Simple atomic quantum memory suitable for semiconductor quantum dot single photons

Quantum memories matched to single photon sources will form an important cornerstone of future quantum network technology. We demonstrate such a memory in warm Rb vapor with on-demand storage and retrieval, based on electromagnetically induced transparency. With an acceptance bandwidth of $\delta f$ = 0.66~GHz the memory is suitable for single photons emitted by semiconductor quantum dots. In this regime, vapor cell memories offer an excellent compromise between storage efficiency, storage time, noise level, and experimental complexity, and atomic collisions have negligible influence on the optical coherences. Operation of the memory is demonstrated using attenuated laser pulses on the single photon level. For 50 ns storage time we measure $\eta_{\textrm{e2e}}^{\textrm{50ns}} = 3.4(3)\%$ \emph{end-to-end efficiency} of the fiber-coupled memory, with an \emph{total intrinsic efficiency} $\eta_{\textrm{int}} = 17(3)\%$. Straightforward technological improvements can boost the end-to-end-efficiency to $\eta_{\textrm{e2e}} \approx 35\%$; beyond that increasing the optical depth and exploiting the Zeeman substructure of the atoms will allow such a memory to approach near unity efficiency. In the present memory, the unconditional readout noise level of $9\cdot 10^{-3}$ photons is dominated by atomic fluorescence, and for input pulses containing on average $\mu_{1}=0.27(4)$ photons the signal to noise level would be unity

Design of a parallel robot with additively manufactured flexure hinges for a cryogenic work environment

Automation is ubiquitous in today's industrial landscape and is finding its way into more and more highly specialised applications - also in the field of cryopreservation. The extreme work conditions in cryobanks place exceptionally high demands on the mechanical and electronic components used. The preservation and storage of biological samples take place at temperatures between -130 °C and -196 °C using liquid nitrogen as a cooling medium. The bearings and joints used in industrial parallel kinematic robots (for example, ball bearings or Cardan joints) jam at these ambient parameters and are unsuitable for an application within a cryobank. We, therefore, develop methods and technologies to enable fully automated handling of biological samples under cryogenic working conditions. The basis for this is a parallel kinematic robot structure that allows the drives to be placed outside the cold environment. In contrast, the rest of the robot structure can be actuated in a cryogenic container. In this context, the passive joints for this parallel robot are designed as additively manufactured monolithic flexure hinges. This paper presents the design, simulation, and construction of the parallel robot and focuses on the flexure hinges fabricated using the selective laser melting process (SLM). We describe the design of the flexure hinges, their intended use in the robot, and the experimental setup used for their validation. We also compare the operating parameters recorded in experiments (such as bending angle, bending moment) with the data obtained in finite element method simulations (FEM). In addition, we describe the geometric constraints and deviations of the manufactured joints due to the manufacturing process

Verkapselung von Zellen in Poly(N-Isopropylacrylamid) zur skalierbaren Generierung von Mikrogeweben Inaugural-Dissertation zur Erlangung der Doktorwürde der Medizinischen Fakultät der Universität zu Köln

Multizelluläre Mikrogewebe in Form von unregelmäßig geformten oder kugelförmigen Clustern können Zell-Zell-Interaktionen abbilden und gewinnen in verschiedenen Bereichen, darunter auch in der Herz-Kreislauf-Forschung, zunehmend an Bedeutung. Kardiale Mikrogewebe entwickeln sich zu hervorragenden Modellsystemen für die Arzneimittelprüfung in vitro („organ-on-a-chip“), werden als Gewebebausteine in 3D Druckverfahren verwendet und ebnen den Weg für verbesserte Zellersatztherapien in vivo. Mikrogewebe werden zum Beispiel im hängenden Tropfen oder speziellen Mikrotiterplatten kultiviert. Wirklich skalierbare Methoden zur Herstellung von Mikrogeweben sind bisher nicht verfügbar. In dieser Studie wird eine neue Methode zur Verkapselung von Zellen in Sphäroiden aus Poly N Isopropylacrylamid (PNIPAAm) vorgestellt. PNIPAAm fällt oberhalb seiner unteren kritischen Lösetemperatur (LCST) zu einem wasserunlöslichen, physikalisch verknüpften Gel aus und schrumpft durch den Ausstoß von Wasser. Aus murinen induziert pluripotenten Stammzellen gewonnene Kardiomyozyten und aus murinem Knochenmark gewonnene mesenchymale Stammzellen wurden in einem Mikrofluidiksystem in PNIPAAm verkapselt. Indem die Temperatur über die (LCST) von 32 °C erhöht wurde, wurden die Zellen in einem kollabierenden Polymernetzwerk eingeschlossen und die Zelldichte in den Polymerkapseln um den Faktor 2,7 erhöht. Innerhalb von 24 Stunden wurden zunächst stabile Herzmikrogewebe gebildet und später durch Auswaschen des PNIPAAm bei Temperaturen unterhalb der LCST aus ihrer Polymerhülle gelöst. Rhythmisch kontrahierende Mikrogewebe zeigten eine homogene Zellverteilung, eine altersabhängige Sarkomerorganisation und die Erzeugung von Aktionspotenzialen. Der neuartige Ansatz ist für die Bildung vitaler Mikrogewebe homogener Eigenschaften geeignet. Außerdem können verschiedene Zelltypen durch das Polymer in ein Gewebe zusammengeführt werden. Das ermöglicht eine Steigerung in der Vielfalt der modellierbaren Gewebe. In Zukunft kann die entwickelte Technik in skalierbare Arbeitsabläufe integriert werden

An artificial rubidium atom

Harnessing quantum mechanics to revolutionize various fields of established technology has fueled research activities in recent years. Especially the prospect of inherently secure quantum communication channels has become increasingly desirable to businesses, politicians and society as a whole to protect sensitive information. At heart, quantum communication relies on the distribution of entangled quantum states that make the communication impervious to eavesdroppers. Naturally single photons or photon pairs are an excellent choice for distributing entangled states at the speed-of-light through existing fiber-networks. One of the most promising quantum light sources constitute epitaxial quantum dots. The high oscillator strength renders them exceptionally bright, while still emitting nearly indistinguishable single photons with quantum efficiencies close to unity, an important prerequisite for high-fidelity photonic quantum applications. By carefully manufacturing the semiconductor heterostructure, the optical environment can be individually tailored, utilizing Purcell enhancement, by embedding the emitter in a semiconductor cavity or exploiting wave guiding properties in form of micropillars or nanowires to enhance the extraction efficiency. Inevitable optical attenuation in fiber networks however necessitates the overall communication channel length to be divided into subsections with nodes that can temporarily store the quantum information. Naturally this requires a quantum memory which can efficiently store the quantum state for a sufficiently long time and subsequently recreate the stored photon on demand. In this framework, atomic memories represent the established benchmark, unrivaled by quantum dots spin states which remain intrinsically limited by the decoherence-inducing interaction with the solid state environment. Combining the excellent single-photon source of a quantum dot with the superior coherence properties of alkali quantum memories in a hybrid system at each quantum node offers the best of both worlds, promising exponential speed-up of truly secure communication. This PhD thesis focuses on the requirements imposed on the quantum dot in such a hybrid quantum network and shows how these challenges can be overcome. The first part of the introduction aims to give a detailed overview on the underlying quantum communication protocol of a hybrid quantum network and how it fares against the more established DLCZ protocol. Next, single-photon sources, and quantum dots in particular, will be outlined and the growth mechanism and optical properties of epitaxial GaAs quantum dots discussed in detail. Lastly, to illustrate the framework in which a quantum dot can efficiently be paired with alkali atoms and to understand the challenges that arise, the mode of operation and attributes of the state-of-art broadband quantum memory will be summarized in chapter 2. The third chapter investigates the optical properties of an epitaxial GaAs quantum dot spectrally matched to rubidium. By means of strain-tuning, the quantum dot can address all hyperfine transitions of the rubidium D2 line and a first interaction with atomic vapors is shown in a transmission measurement. In conjunction with other optical measurements, true Fourier-limited emission of single photons is demonstrated. Furthermore, we establish a possible route to overcome the bandwidth mismatch of the two systems in form of the coherent-scattering regime. While this coherent-scattering regime offers quantum dot single-photons with sub-natural bandwidths, in form of elastically scattered single photons that predominately retain the small linewidth of the excitation laser, the emission is highly probabilistic and relies on continuous-wave excitation or weak, resonant laser pulses of durations exceeding the exciton lifetime. The fourth chapter demonstrates the generation of true on-demand single photons with tailored temporal waveform envelopes between 14 and 245 ns, overcoming the temporal limitations imposed by the exciton two-level system. The photonic bandwidth is reduced by almost one order of magnitude. In the following, the decay dynamics of a positively charged exciton in an GaAs quantum dot will be investigated by time-resolved photolumincescence and resonance fluorescence measurements (chapter 5). In Chapter 6 the optical properties of GaAs quantum dots in 500 nm thick membranes are characterized. Finally, an outlook into future developments and the solutions to remaining challenges will be presented

Krisenprävention jetzt: eine Geberkonferenz für den Sudan

Der Übergangsprozess im Sudan hängt am seidenen Faden - und jetzt kommt noch Covid-19 hinzu. Eine Rückkehr zu Repression und organisierter Gewalt scheint möglich. Die Organisation einer Geberkonferenz durch die Bundesregierung ist ein wichtiger erster Schritt, dieses Szenario zu verhindern

Numerical simulation and statistical analysis of a cascaded flexure hinge for use in a cryogenic working environment

Due to their many advantages, flexible structures are increasingly being used as guide and transmission elements in handling systems. Prismatic solid-state joints with a concentrated cross-sectional reduction are predominantly used as flexure pivots for both microscopic and macroscopic designs. A transfer of these geometries to applications in cryogenic working environments is not easily possible at temperatures below -130 °C due to the changed material properties. In this paper, the further development of swivel joints as cascaded solid state joints for such a cryogenic environment is illustrated by the targeted adaptation of certain joint parameters and dimensions. By means of a comprehensive FEM simulation, it can be shown how the influence of specific parameters affects movement accuracy, process forces and shape stability and to what extent these geometric parameters influence each other in their effect

Open access uptake in Germany 2010-2018: adoption in a diverse research landscape

This study investigates the development of open access (OA) to journal articles from authors affiliated with German universities and non-university research institutions in the period 2010-2018. Beyond determining the overall share of openly available articles, a systematic classification of distinct categories of OA publishing allowed us to identify different patterns of adoption of OA. Taking into account the particularities of the German research landscape, variations in terms of productivity, OA uptake and approaches to OA are examined at the meso-level and possible explanations are discussed. The development of the OA uptake is analysed for the different research sectors in Germany (universities, non-university research institutes of the Helmholtz Association, Fraunhofer Society, Max Planck Society, Leibniz Association, and government research agencies). Combining several data sources (incl. Web of Science, Unpaywall, an authority file of standardised German affiliation information, the ISSN-Gold-OA 3.0 list, and OpenDOAR), the study confirms the growth of the OA share mirroring the international trend reported in related studies. We found that 45% of all considered articles during the observed period were openly available at the time of analysis. Our findings show that subject-specific repositories are the most prevalent type of OA. However, the percentages for publication in fully OA journals and OA via institutional repositories show similarly steep increases. Enabling data-driven decision-making regarding the implementation of OA in Germany at the institutional level, the results of this study furthermore can serve as a baseline to assess the impact recent transformative agreements with major publishers will likely have on scholarly communication. Open Access uptake in Germany 2010-2018: Interactive Supplement: s. https://subugoe.github.io/oauni/articles/supplement.htm

No Deal: German Researchers' Publishing and Citing Behaviours after Big Deal Negotiations with Elsevier

In 2014, a union of German research organisations established Projekt DEAL, a national-level project to negotiate licensing agreements with large scientific publishers. Negotiations between DEAL and Elsevier began in 2016, and broke down without a successful agreement in 2018; in this time, around 200 German research institutions cancelled their license agreements with Elsevier, leading Elsevier to restrict journal access at those institutions. We investigated the effect on researchers' publishing and citing behaviours from a bibliometric perspective, using a dataset of ∼400,000 articles published by researchers at DEAL institutions between 2012-2020. We further investigated these effects with respect to the timing of contract cancellations, research disciplines, collaboration patterns, and article open-access status. We find evidence for a decrease in Elsevier’s market share of articles from DEAL institutions, with the largest year-on-year market share decreases occuring from 2018 to 2020 following the implementation of access restrictions. We also observe year-on-year decreases in the proportion of citations, although the decrease is smaller. We conclude that negotiations with Elsevier and access restrictions have led to some reduced willingness to publish in Elsevier journals, but that researchers are not strongly affected in their ability to cite Elsevier articles, implying that researchers use other methods to access scientific literature.Im Jahr 2014 gründete ein Zusammenschluss deutscher Forschungsorganisationen das Projekt DEAL, ein Projekt auf nationaler Ebene zur Aushandlung von Lizenzverträgen mit großen Wissenschaftsverlagen. Die Verhandlungen zwischen DEAL und Elsevier begannen 2016 und scheiterten 2018 ohne eine erfolgreiche Einigung; in dieser Zeit kündigten rund 200 deutsche Forschungseinrichtungen ihre Lizenzverträge mit Elsevier, was Elsevier dazu veranlasste, den Zeitschriftenzugang für diese Einrichtungen zu beschränken. Die Autor*innen untersuchten die Auswirkungen auf das Publikations- und Zitierverhalten von Forschenden aus bibliometrischer Sicht anhand eines Datensatzes von ∼400.000 Artikeln, die von Forschenden an DEAL-Einrichtungen zwischen 2012-2020 veröffentlicht wurden

An artificial Rb atom in a semiconductor with lifetime-limited linewidth

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 (1.48 GHz) with both resonant and non-resonant 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 upper state dephasing of the QD transition. We therefore establish a close-to-ideal solid-state source of single photons at a key wavelength for quantum technologies

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

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 $\mu$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