21 research outputs found

    Correlations between Optical Properties and Voronoi-Cell Area of Quantum Dots

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    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 nano-holes, 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 Ό\mum-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

    Excitons in InGaAs Quantum Dots without Electron Wetting Layer States

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    The Stranski-Krastanov (SK) growth-mode facilitates the self-assembly of quantum dots (QDs) using lattice-mismatched semiconductors, for instance InAs and GaAs. SK QDs are defect-free and can be embedded in heterostructures and nano-engineered devices. InAs QDs are excellent photon emitters: QD-excitons, electron-hole bound pairs, are exploited as emitters of high quality single photons for quantum communication. One significant drawback of the SK-mode is the wetting layer (WL). The WL results in a continuum rather close in energy to the QD-confined-states. The WL-states lead to unwanted scattering and dephasing processes of QD-excitons. Here, we report that a slight modification to the SK-growth-protocol of InAs on GaAs -- we add a monolayer of AlAs following InAs QD formation -- results in a radical change to the QD-excitons. Extensive characterisation demonstrates that this additional layer eliminates the WL-continuum for electrons enabling the creation of highly charged excitons where up to six electrons occupy the same QD. Single QDs grown with this protocol exhibit optical linewidths matching those of the very best SK QDs making them an attractive alternative to standard InGaAs QDs

    Low-Noise GaAs Quantum Dots for Quantum Photonics

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    Quantum dots are both excellent single-photon sources and hosts for single spins. This combination enables the deterministic generation of Raman-photons -- bandwidth-matched to an atomic quantum-memory -- and the generation of photon cluster states, a resource in quantum communication and measurement-based quantum computing. GaAs quantum dots in AlGaAs can be matched in frequency to a rubidium-based photon memory, and have potentially improved electron spin coherence compared to the widely used InGaAs quantum dots. However, their charge stability and optical linewidths are typically much worse than for their InGaAs counterparts. Here, we embed GaAs quantum dots into an nn-ii-pp-diode specially designed for low-temperature operation. We demonstrate ultra-low noise behaviour: charge control via Coulomb blockade, close-to lifetime-limited linewidths, and no blinking. We observe high-fidelity optical electron-spin initialisation and long electron-spin lifetimes for these quantum dots. Our work establishes a materials platform for low-noise quantum photonics close to the red part of the spectrum.Comment: (19 pages, 12 figures, 1 table

    Sensing dot with high output swing for scalable baseband readout of spin qubits

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    A key requirement for quantum computing, in particular for a scalable quantum computing architecture, is a fast and high-fidelity qubit readout. For semiconductor based qubits, one limiting factor is the output swing of the charge sensor. We demonstrate GaAs and Si/SiGe asymmetric sensing dots (ASDs), which exceed the response of a conventional charge sensing dot by more than ten times, resulting in a boosted output swing of 3 mV3\,\text{mV}. This substantially improved output signal is due to a device design with a strongly decoupled drain reservoir from the sensor dot, mitigating negative feedback effects of conventional sensors. The large output signal eases the use of very low-power readout amplifiers in close proximity to the qubit and will thus render true scalable qubit architectures with semiconductor based qubits possible in the future.Comment: 8 pages, 7 figure

    Tailoring potentials by simulation-aided design of gate layouts for spin qubit applications

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    Gate-layouts of spin qubit devices are commonly adapted from previous successful devices. As qubit numbers and the device complexity increase, modelling new device layouts and optimizing for yield and performance becomes necessary. Simulation tools from advanced semiconductor industry need to be adapted for smaller structure sizes and electron numbers. Here, we present a general approach for electrostatically modelling new spin qubit device layouts, considering gate voltages, heterostructures, reservoirs and an applied source-drain bias. Exemplified by a specific potential, we study the influence of each parameter. We verify our model by indirectly probing the potential landscape of two design implementations through transport measurements. We use the simulations to identify critical design areas and optimize for robustness with regard to influence and resolution limits of the fabrication process.Comment: 10 pages, 6 figure

    Semiconductor membranes for electrostatic exciton trapping in optically addressable quantum transport devices

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    Combining the capabilities of gate defined quantum transport devices in GaAs-based heterostructures and of optically addressed self-assembled quantum dots could open broad perspectives for new devices and functionalities. For example, interfacing stationary solid-state qubits with photonic quantum states would open a new pathway towards the realization of a quantum network with extended quantum processing capacity in each node. While gated devices allow very flexible confinement of electrons or holes, the confinement of excitons without some element of self-assembly is much harder. To address this limitation, we introduce a technique to realize exciton traps in quantum wells via local electric fields by thinning a heterostructure down to a 220 nm thick membrane. We show that mobilities over 1×1061 \times 10^{6} cm2^{2}V−1^{-1}s−1^{-1} can be retained and that quantum point contacts and Coulomb oscillations can be observed on this structure, which implies that the thinning does not compromise the heterostructure quality. Furthermore, the local lowering of the exciton energy via the quantum-confined Stark effect is confirmed, thus forming exciton traps. These results lay the technological foundations for devices like single photon sources, spin photon interfaces and eventually quantum network nodes in GaAs quantum wells, realized entirely with a top-down fabrication process.Comment: v2: added missing acknowledgement. v3: fixed typos in acknolwedgemen

    Optically driving the radiative Auger transition

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    In a radiative Auger process, optical decay is accompanied by simultaneous excitation of other carriers. The radiative Auger process gives rise to weak red-shifted satellite peaks in the optical emission spectrum. These satellite peaks have been observed over a large spectral range: in the X-ray emission of atoms; close to visible frequencies on donors in semiconductors and quantum emitters; and at infrared frequencies as shake-up lines in two-dimensional systems. So far, all the work on the radiative Auger process has focussed on detecting the spontaneous emission. However, the fact that the radiative Auger process leads to photon emission suggests that the transition can also be optically excited. In such an inverted radiative Auger process, excitation would correspond to simultaneous photon absorption and electronic de-excitation. Here, we demonstrate optical driving of the radiative Auger transition on a trion in a semiconductor quantum dot. The radiative Auger and the fundamental transition together form a Λ\Lambda-system. On driving both transitions of this Λ\Lambda-system simultaneously, we observe a reduction of the fluorescence signal by up to 70%70\%. Our results demonstrate a type of optically addressable transition connecting few-body Coulomb interactions to quantum optics. The results open up the possibility of carrying out THz spectroscopy on single quantum emitters with all the benefits of optics: coherent laser sources, efficient and fast single-photon detectors. In analogy to optical control of an electron spin, the Λ\Lambda-system between the radiative Auger and the fundamental transitions allows optical control of the emitters' orbital degree of freedom.Comment: 8 pages, 6 figure

    Charge Tunable GaAs Quantum Dots in a Photonic n-i-p Diode

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    In this submission, we discuss the growth of charge-controllable GaAs quantum dots embedded in an n-i-p diode structure, from the perspective of a molecular beam epitaxy grower. The QDs show no blinking and narrow linewidths. We show that the parameters used led to a bimodal growth mode of QDs resulting from low arsenic surface coverage. We identify one of the modes as that showing good properties found in previous work. As the morphology of the fabricated QDs does not hint at outstanding properties, we attribute the good performance of this sample to the low impurity levels in the matrix material and the ability of n- and p-doped contact regions to stabilize the charge state. We present the challenges met in characterizing the sample with ensemble photoluminescence spectroscopy caused by the photonic structure used. We show two straightforward methods to overcome this hurdle and gain insight into QD emission properties

    Untersuchung des lokalen TropfenÀtzens mit Metalltropfen auf (111)A-orientierten GaAs-OberflÀchen mittels Molekularstrahlepitaxie und der dabei entstehenden Halbleiterheterostrukturen

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    Das Lokale TropfenÀtzen (LDE, eng. local droplet etching\textit {local droplet etching}) ist eine in situ Methode zur Nanostrukturierung von HalbleiteroberflÀchen. In der Arbeit wird ein Transfer dieser Technologie auf (111)A-orientierte GaAs-OberflÀchen vollzogen. Der Transfer ist von Interesse, da solche OberflÀchen die Herstellung von rotationssymmetrischen Nanostrukturen erlauben. Neben der OberflÀchenmanipulation lassen sich mittels LDE ringförmige Halbleiterheteronanostrukturen aus Indiumgalliumarsenid sowie Quantenpunkte erzeugen. Diese werden in der Arbeit unter Einsatz von Photolumineszenz- und Kathodolumineszenzspektroskopie auf ihre Legierungszusammensetzung und energetische Struktur hin untersucht

    Qubit control using a CMOS DAC at mK temperatures

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    Scaling up a quantum processor to tackle real-world problems requires qubit numbers in the millions. Scaleable semiconductor-based architectures have been proposed, many of them relying on integrated control instead of room-temperature electronics. However, it has not yet been shown that this can be achieved. For developing a high-density, low-cost wiring solution, it is highly advantageous for the electronics to be placed at the same temperature as the qubit chip. Therefore, tight integration of the qubit chip with ultra low power CMOS electronics presents a promising route. We demonstrate DC biasing qubit electrodes using a custom-designed 65nm CMOS capacitive DAC operating below 100mK [1]. Our chip features a complete proof of principle solution including interface, DAC memory and logic, the capacitive DAC, and sample-and-hold structures to provide voltages for multiple qubit gates. The bias DAC is combined with the qubit using a silicon interposer chip, enabling flexible routing and tight integration. Voltage stability, noise performance, and temperature are benchmarked using the qubit chip. Our results validate the potential of very low power qubit biasing using highly integrated circuits.[1] P. Vliex et al., IEEE Solid-State Circuits Letters, vol. 3, pp. 218-221, 202
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