12 research outputs found
Advanced interferometry and entanglement measurement of quantum light from GaAs quantum dots
No full textFür Anwendungen im Bereich der Quanten-Information, Prozessierung und Kommunikation werden Qubits mit hoher Ununterscheidbarkeit und hohem Verschränkungsgrad benötigt. In dieser Arbeit untersuchen und besprechen wir die Eigenschaften von Qubits in der Form von Photonen, die von optisch angeregten GaAs Quantenpunkten emittiert werden. Wir beleuchten die zeitabhängige Verbreiterung der Emission-Linien und beschreiben die Messung dieser durch die Methode der Photonen-Korrelations-Fourier-Spektroskopie (PCFS), deren Ergebnisse wir mit den Messungen von Michelson- und Fabry-Perot-Interferometrie vergleichen. Diese zeitliche Entwicklung der Linienbreite vergleichen wir weiters mit der Ununterscheidbarkeit von aufeinanderfolgend emittierten Photonen durch den Hong-Ou-Mandel (HOM) Aufbau für unterschiedliche Zeitverzögerungen. Dabei zeigen wir die Messungen einer Probe, welche bis in den Bereich von Mikrosekunden keine Änderung der HOM Sichtbarkeit aufweist. Diese Quantenpunkte sind daher vielversprechend mögliche Quellen von Qubits in Quantum-Netzwerken. Wir besprechen weiters die Möglichkeit, mit GaAs Quantenpunkten hoch verschränkte Photonenzustände zu erzeugen, welche durch verschiedene Faktoren, insbesondere der Temperatur, den Grad der Verschränkung ändern.Applications in quantum-information, -processing and -communication require high degrees of indistinguishability and entanglement of the used qubits. In this thesis, we investigate and discuss the properties of qubits in the form of photons emitted from optically excited GaAs quantum dots. We look into the time-dependent broadening of the emission linewidth and how it can be measured using the photon-correlation-Fourier-spectroscopy (PCFS) technique, and compare the results with Michelson- and Fabry-Perot interferometry measurements. Further we connect the time evolution of the linewidth to the indistinguishability of consecutively emitted photons using the Hong-Ou-Mandel (HOM) setup for different time delays between the photons and demonstrate a sample where the HOM interference visibility remains unaltered up to the microseconds scale. These quantum dots are therefore promising candidates as sources for quantum networks. We also discuss the possibility to create highly entangled photon pairs using from GaAs quantum dots sources and how the degree of entanglement changes with with temperature, among other factors.Author Gabriel Undeutsch, BScMasterarbeit Universität Linz 202
GaAs quantum dots grown by droplet etching epitaxy as quantum light sources
No full textThis Perspective presents an overview 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 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. (C) 2021 Author(s)
Hyperfine-interaction limits polarization entanglement of photons from semiconductor quantum dots
No full textExcitons in quantum dots are excellent sources of polarization-entangled
photon pairs, but a quantitative understanding of their interaction with the
nuclear spin bath is still missing. Here we investigate the role of hyperfine
energy shifts using experimentally accessible parameters and derive an upper
limit to the achievable entanglement fidelity. Our results are consistent with
all available literature, indicate that spin-noise is often the dominant
process limiting the entanglement in InGaAs quantum dots, and suggest routes to
alleviate its effect
Nano Letters / Electric-Field Control of Photon Indistinguishability in Cascaded Decays in Quantum Dots
No full textHORIZON EUROPE European Innovation Council 101115575 891366 906046 10.55776/COE1 F7113 I 4320 I 4380 2024/08527-2 101017733 87113
Beyond the Four-Level Model: Dark and Hot States in Quantum Dots Degrade Photonic Entanglement
No full textEntangled photon pairs are essential for a multitude of quantum photonic applications. To date, the best performing solid-state quantum emitters of entangled photons are semiconductor quantum dots operated around liquid-helium temperatures. To favor the widespread deployment of these sources, it is important to explore and understand their behavior at temperatures accessible with compact Stirling coolers. Here we study the polarization entanglement among photon pairs from the biexciton–exciton cascade in GaAs quantum dots at temperatures up to ∼65 K. We observe entanglement degradation accompanied by changes in decay dynamics, which we ascribe to thermal population and depopulation of hot and dark states in addition to the four levels relevant for photon pair generation. Detailed calculations considering the presence and characteristics of the additional states and phonon-assisted transitions support the interpretation. We expect these results to guide the optimization of quantum dots as sources of highly entangled photons at elevated temperatures
Beyond the Four-Level Model: Dark and Hot States in Quantum Dots Degrade Photonic Entanglement
No full textEntangled photon
pairs are essential for a multitude of quantum
photonic applications. To date, the best performing solid-state quantum
emitters of entangled photons are semiconductor quantum dots operated
around liquid-helium temperatures. To favor the widespread deployment
of these sources, it is important to explore and understand their
behavior at temperatures accessible with compact Stirling coolers.
Here we study the polarization entanglement among photon pairs from
the biexciton–exciton cascade in GaAs quantum dots at temperatures
up to ∼65 K. We observe entanglement degradation accompanied
by changes in decay dynamics, which we ascribe to thermal population
and depopulation of hot and dark states in addition to the four levels
relevant for photon pair generation. Detailed calculations considering
the presence and characteristics of the additional states and phonon-assisted
transitions support the interpretation. We expect these results to
guide the optimization of quantum dots as sources of highly entangled
photons at elevated temperatures
Post-fabrication tuning of circular Bragg resonators for enhanced emitter-cavity coupling
No full textSolid-state quantum emitters embedded in circular Bragg resonators are
attractive due to their ability to emit quantum states of light with high
brightness and low multi-photon probability. As for any emitter-microcavity
system, fabrication imperfections limit the spatial and spectral overlap of the
emitter with the cavity mode, thus limiting their coupling strength. Here, we
show that an initial spectral mismatch can be corrected after device
fabrication by repeated wet chemical etching steps. We demonstrate ~16 nm
wavelength tuning for optical modes in AlGaAs resonators on oxide, leading to a
4-fold Purcell enhancement of the emission of single embedded GaAs quantum
dots. Numerical calculations reproduce the observations and suggest that the
achievable performance of the resonator is only marginally affected in the
explored tuning range. We expect the method to be applicable also to circular
Bragg resonators based on other material platforms, thus increasing the device
yield of cavity-enhanced solid-state quantum emitters
Postfabrication Tuning of Circular Bragg Resonators for Enhanced Emitter-Cavity Coupling
No full textSolid-state quantum
emitters embedded in circular Bragg resonators
are attractive due to their ability to emit quantum light with high
brightness and low multiphoton probability. As for any emitter-microcavity
system, fabrication imperfections limit the spatial and spectral overlap
of the emitter with the cavity mode, thus limiting their coupling
strength. Here, we show that an initial spectral mismatch can be corrected
after device fabrication by repeated wet chemical etching steps. We
demonstrate an ∼16 nm wavelength tuning for optical modes in
AlGaAs resonators on oxide, leading to a 4-fold Purcell enhancement
of the emission of single embedded GaAs quantum dots. Numerical calculations
reproduce the observations and suggest that the achievable performance
of the resonator is only marginally affected in the explored tuning
range. We expect the method to be applicable also to circular Bragg
resonators based on other material platforms, thus increasing the
device yield of cavity-enhanced solid-state quantum emitters
Quality indicators for the prevention of type 2 diabetes in Europe--IMAGE.
No full textBACKGROUND: The marked increase of type 2 diabetes necessitates active development and implementation of efficient prevention programs. A European level action has been taken by launching the IMAGE project to unify and improve the various prevention management concepts, which currently exist within the EU. This report describes the background and the methods used in the development of the IMAGE project quality indicators for diabetes primary prevention programs. It is targeted to the persons responsible for diabetes prevention at different levels of the health care systems. METHODS: Development of the quality indicators was conducted by a group of specialists representing different professional groups from several European countries. Indicators and measurement recommendations were produced by the expert group in consensus meetings and further developed by combining evidence and expert opinion. RESULTS: The quality indicators were developed for different prevention strategies: population level prevention strategy, screening for high risk, and high risk prevention strategy. Totally, 22 quality indicators were generated. They constitute the minimum level of quality assurance recommended for diabetes prevention programs. In addition, 20 scientific evaluation indicators with measurement standards were produced. These micro level indicators describe measurements, which should be used if evaluation, reporting, and scientific analysis are planned. CONCLUSIONS: We hope that these quality tools together with the IMAGE guidelines will provide a useful tool for improving the quality of diabetes prevention in Europe and make different prevention approaches comparable
A European Evidence-Based Guideline for the Prevention of Type 2 Diabetes
No full textBACKGROUND: The prevalence and socioeconomic burden of type 2 diabetes (T2DM) and associated co-morbidities are rising worldwide. AIMS: This guideline provides evidence-based recommendations for preventing T2DM. METHODS: A European multidisciplinary consortium systematically reviewed the evidence on the effectiveness of screening and interventions for T2DM prevention using SIGN criteria. RESULTS: Obesity and sedentary lifestyle are the main modifiable risk factors. Age and ethnicity are non-modifiable risk factors. Case-finding should follow a step-wise procedure using risk questionnaires and oral glucose tolerance testing. Persons with impaired glucose tolerance and/or fasting glucose are at high-risk and should be prioritized for intensive intervention. Interventions supporting lifestyle changes delay the onset of T2DM in high-risk adults (number-needed-to-treat: 6.4 over 1.8-4.6 years). These should be supported by inter-sectoral strategies that create health promoting environments. Sustained body weight reduction by >or= 5 % lowers risk. Currently metformin, acarbose and orlistat can be considered as second-line prevention options. The population approach should use organized measures to raise awareness and change lifestyle with specific approaches for adolescents, minorities and disadvantaged people. Interventions promoting lifestyle changes are more effective if they target both diet and physical activity, mobilize social support, involve the planned use of established behaviour change techniques, and provide frequent contacts. Cost-effectiveness analysis should take a societal perspective. CONCLUSIONS: Prevention using lifestyle modifications in high-risk individuals is cost-effective and should be embedded in evaluated models of care. Effective prevention plans are predicated upon sustained government initiatives comprising advocacy, community support, fiscal and legislative changes, private sector engagement and continuous media communication
