Search

Integration of entangled photon sources in a quantum photonic chip has enabled the most envisioned quantum photonic technologies to be performed in a compact platform with enhanced complexity and stability as compared to bulk optics. However, the technology to generate entangled photon states in a quantum photonic chip that are neither probabilistic nor restricted to low efficiency is still missing. Here, we introduce a hybrid quantum photonic chip where waveguide-coupled self-assembled quantum dots (QDs) are heterogeneously integrated onto a piezoelectric actuator. By exerting an anisotropic stress, we experimentally show that the fine structure splitting of waveguide-coupled quantum dots can be effectively eliminated. This allows for the demonstration of chip-integrated self-assembled QDs for generating and routing polarization-entangled photon pairs. Our results presented here would open up an avenue for implementing on-demand quantum information processing in a quantum photonic chip by employing all-solid-state self-assembled quantum dot emitters.

Quantum walks are the quantum mechanical analogue of classical random walks and an extremely powerful tool in quantum simulations, quantum search algorithms, and even for universal quantum computing. In our work, we have designed and fabricated an 8x8 two-dimensional square superconducting qubit array composed of 62 functional qubits. We used this device to demonstrate high fidelity single and two particle quantum walks. Furthermore, with the high programmability of the quantum processor, we implemented a Mach-Zehnder interferometer where the quantum walker coherently traverses in two paths before interfering and exiting. By tuning the disorders on the evolution paths, we observed interference fringes with single and double walkers. Our work is an essential milestone in the field, brings future larger scale quantum applications closer to realization on these noisy intermediate-scale quantum processors., 13 pages, 4 figures, and supplementary materials with 21 pages, 13 figures and 1 table

We conduct a combined experimental and theoretical study of the quantum-confined Stark effect in GaAs/AlGaAs quantum dots obtained with the local droplet etching method. In the experiment, we probe the permanent electric dipole and polarizability of neutral and positively charged excitons weakly confined in GaAs quantum dots by measuring their light emission under the influence of a variable electric field applied along the growth direction. Calculations based on the configuration-interaction method show excellent quantitative agreement with the experiment and allow us to elucidate the role of Coulomb interactions among the confined particles and -- even more importantly -- of electronic correlation effects on the Stark shifts. Moreover, we show how the electric field alters properties such as built-in dipole, binding energy, and heavy-light hole mixing of multiparticle complexes in weakly confining systems, underlining the deficiencies of commonly used models for the quantum-confined Stark effect.

In the quest to realize a scalable quantum network, semiconductor quantum dots (QDs) offer distinct advantages including high single-photon efficiency and indistinguishability, high repetition rate (tens of GHz with Purcell enhancement), interconnectivity with spin qubits, and a scalable on-chip platform. However, in the past two decades, the visibility of quantum interference between independent QDs rarely went beyond the classical limit of 50$\%$ and the distances were limited from a few meters to kilometers. Here, we report quantum interference between two single photons from independent QDs separated by 302 km optical fiber. The single photons are generated from resonantly driven single QDs deterministically coupled to microcavities. Quantum frequency conversions are used to eliminate the QD inhomogeneity and shift the emission wavelength to the telecommunication band. The observed interference visibility is 0.67$\pm$0.02 (0.93$\pm$0.04) without (with) temporal filtering. Feasible improvements can further extend the distance to 600 km. Our work represents a key step to long-distance solid-state quantum networks., 14 pages, 5 figures

Scaling up to a large number of qubits with high-precision control is essential in the demonstrations of quantum computational advantage to exponentially outpace the classical hardware and algorithmic improvements. Here, we develop a two-dimensional programmable superconducting quantum processor, Zuchongzhi, which is composed of 66 functional qubits in a tunable coupling architecture. To characterize the performance of the whole system, we perform random quantum circuits sampling for benchmarking, up to a system size of 56 qubits and 20 cycles. The computational cost of the classical simulation of this task is estimated to be 2-3 orders of magnitude higher than the previous work on 53-qubit Sycamore processor [Nature 574, 505 (2019)NATUAS0028-083610.1038/s41586-019-1666-5. We estimate that the sampling task finished by Zuchongzhi in about 1.2 h will take the most powerful supercomputer at least 8 yr. Our work establishes an unambiguous quantum computational advantage that is infeasible for classical computation in a reasonable amount of time. The high-precision and programmable quantum computing platform opens a new door to explore novel many-body phenomena and implement complex quantum algorithms.

Heralded entangling quantum gates are an essential element for the implementation of large-scale optical quantum computation. Yet, the experimental demonstration of genuine heralded entangling gates with free-flying output photons in linear optical system, was hindered by the intrinsically probabilistic source and double-pair emission in parametric down-conversion. Here, by using an on-demand single-photon source based on a semiconductor quantum dot embedded in a micro-pillar cavity, we demonstrate a heralded controlled-NOT (CNOT) operation between two single photons for the first time. To characterize the performance of the CNOT gate, we estimate its average quantum gate fidelity of ($87.8\pm1.2$)%. As an application, we generated event-ready Bell states with a fidelity of ($83.4\pm2.4$)%. Our results are an important step towards the development of photon-photon quantum logic gates., main text: 5 pages, 3 figures; supplement: 18 pages, 2 figure, 2 tables; comments are welcome!

This work was supported by the National Natural Science Foundation of China (Grants No. 11575174, No. 11674308, No. 11704424, and No. 11774326), the Chinese Academy of Sciences, and the National Key Research and Development Program of China. Photonic graph states are underlying resources for one-way optical quantum computation, quantum error correction, fundamental testing of quantum mechanics, and quantum communication networks. Most existing works, however, are based on the spontaneous parametric down-conversion sources that intrinsically suffer from probabilistic generation and double pair components. Here, we create two important classes of graph states, a polarization-encoded four-photon Greenberger–Horne–Zeilinger (GHZ) state and a linear cluster state, by actively demultiplexing a deterministic single-photon source from a semiconductor quantum dot embedded in a micropillar. A state fidelity of 0.790 ± 0.009 (0.763 ± 0.004) and a count rate of ∼13 Hz are observed for the four-photon GHZ (cluster) state. The results constitute a new route toward the multiphoton entanglement with deterministic single-photon sources. Postprint

A semiconductor quantum dot (QD) can generate highly indistinguishable single photons at a high rate. For application in quantum communication and integration in hybrid systems, control of the QD optical properties is essential. Understanding the connection between the optical properties of a QD and the growth process is therefore important. Here, we show for GaAs QDs, grown by infilling droplet-etched nanoholes, that the emission wavelength, the neutral-to-charged exciton splitting, and the diamagnetic shift are strongly correlated with the capture-zone area, an important concept from nucleation theory. We show that the capture-zone model applies to the growth of this system even in the limit of a low QD density in which atoms diffuse over $\ensuremath{\mu}\mathrm{m}$ distances. The strong correlations between the various QD parameters facilitate preselection of QDs for applications with specific requirements on the QD properties; they also suggest that a spectrally narrowed QD distribution will result if QD growth on a regular lattice can be achieved.

An optimal single-photon source should deterministically deliver one and only one photon at a time, with no trade-off between the source's efficiency and the photon indistinguishability. However, all reported solid-state sources of indistinguishable single photons had to rely on polarization filtering which reduced the efficiency by 50%, which fundamentally limited the scaling of photonic quantum technologies. Here, we overcome this final long-standing challenge by coherently driving quantum dots deterministically coupled to polarization-selective Purcell microcavities--two examples are narrowband, elliptical micropillars and broadband, elliptical Bragg gratings. A polarization-orthogonal excitation-collection scheme is designed to minimize the polarization-filtering loss under resonant excitation. We demonstrate a polarized single-photon efficiency of 0.60+/-0.02 (0.56+/-0.02), a single-photon purity of 0.975+/-0.005 (0.991+/-0.003), and an indistinguishability of 0.975+/-0.006 (0.951+/-0.005) for the micropillar (Bragg grating) device. Our work provides promising solutions for truly optimal single-photon sources combining near-unity indistinguishability and near-unity system efficiency simultaneously., 28 pages, 16 figures. partial overlap with arXiv:1809.10992 which was unpublished

An outstanding goal in quantum optics and scalable photonic quantum technology is to develop a source that each time emits one and only one entangled photon pair with simultaneously high entanglement fidelity, extraction efficiency, and photon indistinguishability. By coherent two-photon excitation of a single InGaAs quantum dot coupled to a circular Bragg grating bullseye cavity with broadband high Purcell factor up to 11.3, we generate entangled photon pairs with a state fidelity of 0.90(1), pair generation rate of 0.59(1), pair extraction efficiency of 0.62(6), and photon indistinguishability of 0.90(1) simultaneously. Our work will open up many applications in high-efficiency multi-photon experiments and solid-state quantum repeaters., 7 pages, 7 figures, PRL to appear

Funding: National Natural Science Foundation of China (11575174, 11674308, 11704424, 11774326, 11874346); Chinese Academy of Sciences; National Key Research and Development Program of China. We report a proof-of-principle demonstration of Shor’s algorithm with photons generated by an on-demand semiconductor quantum dot single-photon source for the first time. A fully compiled version of Shor’s algorithm for factoring 15 has been accomplished with a significantly reduced resource requirement that employs the four-photon cluster state. Genuine multiparticle entanglement properties are confirmed to reveal the quantum character of the algorithm and circuit. The implementation realizes the Shor’s algorithm with deterministic photonic qubits, which opens new applications for cluster state beyond one-way quantum computing. Publisher PDF

Hybrid systems consisting of a quantum emitter coupled to a mechanical oscillator are receiving increasing attention for fundamental science and potential applications in quantum technologies. In contrast to most of the presented works in this field, in which the oscillator eigenfrequencies are irreversibly determined by the fabrication process, we present here a simple approach to obtain frequency-tunable mechanical resonators based on suspended nanomembranes. The method relies on a micromachined piezoelectric actuator, which we use both to drive resonant oscillations of a suspended Ga(Al)As membrane with embedded quantum dots and to fine-tune their mechanical eigenfrequencies. Specifically, we excite oscillations with frequencies of at least 60 MHz by applying an AC voltage to the actuator and tune the eigenfrequencies by at least 25 times their linewidth by continuously varying the elastic stress state in the membranes through a DC voltage. The light emitted by optically excited quantum dots is used as a sensitive local strain gauge to monitor the oscillation frequency and amplitude. We expect that our method has the potential to be applicable to other optomechanical systems based on dielectric and semiconductor membranes possibly operating in the quantum regime., This work was supported by the FWF (P 29603), the Linz Institute of Technology (LIT), the LIT Secure and Correct Systems Lab funded by the state of Upper Austria, the EU project HANAS (No. 601126210), AWS Austria Wirtschaftsservice (PRIZE Programme, Grant No. P1308457), the European Research council (ERC) under the European Union's Horizon 2020 Research and Innovation Programme (SPQRel, Grant Agreement No. 679183), and the German Excellence Initiative via the Cluster of Excellence Nanosystems Initiative Munich (NIM). X. Yuan acknowledges support of the China Scholarship Council (CSC, No. 201306090010). Y. Huo thanks support of NSFC (No. 11774326) and STCSM (Nos. 17ZR1443900 and 17PJ1409900). J.M.-S. acknowledges support through the Clarín Programme from the Government of the Principality of Asturias and a Marie Curie-COFUND European grant (No. PA-18-ACB17-29).

The optical selection rules in epitaxial quantum dots are strongly influenced by the orientation of their natural quantization axis, which is usually parallel to the growth direction. This configuration is well suited for vertically emitting devices, but not for planar photonic circuits because of the poorly controlled orientation of the transition dipoles in the growth plane. Here we show that the quantization axis of gallium arsenide dots can be flipped into the growth plane via moderate in plane uniaxial stress. By using piezoelectric strain actuators featuring strain-amplification we study the evolution of the selection rules and excitonic fine-structure in a regime, in which quantum confinement can be regarded as a perturbation compared to strain in determining the symmetry properties of the system. The experimental and computational results suggest that uniaxial stress, may be the right tool to obtain quantum light sources with ideally oriented transition dipoles and enhanced oscillator strengths for integrated quantum photonics., Comment: 44 pages, 17 figures (including supplementary information)

We demonstrate a semiconductor based broadband near-field superlens in the mid-infrared regime. Here, the Drude response of a highly doped n-GaAs layer induces a resonant enhancement of evanescent waves accompanied by a significantly improved spatial resolution at radiation wavelengths around lambda = 20 mu m, adjustable by changing the doping concentration. In our experiments, gold stripes below the GaAs superlens are imaged with a lambda/6 subwavelength resolution by an apertureless near-field optical microscope utilizing infrared radiation from a free-electron laser. The resonant behavior of the observed superlensing effect is in excellent agreement with simulations based on the Drude-Lorentz model. Our results demonstrate a rather simple superlens implementation for infrared nanospectroscopy.

We demonstrate for the first time on-demand and wavelength-tunable single-photon emission from light-hole (LH) excitons in strain engineered GaAs quantum dots (QDs). The LH photon emission from tensile-strained GaAs QDs is systematically investigated with polarization-resolved, power-dependent photoluminescence spectroscopy, and photon-correlation measurements. By integrating QD-containing nanomembranes onto a piezo-actuator and driving single QDs with picosecond laser pulses, we achieve triggered and wavelength-tunable LH single-photon emission. Fourier transform spectroscopy is also performed, from which the coherence time of the LH single-photon emission is studied. We envision that this new type of LH exciton-based single-photon source (SPS) can be applied to realize an all-semiconductor based quantum interface in distributed quantum networks [Phys. Rev. Lett. 2008, 100, 096602].

Quantum photonics holds great promise for future technologies such as secure communication, quantum computation, quantum simulation, and quantum metrology. An outstanding challenge for quantum photonics is to develop scalable miniature circuits that integrate single-photon sources, linear optical components, and detectors on a chip. Plasmonic nanocircuits will play essential roles in such developments. However, for quantum plasmonic circuits, integration of stable, bright, and narrow-band single photon sources in the structure has so far not been reported. Here we present a plasmonic nanocircuit driven by a self-assembled GaAs quantum dot. Through a planar dielectric-plasmonic hybrid waveguide, the quantum dot efficiently excites narrow-band single plasmons that are guided in a two-wire transmission line until they are converted into single photons by an optical antenna. Our work demonstrates the feasibility of fully on-chip plasmonic nanocircuits for quantum optical applications.

Quantum photonics holds great promise for future technologies such as secure communication, quantum computation, quantum simulation, and quantum metrology. An outstanding challenge for quantum photonics is to develop scalable miniature circuits that integrate single-photon sources, linear optical components, and detectors on a chip. Plasmonic nanocircuits will play essential roles in such developments. Plasmonic components feature ultracompact geometries and can be controlled more flexibly and more energy-efficiently compared to conventional dielectric components due to strong field confinement and enhancement. Moreover, plasmonic components are compatible with electronic circuits, thanks to their deep subwavelength sizes as well as their electrically conducting materials. However, for quantum plasmonic circuits, integration of stable, bright, and narrow-band single photon sources in the structure has so far not been reported. Here we present a quantum plasmonic nanocircuit driven by a self-assembled GaAs quantum dot. The quantum dot efficiently excites narrow-band single plasmons that are guided in a two-wire transmission line until they are converted into single photons by an optical antenna. Our work demonstrates the feasibility of fully on-chip plasmonic nanocircuits for quantum optical applications.

The development of scalable sources of non-classical light is fundamental to unlock the technological potential of quantum photonics\cite{Kimble:Nat2008}. Among the systems under investigation, semiconductor quantum dots are currently emerging as near-optimal sources of indistinguishable single photons. However, their performances as sources of entangled-photon pairs are in comparison still modest. Experiments on conventional Stranski-Krastanow InGaAs quantum dots have reported non-optimal levels of entanglement and indistinguishability of the emitted photons. For applications such as entanglement teleportation and quantum repeaters, both criteria have to be met simultaneously. In this work, we show that this is possible focusing on a system that has received limited attention so far: GaAs quantum dots grown via droplet etching. Using a two-photon resonant excitation scheme, we demonstrate that these quantum dots can emit triggered polarization-entangled photons with high purity (g^(2)(0)=0.002 +/-0.002), high indistinguishability (0.93 +/-0.07) and high entanglement fidelity (0.94 +/-0.01). Such unprecedented degree of entanglement, which in contrast to InGaAs can theoretically reach near-unity values, allows Bell's inequality (2.64 +/-0.01) to be violated without the aid of temporal or spectral filtering. Our results show that if quantum-dot entanglement resources are to be used for future quantum technologies, GaAs might be the system of choice., Comment: 10 pages, 3 figures

Dark excitons (DEs) confined in epitaxial quantum dots (QDs) are interesting because of their long lifetime compared to bright excitons (BEs). For the same reason they are usually difficult to access in optical experiments. Here we report on the observation of vertically polarized light emission from DEs confined in high-quality epitaxial GaAs/AlGaAs QDs located in proximity of a cleaved facet of the QD specimen. Calculations based on the eight-band k\ifmmode\cdot\else\textperiodcentered\fi{}p method and configuration interaction allow us to attribute the brightening of the DE to the anisotropic strain present at the sample edge, which breaks the symmetry of the system and enhances valence-band mixing. The mechanism of DE brightening is discussed in detail by inspecting both the Bloch and envelope wave functions of the involved hole states. In addition, by investigating experimentally and theoretically QDs with different sizes, we find that the energy separation between DE and BEs tends to decrease with increasing QD height. Finally, the presence of a cleaved facet is found also to enhance the BE fine structure splitting. This work provides a simple method to optically probe dark excitonic states in QDs and shows that the properties of QDs can be significantly affected by the presence of nearby edges.

We investigate the effect of blinking on the two-photon interference measurement from two independent quantum emitters. We find that blinking significantly alters the statistics in the Hong-Ou-Mandel second-order intensity correlation function ${\mathrm{g}}^{(2)}(\ensuremath{\tau})$ and the outcome of two-photon interference measurements performed with independent quantum emitters. We theoretically demonstrate that the presence of blinking can be experimentally recognized by a deviation from the ${\mathrm{g}}_{D}^{(2)}(0)=0.5$ value when distinguishable photons from two emitters impinge on a beam splitter. Our findings explain the significant differences between linear losses and blinking for correlation measurements between independent sources and are experimentally verified using a parametric down-conversion photon-pair source. We show that blinking imposes a mandatory cross-check measurement to correctly estimate the degree of indistinguishability of photons emitted by independent quantum emitters.

We report on a fast, bandwidth-tunable single-photon source based on an epitaxial GaAs quantum dot. Exploiting spontaneous spin-flip Raman transitions, single photons at $780\,$nm are generated on-demand with tailored temporal profiles of durations exceeding the intrinsic quantum dot lifetime by up to three orders of magnitude. Second-order correlation measurements show a low multi-photon emission probability ($g^{2}(0)\sim\,0.10-0.15$) at a generation rate up to $10\,$MHz. We observe Raman photons with linewidths as low as $200\,$MHz, narrow compared to the $1.1\,$GHz linewidth measured in resonance fluorescence. The generation of such narrow-band single photons with controlled temporal shapes at the rubidium wavelength is a crucial step towards the development of an optimized hybrid semiconductor-atom interface., Comment: 8 pages, 4 figures

We demonstrate an all-electrically operated wavelength-tunable on demand single-photon source for the first time. The device consists of a light-emitting diode in the form of a semiconductor nanomembrane containing self-assembled quantum dots integrated onto a piezoelectric crystal. Triggered single photons are generated via injection of ultrashort electrical pulses into the diode, while their energy can be precisely tuned over a broad range by varying the voltage applied to the piezoelectric crystal. High speed operation of this single-photon-emitting diode up to 0.8 GHz is demonstrated. These results represent an important step toward the realization of electrically driven sources of indistinguishable photons on demand.

Triggered sources of entangled photon pairs are key components in most quantum communication protocols. For practical quantum applications, electrical triggering would allow the realization of compact and deterministic sources of entangled photons. Entangled-light-emitting-diodes based on semiconductor quantum dots are among the most promising sources that can potentially address this task. However, entangled-light-emitting-diodes are plagued by a source of randomness, which results in a very low probability of finding quantum dots with sufficiently small fine structure splitting for entangled-photon generation (∼10(-2)). Here we introduce strain-tunable entangled-light-emitting-diodes that exploit piezoelectric-induced strains to tune quantum dots for entangled-photon generation. We demonstrate that up to 30% of the quantum dots in strain-tunable entangled-light-emitting-diodes emit polarization-entangled photons. An entanglement fidelity as high as 0.83 is achieved with fast temporal post selection. Driven at high speed, that is 400 MHz, strain-tunable entangled-light-emitting-diodes emerge as promising devices for high data-rate quantum applications.

Self-assembled quantum dots (QDs) have emerged as attractive quantum systems for the generation of single and entangled photons for quantum information processing [1]. However, owing to the mesoscopic shape, strain and composition anisotropy, self-assembled QDs are subject to an inhomogeneous broadening and an energetic separation between two bright exciton states known as fine-structure-splitting (FSS). These hurdles hamper not only the realization of indistinguishable single photons from different QDs sources, but also the generation of entangled-photon pairs from the biexciton cascade of QDs. Here, we demonstrate a strain-tunable quantum-LED for electrical generation of ultrafast wavelength-tunable single photons and entangled-photon pairs. The device consists of a p-i-n nanomembrane containing InGaAs QDs integrated onto a piezoactuator capable of delivering biaxial or anisotropic strain fields. Triggered single photons are generated via injection of ultra-short electrical pulses into the diode as their energy is tuned over a broad range of ∼ 5meV by varying the voltage applied to the piezoactuator. High speed operation of this single-photon emitting diode up to 0.8 GHz has been demonstrated [2]. Alternatively, application of the anisotropic strain fields enables cancellation of the fine-structure-splitting (FSS) in QDs. We realize a high yield (∼ 30%) of QDs tuned to be suitable for generation of entangled-photon pairs, and the highest speed generation of entangled-photon, that is, 400MHz [3]. The device constitutes an important step towards the practical exploitation of high data-rate quantum applications involving a large number of all-electrically operated indistinguishable single- and entangled-photon sources.

Triggered sources of entangled photon pairs are key components in most quantum communication protocols. For practical quantum applications, electrical triggering would allow the realization of compact and deterministic sources of entangled photons. Entangled-light-emitting-diodes based on semiconductor quantum dots are among the most promising sources that can potentially address this task. However, entangled-light-emitting-diodes are plagued by a source of randomness, which results in a very low probability of finding quantum dots with sufficiently small fine structure splitting for entangled-photon generation (∼10−2). Here we introduce strain-tunable entangled-light-emitting-diodes that exploit piezoelectric-induced strains to tune quantum dots for entangled-photon generation. We demonstrate that up to 30% of the quantum dots in strain-tunable entangled-light-emitting-diodes emit polarization-entangled photons. An entanglement fidelity as high as 0.83 is achieved with fast temporal post selection. Driven at high speed, that is 400 MHz, strain-tunable entangled-light-emitting-diodes emerge as promising devices for high data-rate quantum applications., Quantum communications require sources of entangled photons. Electrically triggered sources usually suffer from low entangled-emission efficiency. Here, the authors use piezoelectric strains to tune the fine structure of quantum dot emitters, and increase the entanglement probability and fidelity.

We have demonstrated a two-contact quantum well infrared photodetector (QWIP) exhibiting simultaneous photoresponse in both the mid- and the long-wavelength atmospheric windows of 3-5 mu m and of 8-12 mu m. The structure of the device was achieved by sequentially growing a mid-wavelength QWIP part followed by a long-wavelength QWIP part separated by an n-doped layer. Compared with the conventional dual-band QWIP device utilizing three ohmic contacts, our QWIP is promising to greatly facilitate two-color focal plane array (FPA) fabrication by reducing the number of the indium bumps per pixel from three to one just like a monochromatic FPA fabrication and to increase the FPA fill factor by reducing one contact per pixel; another advantage may be that this QWIP FPA boasts broadband detection capability in the two atmospheric windows while using only a monochromatic readout integrated circuit. We attributed this simultaneous broadband detection to the different distributions of the total bias voltage between the mid- and long-wavelength QWIP parts.

We report results important for the creation of a best-of-both-worlds quantum hybrid system consisting of a solid-state source of single photons and an atomic ensemble as quantum memory. We generate single photons from a GaAs quantum dot (QD) frequency matched to the Rb $D2$ transitions and then use the Rb transitions to analyze spectrally the quantum dot photons. We demonstrate lifetime-limited QD linewidths (1.42 GHz) with both resonant and nonresonant excitation. The QD resonance fluorescence in the low power regime is dominated by Rayleigh scattering, a route to match quantum dot and Rb atom linewidths and to shape the temporal wave packet of the QD photons. Noise in the solid-state environment is relatively benign: there is a blinking of the resonance fluorescence at MHz rates but negligible dephasing of the QD excitonic transition. We therefore demonstrate significant progress towards the realization of an ideal solid-state source of single photons at a key wavelength for quantum technologies.

We fabricated an acousto-optic semiconductor hybrid device for strong optomechanical coupling of individual quantum emitters and a surface acoustic wave. Our device comprises a surface acoustic wave chip made from highly piezoelectric LiNbO$_3$ and a GaAs-based semiconductor membrane with an embedded layer of quantum dots. Employing multi-harmonic transducers, we generated sound waves on LiNbO$_3$ over a wide range of radio frequencies. We monitored their coupling to and propagation across the semiconductor membrane both in the electrical and optical domain. We demonstrate enhanced optomechanical tuning of the embedded quantum dots with increasing frequencies. This effect was verified by finite element modelling of our device geometry and attributed to an increased localization of the acoustic field within the semiconductor membrane. For moderately high acoustic frequencies, our simulations predict strong optomechanical coupling making our hybrid device ideally suited for applications in semiconductor based quantum acoustics., Comment: revised manuscript. unit conversion error fixed

We report on two-photon interferences from highly indistinguishable single photons emitted by a quantum dot. Stricly resonant excitation with picosecond laser pulses allows coherent state preparation with a signifi- cantly increased coherence time (T2 \sim 1 ns) and reduced lifetime (T1 \sim 650 ps), as compared to a non-resonant excitation scheme. Building-up the Hong-Ou-Mandel dip without post-selection of the interfering photons, visi- bilities greater than 70 % have been observed. Near-unity indistinguishable photons could be achieved for every dot if charge noise is controlled. Indeed, the remaining decoherence mechanism is likely due to the fluctuating electrostatic environment of the dots.

An electron and a hole trapped in the same quantum dot couple together to form an exciton. Conventionally the hole involved is a heavy hole. Light-hole excitons are now observed by applying elastic stress to initially unstrained gallium arsenide-based dots. The quasiparticles are identified by their optical emission signature, and could be used in future quantum technologies.

We demonstrate experimentally the possibility to manipulate the coupling strength in an asymmetric pair of electronically coupled InGaAs quantum dots by using externally induced strain fields. The coupling strength of holes confined in the dots increases linearly with increasing tensile strain. A model based on $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ theory explains the effect in terms of modified weight of the light hole component mediating the coupling in the barrier. Our results are relevant to the creation and control of entangled states in optically active quantum dots.

We combine micro-photoluminescence (PL) with terahertz excitation to investigate the response of single self-assembled InAs/GaAs quantum dots to intense terahertz pulses tuned to the s-to-p transition. Spectra and transients of single photoluminescence lines reveal the dynamics of electrons upon excitation and subsequent relaxation back into the initial state. Under certain circumstances, the terahertz pulse can release trapped charge carriers, which relax into the quantum dot. Furthermore, we demonstrate near-total depletion of the positive trion PL by an intense terahertz pulse.

Using terahertz-time-domain spectroscopy, it has been demonstrated before that an intersubband transition in photoexcited undoped quantum wells reveals a Fano-like line shape in transmission spectra due to the phase-sensitive coupling of ponderomotive and intersubband currents [ D. Golde et al. Phys. Rev. Lett. 102 127403 (2009)]. In the present experimental study on GaAs/AlGaAs quantum wells we attempt to delineate the observability conditions of this phenomenon. We find that intensity-based Fourier-transform infrared (FTIR) spectroscopy cannot uncover these ponderomotive effects. However, for time-domain spectroscopy they are shown not to be limited to the case of optically excited electrons, but can be seen in doped samples as well.

We demonstrate efficient coupling of excitons in near-surface GaAs quantum dots (QDs) to surface-plasmon polaritons. We observe distinct changes in the photoluminescence of the emitters as the distance between the QDs and the gold interface decreases. Based on an electric point-dipole model, we identify the surface plasmon launching rates for different QD-surface distances. While in conventional far-field experiments only a few percent of the emitted photons can be collected due to the high refractive index semiconductor substrate, already for distances around 30 nm the plasmon launching-rate becomes comparable to the emission rate into bulk photon modes, thus much larger than the photon collection rate. For even smaller distances, the degrading optical properties of the emitter counterweight the increasing coupling efficiency to plasmonic modes.

We prepare symmetry-controlled GaAs/AlGaAs quantum dots (QDs) on (001) GaAs substrates by infilling GaAs into AlGaAs nanoholes. For the most symmetric QDs, we measure an average excitonic fine structure splitting (FSS) of only (3.9 ± 1.8) μeV. The FSS and polarization direction of the two bright excitonic recombination lines directly reflect the degree of the QD symmetry. Since the FSS is comparable to typical homogeneous linewidths of excitonic recombination, these strain-free GaAs/AlGaAs QDs might offer a practical platform to generate entangled photons in future quantum devices.

We report a modified dots-in-a-well (DWELL) infrared photodetector by inserting some very thin GaAs or AlGaAs layers into the InAs dots. The photoluminescence (PL) measurements indicate that the modified DWELL structure with the insertion layers (ILs) of GaAs has a larger peak intensity and a narrower PL linewidth than that without the ILs. For the modified DWELL detector with AlGaAs ILs, the peak detection wavelength reaches very long infrared window of 14.1 μm. The peak detectivity D∗ is 1.1×108 cm Hz1/2/W at 77 K under normal incidence infrared irradiation.

We report a two-color quantum dot infrared photodetector (QDIP) using double tunneling barriers (DTBs) on one side of the InGaAs/AlGaAs dots. The two-color detection is achieved by changing the polarity of the applied bias voltages. In contrast, the same QDIP structure without the DTBs does not exhibit this detection wavelength tunability by switching the bias polarity. The two-color detection is ascribed to a different escape mechanism of electrons between positive and negative biases. The electrons escape out of the quantum well through resonant tunneling for a positive bias voltage while tunnel through a triangular barrier for a negative bias voltage.

We report a strain-tunable quantum light-emitting diode for generation of electrically triggered and energy-tunable single photons. The device consists of a p-i-n quantum dots containing nanomembrane integrated onto a piezoelectric actuator. By injecting ultrafast voltage pulses, electrically driven light emission is realized and the quantum nature of single-photon emission is revealed in the second-order time correlation measurements. In addition to electrically pulsed excitation, we achieved a broad tuning range ( $${\sim }$$ 9 meV) to the energy of the emitted single photons by imposing giant biaxial strain fields on quantum dots within the diode. We further show that energy of the single-photon emission emitted from different quantum dots can be tuned to degeneracy, thus rendering the quantum interference on which most of existing photonic quantum technologies rely. The device reported in this work represents an important step toward the realization of quantum repeater by using electrically triggered quantum light sources of indistinguishable photons.