resonator

Semiconductor quantum dots are promising candidates for the generation of nonclassical light. Coupling a quantum dot to a device capable of providing polarization-selective enhancement of optical transitions is highly beneficial for advanced functionalities, such as efficient resonant driving schemes or applications based on optical cyclicity. Here, we demonstrate broadband polarization-selective enhancement by coupling a quantum dot emitting in the telecom O-band to an elliptical bullseye resonator. We report bright single-photon emission with a degree of linear polarization of 96%, Purcell factor of 3.9 ± 0.6, and count rates up to 3 MHz. Furthermore, we present a measurement of two-photon interference without any external polarization filtering. Finally, we demonstrate compatibility with compact Stirling cryocoolers by operating the device at temperatures up to 40 K. These results represent an important step toward practical integration of optimal quantum dot photon sources in deployment-ready setups.

Robust parallel laser driving of quantum dots for multiplexing of quantum light sources

Deterministic sources of quantum light (i.e. single photons or pairs of entangled photons) are required for a whole host of applications in quantum technology, including quantum imaging, quantum cryptography and the long-distance transfer of quantum information in future quantum networks. Semiconductor quantum dots are ideal candidates for solid-state quantum emitters as these artificial atoms have large dipole moments and a quantum confined energy level structure, enabling the realization of single photon sources with high repetition rates and high single photon purity. Quantum dots may also be triggered using a laser pulse for on-demand operation. The naturally-occurring size variations in ensembles of quantum dots offers the potential to increase the bandwidth of quantum communication systems through wavelength-division multiplexing, but conventional laser triggering schemes based on Rabi rotations are ineffective when applied to inequivalent emitters. Here we report the demonstration of the simultaneous triggering of >10 quantum dots using adiabatic rapid passage. We show that high-fidelity quantum state inversion is possible in a system of quantum dots with a 15 meV range of optical transition energies using a single broadband, chirped laser pulse, laying the foundation for high-bandwidth, multiplexed quantum networks.

Experimental Quantum Non-Gaussian Coincidences of Entangled Photons

Quantum non-Gaussianity, a more potent and highly useful form of nonclassicality, excludes all convex mixtures of Gaussian states and Gaussian parametric processes generating them. Here, for the first time, we conclusively test quantum non-Gaussian coincidences of entangled photon pairs with the Clauser-Horne-Shimony-Holt-Bell factor S=2.328±0.004 from a single quantum dot with a depth up to 0.94±0.02  dB. Such deterministically generated photon pairs fundamentally overcome parametric processes by reducing crucial multiphoton errors. For the quantum non-Gaussian depth of the unheralded (heralded) single-photon state, we achieve the value of 8.08±0.05  dB (19.06±0.29  dB). Our Letter experimentally certifies the exclusive quantum non-Gaussianity properties highly relevant for optical sensing, communication, and computation.