The controlled integration of quantum dots (QDs) as single-photon emitters into quantum light sources is essential for the implementation of large-scale quantum networks. In this study, we employ the deterministic in situ electron-beam lithography (iEBL) nanotechnology platform to integrate individual QDs with high accuracy and process yield into a circular Bragg grating (CBG) resonators. Notably, CBG devices comprising just 3 to 4 rings exhibit photon extraction efficiencies comparable to those of structures with more rings. This facilitates faster fabrication, reduces the device footprint, and enables compatibility with electrical contacting. To demonstrate the scalability of this process, we present results of 95 optically active QD-CBG devices fabricated across two lithography sessions. These devices exhibit bright, narrow-linewidth single-photon emission with excellent optical quality. To evaluate QD placement accuracy, we apply a powerful characterization technique that combines cathodoluminescence (CL) mapping and scanning electron microscopy. Statistical analysis of these devices reveals that our iEBL approach enables high alignment accuracy and a process yield of over across various CBG geometries. Our findings highlight a reliable route toward the scalable fabrication of high-performance QD-based single-photon sources for use in photonic quantum technology applications.

The generation of indistinguishable single photons is a fundamental requirement for photonic quantum technologies. However, spectral fluctuations, often induced by charge noise in epitaxial quantum dots (QDs), lead to exciton dephasing, thereby limiting their practical usage in quantum applications. We present a straightforward approach to mitigate charge noise-induced decoherence in droplet-etched GaAs QDs embedded in an n-i-p diode structure and integrated deterministically into an electrically contacted circular Bragg grating resonator for emission enhancement. The quantum device allows for the stabilization of the charge environment by applying an external electrical field while producing a photon extraction efficiency of (37±2)%. Hong-Ou-Mandel two-photon interference measurements reveal a strong dependence of the exciton dephasing time and interference visibility on the applied bias, in excellent agreement with our theoretical predictions. Notably, the reduction in visibility from a maximum, charge stabilized corrected value of 97% at the optimum bias point follows an inverse square dependence (∝1/I^{2}) with increasing diode current (I) in the forward direction. Under a quasi-resonant excitation scheme, we achieve a maximum exciton dephasing time (T_{2}^{*}) of approximately (6.8±0.5) ns, reaching nearly the Fourier limit (T_{2}=2T_{1}) without the need for complex echo schemes like Ramsey or Carr-Purcell-Meiboom-Gill sequences. These findings are consistent with theoretical predictions from rate equation modeling and quantum optical analysis, as well as voltage-dependent linewidth measurements, demonstrating optimized electrical control of exciton dephasing.

Fiber-pigtailed photonic cavities with embedded semiconductor quantum dots (QDs) are a powerful approach to enable the practical generation and distribution of state-of-the art photonic quantum states, directly suitable for quantum information applications. We report on the simulation, fabrication and quantum optical characterization of fiber-pigtailed hybrid circular Bragg grating cavities with embedded InAs QD emitting in the near-infrared range. The strong Purcell-enhancement thanks to the employed deterministic fabrication approach enables reduced emitter decay times below 80 ps, resulting in pigtailed photon indistinguishability, multi-photon suppression of g(2)(0) < 1% and > 53% of emitted photons per excitation pulse in the pigtailed fiber. The fast decay times furthermore allow for a more than tenfold increase of excitation frequency, allowing for the generation of indistinguishable single photons directly in fiber at 1.28 GHz clock-rates, paving the way for using photonic quantum states at unprecedented rates in a plug-&-play fashion.

Self-assembled optically active quantum dot molecules (QDMs) allow the creation of protected qubits via singlet-triplet spin states. The qubit energy splitting of these states is defined by the tunnel coupling strength and is, therefore, determined by the potential landscape and thus fixed during growth. Applying an in-plane magnetic field increases the confinement of the hybridized wave functions within the quantum dots, leading to a decrease of the tunnel coupling strength. We achieve a tuning of the coupling strength by $(53.4\pm1.7)$ %. The ability to fine-tune this coupling is essential for quantum network and computing applications that require quantum systems with near identical performance.

Optically active quantum dot molecules (QDMs) can host multi-spin quantum states with the potential for the deterministic generation of photonic graph states with tailored entanglement structures. Their usefulness for the generation of such non-classical states of light is determined by orbital and spin decoherence mechanisms, particularly phonon-mediated processes dominant at energy scales up to a few millielectronvolts. Here, we directly measure the spectral function of orbital phonon relaxation in a QDM and benchmark our findings against microscopic kp theory. Our results reveal phonon-mediated relaxation rates exhibiting pronounced resonances and anti-resonances, with rates ranging from several ten ns^-1 to tens of μs^-1. Comparison with a kinetic model reveals the voltage (energy) dependent phonon coupling strength and fully explains the interplay between phonon-assisted relaxation and radiative recombination. These anti-resonances can be leveraged to increase the lifetime of energetically unfavorable charge configurations needed for realizing efficient spin-photon interfaces and multi-dimensional cluster states.

Interfacing light from solid-state single-photon sources with scalable and robust room-temperature quantum memories has been a long-standing challenge in photonic quantum information technologies due to inherent noise processes and time-scale mismatches between the operating conditions of solid-state and atomic systems. Here, we demonstrate storage of single photons from a semiconductor quantum dot (QD) device in a room-temperature atomic vapor memory and their on-demand retrieval. A deterministically fabricated InGaAs QD light source emits single photons at the wavelength of the cesium D1 line at 895 nm which exhibit an inhomogeneously broadened linewidth of 5.1(7) GHz and are subsequently stored in a low-noise ladder-type cesium vapor memory. We show control over the interaction between the single photons and the atomic vapor, allowing for variable retrieval times of up to 19.8(3) ns. A maximum internal efficiency of eta int=0.6(1)% is achieved. Our results expand the application space of both room-temperature vapor memories and semiconductor QDs in future quantum network architectures.

Room‐temperature operable microlasers are essential for advancing quantum photonics and integrated photonic circuits, enabling a wide range of practical applications. In this study, the lasing performance of two types of optically pumped InGaAs quantum dot microcavities – namely, micropillar and photonic‐defect cavities — is systematically compared at elevated temperatures. A comprehensive analysis of device designs through simulations, followed by the fabrication and experimental studies of both structures, allows for a direct performance evaluation. Excitation‐power‐dependent input/output measurements confirm lasing up to 200 K in the micropillar case, where the performance is constrained by sidewall losses, pump power absorption, and inefficient heat dissipation. In contrast, the photonic‐defect cavity demonstrates stable continuous‐wave lasing even at room‐temperature (300 K), attributed mainly to superior thermal management in the quasi‐planar cavity design. Additionally, in the photonic‐defect cavity with low‐absorbing upper mirror, Raman spectroscopy verifies efficient optical pumping, while second‐order autocorrelation measurements provide unambiguous proof of lasing at 300 K. Overall, the quasi‐planar geometry of the photonic‐defect cavity shows high temperature stability and supports flexible fabrication, establishing it as a promising concept for practical microlaser applications.

In laser-based optical gas sensing, to exploit the multi-line response of analyzed gases, one needs to employ light sources with wide spectral tunability. This is a complex requirement, since it requires a gain medium with bright and spectrally broad emission. A matrix of tunable vertical-cavity surface-emitting lasers (VCSELs), with an active region made of epitaxially-grown quantum dots (QDs), might satisfy the abovementioned requirements better than conventional quantum wells with a narrowband gain spectrum. Here, we report on growth development, as well as structural and optical characterization, of atypical self-assembled, low-strain InGaAs/GaAs QDs, obtained by metal-organic chemical vapor deposition and optimized to achieve emission in the range of 935-955 nm, where strong and distinct absorption lines of water are found. As the InGaAs/GaAs QDs do not naturally emit at these wavelengths at room temperature, a special structure design and growth procedure, plus a post-processing step involving rapid thermal annealing, are required. Thanks to these steps, we achieved an emission level below 980 nm, which is a milestone on the way to manufacturing a low-cost VCSEL-based spectroscopic water vapor sensing system crucial for many industrial and environmental applications. (c) 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Ridge-based electrically controlled QD-CBG resonators are realized, achieving high photon extraction efficiency, tunable emissions, high single-photon purity, and photon indistinguishability, for applications in long-distance quantum networks.

We demonstrate on-demand storage and retrieval of quantum dot single photons in a room-temperature atomic vapor memory with variable storage times. This heterogeneous interface promises applications in buffering and conditioning of quantum information in networks.

Solid-state quantum light sources based on semiconductor quantum dots (QDs) are increasingly employed in photonic quantum information applications. Especially when moving towards real-world scenarios outside shielded lab environments, the efficient and robust coupling of nanophotonic devices to single-mode optical fibers offers substantial advantage by enabling “plug-and-play” operation. In this work we present a fiber-pigtailed cavity-enhanced source of flying qubits emitting single indistinguishable photons at clock-rates exceeding 1 GHz. This is achieved by employing a fully deterministic technique for fiber-pigtailing optimized QD-devices based on hybrid circular Bragg grating (hCBG) micro-cavities. The fabricated fiber-pigtailed hCBGs feature emission lifetimes of <80 ${< } 80$ ps, corresponding to a Purcell factor of ∼9, a suppression of multi-photon emission events with g (2)(0) < 1 %, a photon-indistinguishability >80 ${ >} 80$ % and a measured single-photon coupling efficiency of 53 % in a high numerical aperture single-mode fiber, corresponding to 1.2 Megaclicks per second at the single-photon detectors under 80 MHz excitation clock-rates. Furthermore, we show that high multi-photon suppression and indistinguishability prevail for excitation clock-rates exceeding 1 GHz. Our results show that Purcell-enhanced fiber-pigtailed quantum light sources based on hCBG cavities are a prime candidate for applications of quantum information science.

We report on the deterministic fabrication of high-performance hybrid circular Bragg gratings (hCBGs) with embedded InAs/GaAs quantum dots and their direct and permanent fiber-pigtailing to single mode fibers. The devices exhibit spontaneous emission lifetimes <50ps resulting in experimental Purcell factor well beyond 15. The fiber-pigtailed devices show excellent temperature stability and unprecedented performance in terms of the single photon purity.

We present Purcell-enhanced (F P > 25) semiconductor InAs quantum dot decay times of T 1 < 30 ps, enabled by deterministic hybrid circular Bragg gratings (hCBGs). We investigate the benefits of these short T 1-times on the two-photon indistinguishability for quasi-resonant and strictly resonant excitation and observe visibilities of >= 96% at 12.5 ns time delay of consecutively emitted photons. The strongly Purcell-enhanced decay times enable a high degree of indistinguishability for elevated temperatures of up to 30 K and, moreover, allow for excitation of up to 1.28 GHz repetition rate. Our work highlights the prospects of highly Purcell-enhanced solid-state quantum emitters for applications in quantum information and technologies operating at GHz clock rates.

We report an easy-to-fabricate microcavity design to produce optically pumped high-β quantum dot microlasers. Our cavity concept is based on a buried photonic-defect for tight lateral mode confinement in a quasi-planar microcavity system, which includes an upper dielectric distributed Bragg reflector (DBR) as a promising alternative to conventional III–V semiconductor DBRs. The cavities show distinct emission features with a characteristic photonic-defect size-dependent mode separation and Q-factors up to 17 000. Comprehensive investigations further reveal lasing operation with a systematic increase (decrease) of the β-factor (threshold pump power) with the number of mirror pairs in the upper dielectric DBR. Notably, due to the quasi-planar device geometry, the microlasers show high temperature stability, evidenced by the absence of temperature-induced redshift of emission energy and linewidth broadening typically observed for nano- and microlasers at high excitation powers. The device exhibits remarkable lasing performance, maintaining efficacy even under elevated temperatures of up to 260 K.

The realization of efficient quantum light sources relies on the integration of self-assembled quantum dots (QDs) into photonic nanostructures with high spatial positioning accuracy. In this work, we present a comprehensive investigation of the QD position accuracy, obtained using two marker-based QD positioning techniques, photoluminescence (PL) and cathodoluminescence (CL) imaging, as well as using a marker-free in-situ electron beam lithography (in-situ EBL) technique. We employ four PL imaging configurations with three different image processing approaches and compare them with CL imaging. We fabricate circular mesa structures based on the obtained QD coordinates from both PL and CL image processing to evaluate the final positioning accuracy. This yields final position offset of the QD relative to the mesa center of $\mu_x$ = (-40$\pm$58) nm and $\mu_y$ = (-39$\pm$85) nm with PL imaging and $\mu_x$ = (-39$\pm$30) nm and $\mu_y$ = (25$\pm$77) nm with CL imaging, which are comparable to the offset $\mu_x$ = (20$\pm$40) nm and $\mu_y$ = (-14$\pm$39) nm obtained using the in-situ EBL method. We discuss the possible causes of the observed offsets, which are significantly larger than the QD localization uncertainty obtained from simply imaging the QD light emission from an unstructured wafer. Our study highlights the influences of the image processing technique and the subsequent fabrication process on the final positioning accuracy for a QD placed inside a photonic nanostructure.

We report on the epitaxial growth, theoretical modeling, and structural as well as optical investigation of multi-layer, site-controlled quantum dots fabricated using the buried stressor method. This deterministic growth technique utilizes the strain from a partially oxidized AlAs layer to induce site-selective nucleation of InGaAs quantum dots. By implementing strain-induced spectral nano-engineering, we achieve spectral control of emission and a local increase in the emitter density. Furthermore, we achieve a threefold increase in the optical intensity and reduce the inhomogeneous broadening of the ensemble emission by 20% via stacking three layers of site-controlled emitters, which is valuable for using the SCQDs as a gain medium in microlaser applications. Our optimization of site-controlled growth of quantum dots enables the development of high-β microlasers with increased confinement factor.

Cavity-enhanced emission of electrically controlled semiconductor quantum dots (QDs) is essential in the development of bright quantum devices for real-world quantum photonic applications. Combining the circular Bragg grating (CBG) approach with a PIN-diode structure, we propose and implement designs for ridge-based electrically contacted QD-CBG resonators. Through fine-tuning of device parameters in numerical simulations and deterministic nanoprocessing, we produced electrically controlled single QD-CBG resonators with excellent electro-optical emission properties. These include multiple wavelength-tunable emission lines and a photon extraction efficiency (PEE) of up to 30.4(3.4)%, where refined numerical optimization based on experimental findings suggests a substantial improvement, promising PEE > 50%. Additionally, the developed quantum light sources yield single-photon purity reaching 99.2(2)% and photon indistinguishability of 75(5)% under quasi-resonant p-shell excitation. Our results present high-performance quantum devices with combined cavity enhancement and deterministic charge-environment controls, which are relevant for the development of photonic quantum information systems such as complex quantum repeater networks.

Abstract Many applications of InGaAs quantum dots in various fields of photonics, including optoelectronics, quantum technologies, and telecommunications have emerged in recent years. The objective of this work is to evaluate and optimize the growth and fabrication parameters of site-controlled InGaAs quantum dots, grown with the buried stressor method. The aim is to enhance the precision, uniformity and reproducibility of the quantum dot placement and local density for advanced optoelectronic applications, such as low-treshold microlasers and quantum light sources. The influence of the growth and the fabrication processes on the structural and optical properties of site-controlled quantum dots is investigated in depth using cathodoluminescence mapping, scanning electron microscopy, confocal laser scanning microscopy, and photoluminescence spectroscopy.

Whispering gallery mode (WGM) based microlasers have gained significant interest across various fields of nanophotonics, including biosensing, metrology, and on-chip excitation of single quantum dots in integrated quantum photonic structures. In this study, we report a comprehensive diameter-dependent study of whispering gallery mode lasing in quantum dot micropillar cavities. The lasing threshold, mode energy, quality factor, light-matter interaction in terms of the Purcell factor, and the free spectral range, are studied systematically for diameters ranging from 1 to 20 µm at cryogenic temperatures. To describe the experimental data, we use rate equation fitting and numerical simulations based on the finite element method, including realistic loss channels. Our results show a strong and systematic dependence of all lasing properties on the diameter of the micropillars. We also observe significant variations in the lasing properties of nominally identical micropillars, indicating a significant influence of the growth and fabrication imperfections on the output of the devices. The study provides important information on the optical properties of micropillar-based WGM lasing, which can aid in the advancement of optoelectronic applications using these nanophotonic structures.

We present a disseminable single-photon source based on an InGaAs quantum dot in a micro-mesa. This source achieves a maximum photon flux of (2.74 +/- 0.03) x 10(6) photons/s at a wavelength of (929.7 +/- 0.1) nm, with a multiphoton suppression of g((2))(0) = 0.22 +/- 0.02. The semiconductor chip can be installed in any cryogenic system and the compact size of all optical components facilitates easy dissemination for interlaboratory comparisons. The consistency in results regarding single-photon purity and the similarity of the measured photon fluxes across two distinct laboratory environments underscores the robustness and versatility of this single-photon source, which shows its potential to be used as a standard source for interlaboratory comparisons in quantum radiometry. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).