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Epitaxial GaAs quantum dots grown by droplet etching have recently shown excellent properties as sources of single photons as well as entangled photon pairs. Integration in some nanophotonic structures requires surface-to-dot distances of less than 100 nm. This demands a surface passivation scheme, which could be useful to lower the density of surface states. To address this issue, sulphur passivation with dielectric overlayer as an encapsulation is used for surface to QD distances of 40 nm, which results in the partial recovery of emission linewidths to bulk values as well as in the increase of the photoluminescence intensity., Comment: 24 pages, 8 figures

State-of-the-art quantum key distribution systems are based on the BB84 protocol and single photons generated by lasers. These implementations suffer from range limitations and security loopholes, which require expensive adaptation. The use of polarization entangled photon pairs substantially alleviates the security threads while allowing for basically arbitrary transmission distances when embedded in quantum repeater schemes. Semiconductor quantum dots are capable of emitting highly entangled photon pairs with ultra-low multi-pair emission probability even at maximum brightness. Here we report on the first implementation of the BBM92 protocol using a quantum dot source with an entanglement fidelity as high as 0.97(1). For a proof of principle, the key generation is performed between two buildings, connected by 350 metre long fiber, resulting in an average key rate of 135 bits/s and a qubit error rate of 0.019 over a time span of 13 hours, without resorting to time- or frequency-filtering techniques. Our work demonstrates the viability of quantum dots as light sources for entanglement-based quantum key distribution and quantum networks. By embedding them in state-of-the-art photonic structures, key generation rates in the Gbit/s range are at reach., Comment: 7 pages, 2 figures

The long dreamed quantum internet would consist of a network of quantum nodes (solid-state or atomic systems) linked by flying qubits, naturally based on photons, travelling over long distances at the speed of light, with negligible decoherence. A key component is a light source, able to provide single or entangled photon pairs. Among the different platforms, semiconductor quantum dots are very attractive, as they can be integrated with other photonic and electronic components in miniaturized chips. In the early 1990s two approaches were developed to synthetize self-assembled epitaxial semiconductor quantum dots (QDs), or artificial atoms, namely the Stranski-Krastanov (SK) and the droplet epitaxy (DE) method. Because of its robustness and simplicity, the SK method became the workhorse to achieve several breakthroughs in both fundamental and technological areas. The need for specific emission wavelengths or structural and optical properties has nevertheless motivated further research on the DE method and its more recent development, the local-droplet-etching (LDE), as complementary routes to obtain high quality semiconductor nanostructures. The recent reports on the generation of highly entangled photon pairs, combined with good photon indistinguishability, suggest that DE and LDE QDs may complement (and sometime even outperform) conventional SK InGaAs QDs as quantum emitters. We present here a critical survey of the state of the art of DE and LDE, highlighting the advantages and weaknesses, the obtained achievements and the still open challenges, in view of applications in quantum communication and technology., 26 pages, 5 figures

Integrated quantum photonics offers a promising path to scale up quantum optics experiments by miniaturizing and stabilizing complex laboratory setups. Central elements of quantum integrated photonics are quantum emitters, memories, detectors, and reconfigurable photonic circuits. In particular, integrated detectors not only offer optical readout but, when interfaced with reconfigurable circuits, allow feedback and adaptive control, crucial for deterministic quantum teleportation, training of neural networks, and stabilization of complex circuits. However, the heat generated by thermally reconfigurable photonics is incompatible with heat-sensitive superconducting single-photon detectors, and thus their on-chip co-integration remains elusive. Here we show low-power microelectromechanical reconfiguration of integrated photonic circuits interfaced with superconducting single-photon detectors on the same chip. We demonstrate three key functionalities for photonic quantum technologies: 28 dB high-extinction routing of classical and quantum light, 90 dB high-dynamic range single-photon detection, and stabilization of optical excitation over 12 dB power variation. Our platform enables heat-load free reconfigurable linear optics and adaptive control, critical for quantum state preparation and quantum logic in large-scale quantum photonics applications., Integrated photonics are promising to scale up quantum optics. Here the authors combine low-power microelectromechanical control and superconducting single-photon detectors on the same chip and demonstrate routing, high-dynamic-range detection, and power stabilization.

This paper presents an overview and perspectives on the epitaxial growth and optical properties of GaAs quantum dots obtained with the droplet etching method as high-quality sources of quantum light. We illustrate the recent achievements regarding the generation of single photons and polarization entangled photon pairs and the use of these sources in applications of central importance in quantum communication, such as entanglement swapping and quantum key distribution.

Scaling up quantum optics experiments requires on-chip reconfigurable quantum photonics, but their integration with detectors is a challenge. We show microelectromechanical reconfiguration of photonic circuits with on-chip superconducting single–photon detectors and demonstrate key applications.

Epitaxial quantum dots (QDs) have long been identified as promising charge spin qubits offering an efficient interface to quantum light and advanced semiconductor nanofabrication technologies. However, charge spin coherence is limited by interaction with the nanoscale ensemble of atomic nuclear spins, which is particularly problematic in strained self-assembled dots. Here, we use strain-free GaAs/AlGaAs QDs, demonstrating a fully functioning two-qubit quantum register using the nanoscale ensemble of arsenic quadrupolar nuclear spins as its hardware. Tailored radio-frequency pulses allow quantum state storage for up to 20 ms, and are used for few-microsecond single-qubit and two-qubit control gates with fidelities exceeding 97%. Combining long coherence and high-fidelity control with optical initialization and readout, we implement benchmark quantum computations such as Grover’s search and the Deutsch–Jozsa algorithm. Our results identify QD nuclei as a potential quantum information resource, which can complement charge spins and light particles in future QD circuits. Epitaxial quantum dot charge spin qubits offer efficient quantum light links, but their coherence is limited by interactions with the nanoscale ensemble of atomic nuclear spins. Employing nuclear spins instead as its hardware, strain-free GaAs/AlGaAs quantum dots can constitute a fully functional two-qubit quantum register.

We investigate the indistinguishability of single photons generated from strain-free GaAs/AlGaAs quantum dots using pulsed resonance fluorescence techniques. In pulsed two-photon interference measurements we observe a single photon indistinguishability with a raw visibility of 95%. This can be traced back to the short intrinsic lifetime of excitons and trions confined in the GaAs quantum dots and demonstrates that for this material system the generation of single photons is possible with near-unity indistinguishability even without Purcell enhancement.

Epitaxially grown self-assembled quantum dots (QDs) are promising candidates for an efficient single-photon generation. In order to maximize the number of photons extracted from the device, QDs are frequently embedded into photonic structures such as micro-cavities. Once the QD is positioned inside an optical cavity, the Purcell effect ensures that light is emitted predominantly into the cavity mode. In this contribution, we demonstrate results of emission tuning of QDs inserted in micro-cavities. A sample containing an InAs/GaAs QDs embedded in a planar cavity based on Bragg reflectors has been integrated onto the PMN-PT piezo crystal. Subsequently, micro-cavities have been fabricated by electron-beam lithography and reactive ion etching. The application of external stress produces linear shifts of QDs emission which could be tuned into the resonance with fundamental cavity mode and allow enhancement of QD emission due to the Purcell effect.

We investigate the degree of indistinguishability of cascaded photons emitted from a three-level quantum ladder system; in our case the biexciton-exciton cascade of semiconductor quantum dots. For the three-level quantum ladder system we theoretically demonstrate that the indistinguishability is inherently limited for both emitted photons and determined by the ratio of the lifetimes of the excited and intermediate states. We experimentally confirm this finding by comparing the quantum interference visibility of noncascaded emission and cascaded emission from the same semiconductor quantum dot. Quantum optical simulations produce very good agreement with the measurements and allow us to explore a large parameter space. Based on our model, we propose photonic structures to optimize the lifetime ratio and overcome the limited indistinguishability of cascaded photon emission from a three-level quantum ladder system.

By using the photons emitted from a GaAs quantum dot, we inject single hybrid light-matter particles into a 2D GaAs/AlGaAs microcavity-quantum well system, showing the viability of interface these complementary semiconductor photonic platforms.

Semiconductor devices are strong competitors in the race for the development of quantum computational systems. In this work, we interface two semiconductor building blocks of different dimensionalities with complementary properties: (1) a quantum dot hosting a single exciton and acting as a nearly ideal single-photon emitter and (2) a quantum well in a 2D microcavity sustaining polaritons, which are known for their strong interactions and unique hydrodynamic properties, including ultrafast real-time monitoring of their propagation and phase mapping. In the present experiment, we can thus observe how the injected single particles propagate and evolve inside the microcavity, giving rise to hydrodynamic features typical of macroscopic systems despite their genuine intrinsic quantum nature. In the presence of a structural defect, we observe the celebrated quantum interference of a single particle that produces fringes reminiscent of wave propagation. While this behavior could be theoretically expected, our imaging of such an interference pattern, together with a measurement of antibunching, constitutes the first demonstration of spatial mapping of the self-interference of a single quantum particle impinging on an obstacle., Watching a single particle behave like a wave Pioneering experimental observations of the quantum interference of single particles vividly illustrate wave-particle duality, and point the way towards manipulating individual excitations for quantum computing. An international team including Vincenzo Ardizzone and co-workers at CNR Nanotec in Lecce, Italy, generated single photons by directing a pulsed laser onto a nano-sized crystal called a quantum dot. These photons then interacted with excitons in a semiconductor cavity to produce hybrid particles called polaritons, which show great promise as switchable quantum bits (qubits) in future computers. Most interestingly, the researchers observed a single-polariton scattering off a defect in the cavity, leaving wave-like fringes where the particle’s incoming wavefunction interfered with the scattered parts. This observation represents the first two-dimensional mapping of wave-particle duality for a single quantum particle, and is reminiscent of the famous double-slit experiments.

The appearance of single photon sources in atomically thin semiconductors holds great promises for the development of a flexible and ultracompact quantum technology in which elastic strain engineering can be used to tailor their emission properties. Here, we show a compact and hybrid two-dimensional semiconductor-piezoelectric device that allows for controlling the energy of single photons emitted by quantum emitters localized in wrinkled WSe2 monolayers. We demonstrate that strain fields exerted by the piezoelectric device can be used to tune the energy of localized excitons in WSe2 up to 18 meV in a reversible manner while leaving the single photon purity unaffected over a wide range. Interestingly, we find that the magnitude and, in particular, the sign of the energy shift as a function of stress is emitter dependent. With the help of finite element simulations we suggest a simple model that explains our experimental observations and, furthermore, discloses that the type of strain (tensile or compressive) experienced by the quantum emitters strongly depends on their localization across the wrinkles. Our findings are of strong relevance for the practical implementation of single photon devices based on two-dimensional materials as well as for understanding the effects of strain on their emission properties., J.M.-S. acknowledges support through the Cların Programme from the Government of the Principality of Asturias and a Marie Curie-COFUND European grant (PA-18-ACB17-29). J.T.-G. acknowledges support through the Severo Ochoa Program from the Goverment of the Principality of Asturias (PA-18-PF-BP17-126). P.A.-G. acknowledges support from the European Research Council under Starting Grant 715496, 2DNANOPTICA. R.T., D.T., and M.S. acknowledge support by the European Research council (ERC) under the European Union’s Horizon 2020 Research and Innovation Programme (SPQRel, Grant agreement No. 679183). C.S. acknowledges support by the European Research council (ERC) under the European Union’s Horizon 2020 Research and Innovation Programme (UnLiMIt-2D, Grant agreement No. 679288). A.R. acknowledges support from the Linz Institute of Technology (LIT). The Würzburg group acknowledges support by the State of Bavaria. S.T. acknowledges support from NSF DMR-1552220, NSF CMMI-1933214 and NSF DMR-1838443.

Scalable quantum photonic architectures demand highly efficient, high-purity single-photon sources, which can be frequency matched via external tuning. We demonstrate a single-photon source based on an InAs quantum dot embedded in a micropillar resonator, which is frequency tunable via externally-applied stress. Our platform combines the advantages of a Bragg micropillar cavity and the piezo-strain-tuning technique enabling single photon spontaneous emission enhancement via the Purcell effect and quantum dot (QD) with tunable wavelength. Our optomechanical platform has been implemented by integration of semiconductor-based QD-micropillars on a piezoelectric substrate. The fabricated device exhibits spontaneous emission enhancement with a Purcell factor of 4.4$\pm$0.7 and allows for a pure triggered single-photon generation with $g^{(2)}(0)$ < 0.07 under resonant excitation. A quantum dot emission energy tuning range of 0.75 meV for 27 kV/cm applied to the piezo substrate has been achieved. Our results pave the way towards the scalable implementation of single-photon quantum photonic technologies using optoelectronic devices., 16 pages, 4 figures

In fundamental research, surface acoustic waves (SAWs), nanoscale earthquakes on a chip are an extremely versatile tool to dynamically probe and manipulate charge and spin excitations condensed matter systems. When excited on piezoelectric materials both the oscillating mechanical stress of the SAW and its accompanying electric fields can be harnessed to establish strong interactions between single quantum emitters and sound on the nanoscale [1–4]. Here, we report on the integration of single quantum emitters in suspended phononic waveguides. Figure 1 (a) shows an electron micrograph of a 50 μm long and 2 μm wide suspended beam patterned on an approximately 150 nm thick (Al)GaAs heterostructure containing a single layer of GaAs quantum dots (QDs). In our experiment a Rayleigh SAW is excited by a multiharmonic frequency-chirped transducer [5] excited on the unpatterned region and coupled into the suspended beam where it propagates as a Lamb mode of identical frequency [6]. Figure 1 (b) and (c) compare the optomechanical response of a single QD in the unpatterend region and in the suspended beam, respectively. Clearly, the optomechanical of the quantum emitter inside the suspended phononic waveguide is enhanced by a factor of 3.5, compared to that of the emitter strained by the Rayleigh SAW. This finding may arise from an enhancement of the optomechanical coupling between the emitter and the elastic wave in the one-dimensional phononic waveguide. Employing phase resolved spectroscopy on an ensemble of quantum emitters we can unambiguously show, that the observed optomechanical response arises from the coupling between the low phase velocity, anti-symmetric (flexural) Lamb-Mode and the QD. Moreover, our fabricated phononic waveguides exhibit low propagation losses.

The generation of high-quality entangled photon pairs has been a long-sought goal in modern quantum communication and computation. So far, the most widely used entangled photon pairs have been generated from spontaneous parametric down-conversion (SPDC), a process that is intrinsically probabilistic and thus relegated to a regime of low rates of pair generation. In contrast, semiconductor quantum dots can generate triggered entangled photon pairs through a cascaded radiative decay process and do not suffer from any fundamental trade-off between source brightness and multi-pair generation. However, a source featuring simultaneously high photon extraction efficiency, high degree of entanglement fidelity and photon indistinguishability has been lacking. Here, we present an entangled photon pair source with high brightness and indistinguishability by deterministically embedding GaAs quantum dots in broadband photonic nanostructures that enable Purcell-enhanced emission. Our source produces entangled photon pairs with a pair collection probability of up to 0.65(4) (single-photon extraction efficiency of 0.85(3)), entanglement fidelity of 0.88(2), and indistinguishabilities of 0.901(3) and 0.903(3) (brackets indicate uncertainty on last digit). This immediately creates opportunities for advancing quantum photonic technologies.

Scalable quantum photonic applications require wavelength reproducibility and high quality of the emitted photons. To address these issues, we investigate strain-tuning ofself-assembled semiconductor quantum dots embedded into micropillar cavities

Lead–magnesium niobate lead titanate (PMN-PT) has been proven as an excellent material for sensing and actuating applications. The fabrication of advanced ultra-small PMN-PT-based devices relies on the availability of sophisticated procedures for the micro-machining of PMN-PT thin films or bulk substrates. Approaches reported up to date include chemical etching, excimer laser ablation, and ion milling. To ensure an excellent device performance, a key mandatory feature for a micro-machining process is to preserve as far as possible the crystalline quality of the substrates; in other words, the fabrication method must induce a low density of cracks and other kind of defects. In this work, we demonstrate a relatively fast procedure for the fabrication of high-quality PMN-PT micro-machined actuators employing green femtosecond laser pulses. The fabricated devices feature the absence of extended cracks and well-defined edges with relatively low roughness, which is advantageous for the further integration of nanomaterials onto the piezoelectric actuators.

A hybrid device comprising a (Al)GaAs quantum dot heterostructure and a LiNbO$_3$ surface acoustic wave resonator is fabricated by heterointegration. High acoustic quality factors $Q>4000$ are demonstrated for an operation frequency $f\approx 300$ MHz. The measured large quality factor-frequency products $Q\times f>10^{12}$ ensures the suppression of decoherence due to thermal noise for temperatures exceeding $T>50\,\mathrm{K}$. Frequency and position dependent optomechanical coupling of single quantum dots and the resonator modes is observed., Accepted manuscript

Funding: H2020 European Research Council (679183); Austrian Science Fund (P29603); Seventh Framework Programme (601126); Central European Institute of Technology (7AMB17AT044); Horizon 2020 Framework Programme (731473); European Metrology Programme for Innovation and Research (17FUN06); Bundesministerium für Wissenschaft, Forschung und Wirtschaft (CZ 07 / 2017); Ministerstvo Školství, Mládeže a Telovýchovy; QuantERA (Hyper-U-P-S); QuantERA (CUSPIDOR); Linz Institute of Technology (LIT); LIT Secure and Correct Systems Lab; Bayerisches Staatsministerium für Bildung und Kultus, Wissenschaft und Kunst; Deutsche Forschungsgemeinschaft (SCHN1376 5.1). Light emission from solid-state quantum emitters is inherently prone to environmental decoherence, which results in a line broadening and in the deterioration of photon indistinguishability. Here we employ photon correlation Fourier spectroscopy (PCFS) to study the temporal evolution of such a broadening in two prominent systems: GaAs and In(Ga)As quantum dots. Differently from previous experiments, the emitters are driven with short laser pulses as required for the generation of high-purity single photons, the time scales we probe range from a few nanoseconds to milliseconds and, simultaneously, the spectral resolution we achieve can be as small as ∼ 2µeV. We find pronounced differences in the temporal evolution of different optical transition lines, which we attribute to differences in their homogeneous linewidth and sensitivity to charge noise. We analyze the effect of irradiation with additional white light, which reduces blinking at the cost of enhanced charge noise. Due to its robustness against experimental imperfections and its high temporal resolution and bandwidth, PCFS outperforms established spectroscopy techniques, such as Michelson interferometry. We discuss its practical implementation and the possibility to use it to estimate the indistinguishability of consecutively emitted single photons for applications in quantum communication and photonic-based quantum information processing. Publisher PDF

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)

Wide passband interdigital transducers are employed to establish a stable phase-lock between a train of laser pulses emitted by a mode-locked laser and a surface acoustic wave generated electrically by the transducer. The transducer design is based on a multi-harmonic split-finger architecture for the excitation of a fundamental surface acoustic wave and a discrete number of its overtones. Simply by introducing a variation of the transducer's periodicity $p$, a frequency chirp is added. This combination results in wide frequency bands for each harmonic. The transducer's conversion efficiency from the electrical to the acoustic domain was characterized optomechanically using single quantum dots acting as nanoscale pressure sensors. The ability to generate surface acoustic waves over a wide band of frequencies enables advanced acousto-optic spectroscopy using mode-locked lasers with fixed repetition rate. Stable phase-locking between the electrically generated acoustic wave and the train of laser pulses was confirmed by performing stroboscopic spectroscopy on a single quantum dot at a frequency of 320 MHz. Finally, the dynamic spectral modulation of the quantum dot was directly monitored in the time domain combining stable phase-locked optical excitation and time-correlated single photon counting. The demonstrated scheme will be particularly useful for the experimental implementation of surface acoustic wave-driven quantum gates of optically addressable qubits or collective quantum states or for multi-component Fourier synthesis of tailored nanomechanical waveforms., Comment: revised manuscript

Plasmon resonant arrays or meta-surfaces shape both the incoming optical field and the local density of states for emission processes. They provide large regions of enhanced emission from emitters and greater design flexibility than single nanoantennas. This makes them of great interest for engineering optical absorption and emission. Here we study the coupling of a single quantum emitter, a self-assembled semiconductor quantum dot, to a plasmonic meta-surface. We investigate the influence of the spectral properties of the nanoantennas and the position of the emitter in the unit cell of the structure. We observe a resonant enhancement due to emitter-array coupling in the far-field regime and find a clear difference from the interaction of an emitter with a single antenna.

We demonstrate an ab-initio predictive approach to computing the thermal conductivity ($\kappa$) of InAs/GaAs superlattices (SLs) of varying period, thickness, and composition. Our new experimental results illustrate how this method can yield good agreement with experiment when realistic composition profiles are used as inputs for the theoretical model. Due to intrinsic limitations to the InAs thickness than can be grown, bulk-like SLs show limited sensitivity to the details of their composition profile, but the situation changes significantly when finite-thickness effects are considered. If In segregation could be minimized during the growth process, SLs with significantly higher $\kappa$ than that of the random alloy with the same composition would be obtained, with the potential to improve heat dissipation in InAs/GaAs-based devices.

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].

Several semiconductor quantum dot techniques have been investigated for the generation of entangled photon pairs. Among the other techniques, droplet epitaxy enables the control of the shape, size, density, and emission wavelength of the quantum emitters. However, the fraction of the entanglement-ready quantum dots that can be fabricated with this method is still limited to around 5%, and matching the energy of the entangled photons to atomic transitions (a promising route towards quantum networking) remains an outstanding challenge. Here, we overcome these obstacles by introducing a modified approach to droplet epitaxy on a high symmetry (111)A substrate, where the fundamental crystallization step is performed at a significantly higher temperature as compared to previous reports. Our method drastically improves the yield of entanglement-ready photon sources near the emission wavelength of interest, which can be as high as 95% due to the low values of fine structure splitting and radiative lifetime, together with the reduced exciton dephasing offered by the choice of GaAs/AlGaAs materials. The quantum dots are designed to emit in the operating spectral region of Rb-based slow-light media, providing a viable technology for quantum repeater stations., 14 pages, 3 figures

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

Independent tuning of emission energy and decay time of neutral excitons confined in single self-assembled In(Ga)As/GaAs quantum dots is achieved by simultaneously employing vertical electric fields and lateral biaxial strain fields. By locking the emission energy via a closed-loop feedback on the piezoelectric actuator used to control the strain in the quantum dot, we continuously decrease the decay time of an exciton from 1.4 to 0.7 ns. Both perturbations are fully electrically controlled and their combination offers a promising route to engineer the indistinguishability of photons emitted from spatially separated single photon sources. © 2017 Author(s).

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

Plasmonics offers the opportunity of tailoring the interaction of light with single quantum emitters. However, the strong field localization of plasmons requires spatial fabrication accuracy far beyond what is required for other nanophotonic technologies. Furthermore, this accuracy has to be achieved across different fabrication processes to combine quantum emitters and plasmonics. We demonstrate a solution to this critical problem by controlled positioning of plasmonic nanoantennas with an accuracy of 11 nm next to single self-assembled GaAs semiconductor quantum dots, whose position can be determined with nanometer precision. These dots do not suffer from blinking or bleaching or from random orientation of the transition dipole moment as colloidal nanocrystals do. Our method introduces flexible fabrication of arbitrary nanostructures coupled to single-photon sources in a controllable and scalable fashion.

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.

A multilayer of GeSi nanowires separated with Si spacers was readily self-assembled on miscut Si (001) substrates with 8 degrees off toward (110). The nanowires oriented along the miscut direction were very small and compactly arranged on the vicinal surface. Systematic photoluminescence (PL) spectroscopy studies were carried out on the GeSi nanowires. With increasing excitation power, a sublinear power dependent PL intensity and a blue-shift of -7 meV/decade with nearly constant full width of half maximum (FWHM) of PL peaks from multilayer GeSi nanowires was observed. These results indicated a typical type-III band alignment of GeSi nanowires/Si. The blue-shift was attributed to band bending effect with the increase of photon-generated carriers. The nearly independent FWHM of PL peaks with the excitation power was explained in terms of the formation of mini-band due to strong coupling of holes in closely neighboring nanowires. An activation energy of -12 meV was extracted from the temperature-dependent intensity of PL peaks of the nanowires, which was assigned to be the exciton binding energy around the nanowires. Based on Raman spectra, the Ge content in GeSi nanowires was estimated to be -61%.

The prospect of using the quantum nature of light for secure communication keeps spurring the search and investigation of suitable sources of entangled photons. A single semiconductor quantum dot is one of the most attractive, as it can generate indistinguishable entangled photons deterministically and is compatible with current photonic-integration technologies. However, the lack of control over the energy of the entangled photons is hampering the exploitation of dissimilar quantum dots in protocols requiring the teleportation of quantum entanglement over remote locations. Here we introduce quantum dot-based sources of polarization-entangled photons whose energy can be tuned via three-directional strain engineering without degrading the degree of entanglement of the photon pairs. As a test-bench for quantum communication, we interface quantum dots with clouds of atomic vapours, and we demonstrate slow-entangled photons from a single quantum emitter. These results pave the way towards the implementation of hybrid quantum networks where entanglement is distributed among distant parties using optoelectronic devices., Quantum dots can produce entangled photons, but careful control over photon wavelength is required for real-life applications. Here, the authors use three-directional strain engineering to overcome this problem and to interface entangled photons from quantum dots with a cloud of natural atoms.

9th International Conference on Silicon Epitaxy and Heterostructures, May 18-22, 2015, Montreal, QC, Canada

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.

The physical properties of materials critically depend on the interatomic distances of the constituent atoms, which in turn can be tuned by introducing elastic strain (stress). While about four decades ago strain was generally regarded as a feature to be avoided in semiconductors,[1] strain engineering is nowadays ubiquitously used, e.g., to enhance the carrier mobility in transistors[2,3] and to achieve lasing action at reduced current densities in heterostructure lasers.[4] For this reason, its potential impact on our society has been compared to that of chemical alloying.[5] Strain can be used not only to enhance specific material/device properties, but also to impart completely new properties to a given material, thus opening the way to previously inaccessible applications. Examples are Ge turning into a direct-band gap semiconductor suitable for lasers,[6-9] exciton dynamics tailoring in nanowires induced by strain gradients,[10] graphene electronic states engineering and strain-induced giant pseudo-magnetic fields up to 300 T,[11,12] a topological insulator turning into a semiconductor,[13] linear electro-optical effects in Si,[14] bandgap modulation in atomically thin films [15] or surface chemical and electronic states tuning.[16]

The controlled positioning and fabrication of nanostructures on single quantum emitters allows tuning and engineering their properties. Here, we demonstrate two methods to modify the light emission properties of single semiconductor quantum dots using plasmonic nanostructures. First, we have used atomic force microscopy (AFM) based nanomanipulation to position plasmon resonant gold nanoparticles on single near-surface GaAs quantum dots. The particles act as optical antennas significantly enhancing the photoluminescence signal. The increase in luminescence is shown to originate from an increased excitation rate of the emitter. We have also developed a method to fabricate nanostructures aligned to near-surface semiconductor quantum dots using electron beam lithography (EBL). The accuracy of the alignment procedure achieved in this work is better than 30 nm. The flexibility of our approach allows single semiconductor quantum dots to be coupled to complex device structures in order to obtain new functionalities. As a first example we show how excitons in quantum dots can be coupled to plasmonic waveguides. Our results pave the way for realizing integrated nanoscale optical circuits for quantum optics research and applications.

This paper reviews the recent advances obtained by integrating semiconductor epitaxial films with embedded self-assembled quantum dots (QDs) on top of single-crystal piezoelectric substrates made of lead magnesium niobate–lead titanate (PMN-PT). This combination allows us to study in detail the effects produced by variable strains (up to about ± 0.2%) on the excitonic emission of single QDs and to add a powerful “tuning knob” to QDs. Biaxial stress can be used to reversibly shift the emission wavelength of QDs in a spectral range wider than 10 meV and to modify the relative binding energies of excitonic species. Anisotropic stress has instead a strong influence on the fine structure splitting of neutral excitons. Finally, we present experimental results on the effect of biaxial strain on the optical modes of microring optical resonators and show a simple approach enabling the compensation of piezo-creep via a closed-loop system. Schematic illustration of a QD membrane integrated on top of a PMN-PT substrate. Stress provided by the piezoelectric substrate allows broad range tuning of the emission properties of the overlying dots.

Deterministic control of position on a substrate, uniform size and shape are prerequisite for most of the envisioned applications of SiGe islands in electronic and optoelectronic devices. As an example of electronic application, tensile strained Si layers on top of coherent SiGe islands may be used as channels for field effect transistors (FETs) with enhanced electron mobility. For such a kind of application, site-controlled islands are required to allow for their external addressability. In this feature article we investigate the morphological and compositional evolution of site-controlled SiGe islands on pit-patterned Si(001) substrates. We then report on the first demonstrated n-channel FET with enhanced electron mobility based on SiGe islands. Finally, a new approach for further increase of the tensile strain is presented.

We present an overview and a comparison between three different methods of creating low density lateral In(Ga)As quantum dot molecules (QDMs) embedded in a GaAs matrix. Each of them is based on the use of nanoholes to control the dot nucleation site and generate the QDMs. The three methods used to create suitable nanoholes are: (1) In situ excess gallium droplet etching, where the nanohole shape is modified by overgrowth of a thin GaAs buffer to give QDM nucleation. (2) Ex situ electron-beam lithographic patterning and wet-etching, where the patterned nanohole size is critical for formation of QDMs. (3) In situ strain-selective etching of buried InAs quantum dots by AsBr3. The mechanisms of QDM formation, dependence on growth parameters, advantages and disadvantages of each technique and future challenges are discussed.

1 Institut fur Halbleiteroptik und Funktionelle Grenzflachen, Universitat Stuttgart, Allmandring 3, 70569 Stuttgart, Germany 2 Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan 3 Institut fur Integrative Nanowissenschaften IFW Dresden, Helmholtzstrase 20, 01069 Dresden, Germany 4 Max-Planck-Institut fur Festkorperforschung Stuttgart, Heisenbergstrase 1, 70569 Stuttgart, Germany

The controlled interaction of several quantum dots (QDs) mediated by plasmonic or photonic nanostructures promises interesting new functionality in the fields of quantum computing and telecommunication. Ultrafast laser pulses can be used to write and read out the state of the QD. We review ultrafast coherent spectroscopy of single QDs. The focus of this article is on the technique of transient reflection spectroscopy which can be applied to a broad range of samples and devices. It only requires optical access to a single quantum system next to a reflecting surface. We demonstrate the versatility of our approach by presenting several quantum optical studies such as Rabi oscillations, perturbed free induction decay, and quantum beats from an entangled excitonic state in weakly absorbing GaAs QDs. We expect this experimental method to make coherent experiments possible in elaborate devices where quantum emitters are interacting with a complex environment such as plasmonic waveguides or antennas.

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.

More than 80 years passed since the first publication on entangled quantum states. In this period of time the concept of spookily interacting quantum states became an emerging field of science. After various experiments proving the existence of such non-classical states, visionary ideas were put forward to exploit entanglement in quantum information science and technology. These novel concepts have not yet come out of the experimental stage, mostly because of the lack of suitable, deterministic sources of entangled quantum states. Among many systems under investigation, semiconductor quantum dots are particularly appealing emitters of on-demand, single polarization-entangled photon-pairs. Although, it was originally believed that quantum dots must exhibit a limited degree of entanglement related to numerous decoherence effects present in the solid-state. Recent studies invalidated the premise of unavoidable entanglement degrading effects. We review the relevant experiments which have led to these important discoveries and discuss the remaining challenges for the anticipated quantum technologies.