Showing 1–48 of 48 results for author: Strittmatter, A

A Machine-Learning-Compatible Omnibus Test for Treatment Effect Heterogeneity

Authors: Elia Lapenta, Anthony Strittmatter, Pedro Vergara Merino

Abstract: This study proposes a formal, computationally efficient nonparametric omnibus test for treatment-effect heterogeneity that is compatible with a broad class of estimators, including modern machine-learning methods. The test is designed for settings in which identification can rely on high-dimensional controls while heterogeneity is assessed with respect to a low-dimensional subset of covariates. We… ▽ More This study proposes a formal, computationally efficient nonparametric omnibus test for treatment-effect heterogeneity that is compatible with a broad class of estimators, including modern machine-learning methods. The test is designed for settings in which identification can rely on high-dimensional controls while heterogeneity is assessed with respect to a low-dimensional subset of covariates. We derive the test statistic's asymptotic null distribution and develop a bootstrap procedure that is efficient because it avoids re-estimating nuisance parameters in each iteration. The testing approach applies to multiple empirical designs, including randomized experiments, selection-on-observables, difference-in-differences, and instrumental-variables settings. Monte Carlo simulations show that the test attains near-nominal size under the null and exhibits good power against heterogeneous alternatives. We further illustrate the procedure using two empirical applications on retirement savings and trade liberalization. △ Less

Submitted 7 July, 2026; originally announced July 2026.

Comments: 42 pages, 5 figures, 2 tables

seg2med: a bridge from artificial anatomy to multimodal medical images

Authors: Zeyu Yang, Zhilin Chen, Yipeng Sun, Anika Strittmatter, Anish Raj, Ahmad Allababidi, Johann S. Rink, Frank G. Zöllner

Abstract: We present seg2med, a modular framework for anatomy-driven multimodal medical image synthesis. The system integrates three components to enable high-fidelity, cross-modality generation of CT and MR images based on structured anatomical priors. First, anatomical maps are independently derived from three sources: real patient data, XCAT digital phantoms, and synthetic anatomies created by combining… ▽ More We present seg2med, a modular framework for anatomy-driven multimodal medical image synthesis. The system integrates three components to enable high-fidelity, cross-modality generation of CT and MR images based on structured anatomical priors. First, anatomical maps are independently derived from three sources: real patient data, XCAT digital phantoms, and synthetic anatomies created by combining organs from multiple patients. Second, we introduce PhysioSynth, a modality-specific simulator that converts anatomical masks into prior volumes using tissue-dependent parameters (e.g., HU, T1, T2, proton density) and modality-specific signal models. It supports simulation of CT and multiple MR sequences including GRE, SPACE, and VIBE. Third, the synthesized anatomical priors are used to train 2-channel conditional denoising diffusion models, which take the anatomical prior as structural condition alongside the noisy image, enabling generation of high-quality, structurally aligned images. The framework achieves SSIM of 0.94 for CT and 0.89 for MR compared to real data, and FSIM of 0.78 for simulated CT. The generative quality is further supported by a Frechet Inception Distance (FID) of 3.62 for CT synthesis. In modality conversion, seg2med achieves SSIM of 0.91 for MR to CT and 0.77 for CT to MR. Anatomical fidelity evaluation shows synthetic CT achieves mean Dice scores above 0.90 for 11 key abdominal organs, and above 0.80 for 34 of 59 total organs. These results underscore seg2med's utility in cross-modality synthesis, data augmentation, and anatomy-aware medical AI. △ Less

Submitted 12 June, 2025; v1 submitted 12 April, 2025; originally announced April 2025.

Comments: 17 pages, 10 figures Web demo available at https://huggingface.co/spaces/Zeyu0601/frankenstein

Membrane phononic crystals for high-Qm mechanical defect modes in piezoelectric aluminum nitride

Authors: Anastasiia Ciers, Laurentius Radit Nindito, Alexander Jung, Hannes Pfeifer, Armin Dadgar, Andre Strittmatter, Witlef Wieczorek

Abstract: Nanomechanical resonators with exceptionally low dissipation are advancing mechanics-based sensors and quantum technologies. The key for these advances is the engineering of localized phononic modes that are well-isolated from the environment, i.e., that exhibit a high mechanical quality factor, Qm. Membrane phononic crystals fabricated from strained thin films can realize high-Qm single or multip… ▽ More Nanomechanical resonators with exceptionally low dissipation are advancing mechanics-based sensors and quantum technologies. The key for these advances is the engineering of localized phononic modes that are well-isolated from the environment, i.e., that exhibit a high mechanical quality factor, Qm. Membrane phononic crystals fabricated from strained thin films can realize high-Qm single or multiple localized phononic defect modes at MHz frequencies. These defect modes can be efficiently interfaced with out-of-plane light or coupled to a microwave quantum circuit, enabling readout and control of their motion. When membrane phononic crystals are fabricated from a crystalline film, they could offer built-in functionality. We demonstrate a membrane phononic crystal realized in a strained 90 nm-thin film of aluminum nitride (AlN), which is a crystalline piezoelectric material. We engineer a high-Qm localized phononic defect mode at 1.8 MHz with a Qxf-product of 1.5x10^13 Hz at room temperature. In future devices, the built-in piezoelectricity of AlN can be utilized for direct coupling to qubits or in-situ tuning of mechanical mode frequencies, defect mode couplings, or acoustic bandgaps, which can be used as building blocks of tunable phononic circuits or low-noise sensors. △ Less

Submitted 25 June, 2025; v1 submitted 31 January, 2025; originally announced January 2025.

Thickness dependence of the mechanical properties of piezoelectric high-$Q_m$ nanomechanical resonators made from aluminium nitride

Authors: Anastasiia Ciers, Alexander Jung, Joachim Ciers, Laurentius Radit Nindito, Hannes Pfeifer, Armin Dadgar, Jürgen Bläsing, André Strittmatter, Witlef Wieczorek

Abstract: Nanomechanical resonators with high quality factors (\Qm{}) enable mechanics-based quantum technologies, in particular quantum sensing and quantum transduction. High-\Qm{} nanomechanical resonators in the kHz to MHz frequency range can be realized in tensile-strained thin films that allow the use of dissipation dilution techniques to drastically increase \Qm{}. In our work, we study the material p… ▽ More Nanomechanical resonators with high quality factors (\Qm{}) enable mechanics-based quantum technologies, in particular quantum sensing and quantum transduction. High-\Qm{} nanomechanical resonators in the kHz to MHz frequency range can be realized in tensile-strained thin films that allow the use of dissipation dilution techniques to drastically increase \Qm{}. In our work, we study the material properties of tensile-strained piezoelectric films made from aluminium nitride (AlN). We characterize crystalline AlN films with a thickness ranging from \SI{45}{\nano\meter} to \SI{295}{\nano\meter}, which are directly grown on Si(111) by metal-organic vapour-phase epitaxy. We report on the crystal quality and surface roughness, the piezoelectric response, and the residual and released stress of the AlN thin films. Importantly, we determine the intrinsic quality factor of the films at room temperature in high vacuum. We fabricate and characterize AlN nanomechanical resonators that exploit dissipation dilution to enhance the intrinsic quality factor by utilizing the tensile strain in the film. We find that AlN nanomechanical resonators below \SI{200}{\nano\meter} thickness exhibit the highest \Qf{}-product, on the order of $10^{12}$\,Hz. We discuss possible strategies to optimize the material growth that should lead to devices that reach even higher \Qf{}-products. This will pave the way for future advancements of optoelectromechanical quantum devices made from tensile-strained piezoelectric AlN. △ Less

Submitted 13 January, 2025; v1 submitted 4 October, 2024; originally announced October 2024.

Comments: 15 pages, 14 figures

Journal ref: Materials for Quantum Technology 4, 046301 (2024)

Nanomechanical crystalline AlN resonators with high quality factors for quantum optoelectromechanics

Authors: Anastasiia Ciers, Alexander Jung, Joachim Ciers, Laurentius Radit Nindito, Hannes Pfeifer, Armin Dadgar, Andre Strittmatter, Witlef Wieczorek

Abstract: High-\Qm{} mechanical resonators are crucial for applications where low noise and long coherence time are required, as mirror suspensions, quantum cavity optomechanical devices, or nanomechanical sensors. Tensile strain in the material enables the use of dissipation dilution and strain engineering techniques, which increase the mechanical quality factor. These techniques have been employed for hig… ▽ More High-\Qm{} mechanical resonators are crucial for applications where low noise and long coherence time are required, as mirror suspensions, quantum cavity optomechanical devices, or nanomechanical sensors. Tensile strain in the material enables the use of dissipation dilution and strain engineering techniques, which increase the mechanical quality factor. These techniques have been employed for high-\Qm{} mechanical resonators made from amorphous materials and, recently, from crystalline materials such as InGaP, SiC, and Si. A strained crystalline film exhibiting substantial piezoelectricity expands the capability of high-\Qm{} nanomechanical resonators to directly utilize electronic degrees of freedom. In this work we realize nanomechanical resonators with \Qm{} up to $2.9\times 10^{7}$ made from tensile-strained \SI{290}{\nano\meter}-thick AlN, which is an epitaxially-grown crystalline material offering strong piezoelectricity. We demonstrate nanomechanical resonators that exploit dissipation dilution and strain engineering to reach a \Qf-product approaching $10^{13}$\,\SI{}{\hertz} at room temperature. We realize a novel resonator geometry, triangline, whose shape follows the Al-N bonds and offers a central pad that we pattern with a photonic crystal. This allows us to reach an optical reflectivity above 80\% for efficient coupling to out-of-plane light. The presented results pave the way for quantum optoelectromechanical devices at room temperature based on tensile-strained AlN. △ Less

Submitted 19 September, 2024; v1 submitted 19 February, 2024; originally announced February 2024.

Energy-time entanglement from a resonantly driven quantum dot three-level system

Authors: M. Hohn, K. Barkemeyer, M. von Helversen, L. Bremer, M. Gschrey, J. -H. Schulze, A. Strittmatter, A. Carmele, S. Rodt, S. Bounouar, S. Reitzenstein

Abstract: Entanglement is a major resource in advanced quantum technology, where it can enable secure exchange of information over large distances. Energy-time entanglement is particularly attractive for its beneficial robustness in fiber-based quantum communication and can be demonstrated in the Franson interferometer. We report on Franson-type interference from a resonantly driven biexciton cascade under… ▽ More Entanglement is a major resource in advanced quantum technology, where it can enable secure exchange of information over large distances. Energy-time entanglement is particularly attractive for its beneficial robustness in fiber-based quantum communication and can be demonstrated in the Franson interferometer. We report on Franson-type interference from a resonantly driven biexciton cascade under continuous wave excitation. Our measurements yield a maximum visibility of (73 $\pm$ 2)% surpassing the limit of violation of Bell's inequality (70.7%) by more than one standard deviation. Despite being unable to satisfy a loophole free violation, our work demonstrates promising results concerning future works on such a system. Furthermore, our systematical studies on the impact of driving strength indicate that dephasing mechanisms and deviations from the cascaded emission have major impact on the degree of the measured energy-time entanglement. △ Less

Submitted 20 April, 2023; v1 submitted 13 January, 2023; originally announced January 2023.

Speed, Quality, and the Optimal Timing of Complex Decisions: Field Evidence

Authors: Uwe Sunde, Dainis Zegners, Anthony Strittmatter

Abstract: This paper presents an empirical investigation of the relation between decision speed and decision quality for a real-world setting of cognitively-demanding decisions in which the timing of decisions is endogenous: professional chess. Move-by-move data provide exceptionally detailed and precise information about decision times and decision quality, based on a comparison of actual decisions to a co… ▽ More This paper presents an empirical investigation of the relation between decision speed and decision quality for a real-world setting of cognitively-demanding decisions in which the timing of decisions is endogenous: professional chess. Move-by-move data provide exceptionally detailed and precise information about decision times and decision quality, based on a comparison of actual decisions to a computational benchmark of best moves constructed using the artificial intelligence of a chess engine. The results reveal that faster decisions are associated with better performance. The findings are consistent with the predictions of procedural decision models like drift-diffusion-models in which decision makers sequentially acquire information about decision alternatives with uncertain valuations. △ Less

Submitted 26 January, 2022; originally announced January 2022.

Optimal Targeting in Fundraising: A Causal Machine-Learning Approach

Authors: Tobias Cagala, Ulrich Glogowsky, Johannes Rincke, Anthony Strittmatter

Abstract: Ineffective fundraising lowers the resources charities can use to provide goods. We combine a field experiment and a causal machine-learning approach to increase a charity's fundraising effectiveness. The approach optimally targets a fundraising instrument to individuals whose expected donations exceed solicitation costs. Our results demonstrate that machine-learning-based optimal targeting allows… ▽ More Ineffective fundraising lowers the resources charities can use to provide goods. We combine a field experiment and a causal machine-learning approach to increase a charity's fundraising effectiveness. The approach optimally targets a fundraising instrument to individuals whose expected donations exceed solicitation costs. Our results demonstrate that machine-learning-based optimal targeting allows the charity to substantially increase donations net of fundraising costs relative to uniform benchmarks in which either everybody or no one receives the gift. To that end, it (a) should direct its fundraising efforts to a subset of past donors and (b) never address individuals who were previously asked but never donated. Further, we show that the benefits of machine-learning-based optimal targeting even materialize when the charity only exploits publicly available geospatial information or applies the estimated optimal targeting rule to later fundraising campaigns conducted in similar samples. We conclude that charities not engaging in optimal targeting waste significant resources. △ Less

Submitted 29 September, 2021; v1 submitted 10 March, 2021; originally announced March 2021.

The Gender Pay Gap Revisited with Big Data: Do Methodological Choices Matter?

Authors: Anthony Strittmatter, Conny Wunsch

Abstract: The vast majority of existing studies that estimate the average unexplained gender pay gap use unnecessarily restrictive linear versions of the Blinder-Oaxaca decomposition. Using a notably rich and large data set of 1.7 million employees in Switzerland, we investigate how the methodological improvements made possible by such big data affect estimates of the unexplained gender pay gap. We study th… ▽ More The vast majority of existing studies that estimate the average unexplained gender pay gap use unnecessarily restrictive linear versions of the Blinder-Oaxaca decomposition. Using a notably rich and large data set of 1.7 million employees in Switzerland, we investigate how the methodological improvements made possible by such big data affect estimates of the unexplained gender pay gap. We study the sensitivity of the estimates with regard to i) the availability of observationally comparable men and women, ii) model flexibility when controlling for wage determinants, and iii) the choice of different parametric and semi-parametric estimators, including variants that make use of machine learning methods. We find that these three factors matter greatly. Blinder-Oaxaca estimates of the unexplained gender pay gap decline by up to 39% when we enforce comparability between men and women and use a more flexible specification of the wage equation. Semi-parametric matching yields estimates that when compared with the Blinder-Oaxaca estimates, are up to 50% smaller and also less sensitive to the way wage determinants are included. △ Less

Submitted 19 February, 2021; v1 submitted 18 February, 2021; originally announced February 2021.

Thermal stability of emission from single InGaAs/GaAs quantum dots at the telecom O-band

Authors: Paweł Holewa, Marek Burakowski, Anna Musiał, Nicole Srocka, David Quandt, André Strittmatter, Sven Rodt, Stephan Reitzenstein, Grzegorz Sęk

Abstract: Single-photon sources are key building blocks in most of the emerging secure telecommunication and quantum information processing schemes. Semiconductor quantum dots (QD) have been proven to be the most prospective candidates. However, their practical use in fiber-based quantum communication depends heavily on the possibility of operation in the telecom bands and at temperatures not requiring exte… ▽ More Single-photon sources are key building blocks in most of the emerging secure telecommunication and quantum information processing schemes. Semiconductor quantum dots (QD) have been proven to be the most prospective candidates. However, their practical use in fiber-based quantum communication depends heavily on the possibility of operation in the telecom bands and at temperatures not requiring extensive cryogenic systems. In this paper we present a temperature-dependent study on single QD emission and single-photon emission from metalorganic vapour-phase epitaxy-grown InGaAs/GaAs QDs emitting in the telecom O-band. Micro-photoluminescence studies reveal that trapped holes in the vicinity of a QD act as reservoir of carriers that can be exploited to enhance photoluminescence from trion states observed at elevated temperatures up to at least 80 K. The luminescence quenching is mainly related to the promotion of holes to higher states in the valence band and this aspect must be primarily addressed in order to further increase the thermal stability of emission. Photon autocorrelation measurements yield single photon emission with a purity of $g_{50\mathrm{K}}^{(2)}\left(0\right)=0.13$ up to 50 K. Our results imply that these nanostructures are very promising candidates for single-photon sources at elevated temperatures in the telecom O-band and highlight means for improvements in their performance. △ Less

Submitted 19 November, 2020; v1 submitted 20 October, 2020; originally announced October 2020.

Comments: 3 figures

Journal ref: Scientific Reports 10, 21816 (2020)

Identifying causal channels of policy reforms with multiple treatments and different types of selection

Authors: Annabelle Doerr, Anthony Strittmatter

Abstract: We study the identification of channels of policy reforms with multiple treatments and different types of selection for each treatment. We disentangle reform effects into policy effects, selection effects, and time effects under the assumption of conditional independence, common trends, and an additional exclusion restriction on the non-treated. Furthermore, we show the identification of direct- a… ▽ More We study the identification of channels of policy reforms with multiple treatments and different types of selection for each treatment. We disentangle reform effects into policy effects, selection effects, and time effects under the assumption of conditional independence, common trends, and an additional exclusion restriction on the non-treated. Furthermore, we show the identification of direct- and indirect policy effects after imposing additional sequential conditional independence assumptions on mediating variables. We illustrate the approach using the German reform of the allocation system of vocational training for unemployed persons. The reform changed the allocation of training from a mandatory system to a voluntary voucher system. Simultaneously, the selection criteria for participants changed, and the reform altered the composition of course types. We consider the course composition as a mediator of the policy reform. We show that the empirical evidence from previous studies reverses when considering the course composition. This has important implications for policy conclusions. △ Less

Submitted 11 October, 2020; originally announced October 2020.

Decisions and Performance Under Bounded Rationality: A Computational Benchmarking Approach

Authors: Dainis Zegners, Uwe Sunde, Anthony Strittmatter

Abstract: This paper presents a novel approach to analyze human decision-making that involves comparing the behavior of professional chess players relative to a computational benchmark of cognitively bounded rationality. This benchmark is constructed using algorithms of modern chess engines and allows investigating behavior at the level of individual move-by-move observations, thus representing a natural be… ▽ More This paper presents a novel approach to analyze human decision-making that involves comparing the behavior of professional chess players relative to a computational benchmark of cognitively bounded rationality. This benchmark is constructed using algorithms of modern chess engines and allows investigating behavior at the level of individual move-by-move observations, thus representing a natural benchmark for computationally bounded optimization. The analysis delivers novel insights by isolating deviations from this benchmark of bounded rationality as well as their causes and consequences for performance. The findings document the existence of several distinct dimensions of behavioral deviations, which are related to asymmetric positional evaluation in terms of losses and gains, time pressure, fatigue, and complexity. The results also document that deviations from the benchmark do not necessarily entail worse performance. Faster decisions are associated with more frequent deviations from the benchmark, yet they are also associated with better performance. The findings are consistent with an important influence of intuition and experience, thereby shedding new light on the recent debate about computational rationality in cognitive processes. △ Less

Submitted 2 December, 2020; v1 submitted 26 May, 2020; originally announced May 2020.

Entanglement robustness to excitonic spin precession in a quantum dot

Authors: Samir Bounouar, Gabriel Rein, Kisa Barkemeyer, Julian Schleibner, Peter Schnauber, Manuel Gschrey, Jan-Hindrik Schulze, André Strittmatter, Sven Rodt, Andreas Knorr, Alexander Carmele, Stephan Reitzenstein

Abstract: A semiconductor quantum dot (QD) is an attractive resource to generate polarization-entangled photon pairs. We study the excitonic spin precession (flip-flop) in a family of QDs with different excitonic fine-structure splitting (FSS) and its impact on the entanglement of photons generated from the excitonic-biexcitonic radiative cascade. Our results reveal that coherent processes leave the time po… ▽ More A semiconductor quantum dot (QD) is an attractive resource to generate polarization-entangled photon pairs. We study the excitonic spin precession (flip-flop) in a family of QDs with different excitonic fine-structure splitting (FSS) and its impact on the entanglement of photons generated from the excitonic-biexcitonic radiative cascade. Our results reveal that coherent processes leave the time post-selected entanglement of QDs with finite FSS unaffected while changing the eigenstates of the system. The flip-flop's precession is observed via quantum tomography through anomalous oscillations of the coincidences in the rectilinear basis. A theoretical model is constructed with the inclusion of an excitonic flip-flop rate and is compared with a two-photon quantum tomography measurement on a QD exhibiting the spin flip-flop mechanism. A generalization of the theoretical model allows estimating the degree of entanglement as a function of the FSS and the spin-flip rate. For a finite temporal resolution, the negativity is found to be oscillating with respect to both the FSS and the spin-flip rate. This oscillatory behavior disappears for perfect temporal resolution and maximal entanglement is retrieved despite the flip-flop process. △ Less

Submitted 31 January, 2020; originally announced January 2020.

Journal ref: Phys. Rev. B 102, 045304 (2020)

Direct and Indirect Effects based on Changes-in-Changes

Authors: Martin Huber, Mark Schelker, Anthony Strittmatter

Abstract: We propose a novel approach for causal mediation analysis based on changes-in-changes assumptions restricting unobserved heterogeneity over time. This allows disentangling the causal effect of a binary treatment on a continuous outcome into an indirect effect operating through a binary intermediate variable (called mediator) and a direct effect running via other causal mechanisms. We identify aver… ▽ More We propose a novel approach for causal mediation analysis based on changes-in-changes assumptions restricting unobserved heterogeneity over time. This allows disentangling the causal effect of a binary treatment on a continuous outcome into an indirect effect operating through a binary intermediate variable (called mediator) and a direct effect running via other causal mechanisms. We identify average and quantile direct and indirect effects for various subgroups under the condition that the outcome is monotonic in the unobserved heterogeneity and that the distribution of the latter does not change over time conditional on the treatment and the mediator. We also provide a simulation study and an empirical application to the Jobs II programme. △ Less

Submitted 22 October, 2019; v1 submitted 11 September, 2019; originally announced September 2019.

Journal ref: Journal of Business and Economic Statistics, 2020, forthcoming

Sorting on the Used-Car Market After the Volkswagen Emission Scandal

Authors: Anthony Strittmatter, Michael Lechner

Abstract: The disclosure of the VW emission manipulation scandal caused a quasi-experimental market shock to the observable environmental quality of VW diesel vehicles. To investigate the market reaction to this shock, we collect data from a used-car online advertisement platform. We find that the supply of used VW diesel vehicles increases after the VW emission scandal. The positive supply side effects inc… ▽ More The disclosure of the VW emission manipulation scandal caused a quasi-experimental market shock to the observable environmental quality of VW diesel vehicles. To investigate the market reaction to this shock, we collect data from a used-car online advertisement platform. We find that the supply of used VW diesel vehicles increases after the VW emission scandal. The positive supply side effects increase with the probability of manipulation. Furthermore, we find negative impacts on the asking prices of used cars subject to a high probability of manipulation. We rationalize these findings with a model for sorting by the environmental quality of used cars. △ Less

Submitted 26 August, 2019; originally announced August 2019.

Journal ref: Journal of Environmental Economics and Management, 2020, 101

Heterogeneous Earnings Effects of the Job Corps by Gender Earnings: A Translated Quantile Approach

Authors: Anthony Strittmatter

Abstract: Several studies of the Job Corps tend to nd more positive earnings effects for males than for females. This effect heterogeneity favouring males contrasts with the results of the majority of other training programmes' evaluations. Applying the translated quantile approach of Bitler, Hoynes, and Domina (2014), I investigate a potential mechanism behind the surprising findings for the Job Corps. My… ▽ More Several studies of the Job Corps tend to nd more positive earnings effects for males than for females. This effect heterogeneity favouring males contrasts with the results of the majority of other training programmes' evaluations. Applying the translated quantile approach of Bitler, Hoynes, and Domina (2014), I investigate a potential mechanism behind the surprising findings for the Job Corps. My results provide suggestive evidence that the effect of heterogeneity by gender operates through existing gender earnings inequality rather than Job Corps trainability differences. △ Less

Submitted 23 August, 2019; originally announced August 2019.

Journal ref: Labour Economics, 2019, 61

Interplay between emission wavelength and s-p splitting in MOCVD-grown InGaAs/GaAs quantum dots emitting above 1.3 μm

Authors: Paweł Podemski, Anna Musiał, Krzysztof Gawarecki, Aleksander Maryński, Przemysław Gontar, Artem Bercha, Witold A. Trzeciakowski, Nicole Srocka, Tobias Heuser, David Quandt, André Strittmatter, Sven Rodt, Stephan Reitzenstein, Grzegorz Sęk

Abstract: The electronic structure of strain-engineered single InGaAs/GaAs quantum dots emitting in the telecommunication O band is probed experimentally by photoluminescence excitation spectroscopy. Observed resonances can be attributed to p-shell states of individual quantum dots. The determined energy difference between s-shell and p-shell shows an inverse dependence on the emission energy. The experimen… ▽ More The electronic structure of strain-engineered single InGaAs/GaAs quantum dots emitting in the telecommunication O band is probed experimentally by photoluminescence excitation spectroscopy. Observed resonances can be attributed to p-shell states of individual quantum dots. The determined energy difference between s-shell and p-shell shows an inverse dependence on the emission energy. The experimental data are compared with the results of confined states calculations, where the impact of the size and composition in the investigated structures is simulated within the 8-band $\mathbf{k}\cdot\mathbf{p}$ model. On this basis, the experimental observation is attributed mainly to changes in indium content within individual quantum dots, indicating a way of engineering and selecting a desired quantum dot, whose electronic structure is the most suitable for a given nanophotonic application. △ Less

Submitted 17 January, 2020; v1 submitted 14 August, 2019; originally announced August 2019.

Comments: 15 pages, 3 figures, improved manuscript text and figures

Journal ref: Appl. Phys. Lett. 116, 023102 (2020)

Tools for the Performance Optimization of Single-Photon Quantum Key Distribution

Authors: Timm Kupko, Martin v. Helversen, Lucas Rickert, Jan-Hindrik Schulze, André Strittmatter, Manuel Gschrey, Sven Rodt, Stephan Reitzenstein, Tobias Heindel

Abstract: Quantum light sources emitting triggered single photons or entangled photon pairs have the potential to boost the performance of quantum key distribution (QKD) systems. Proof-of-principle experiments affirmed these prospects, but further efforts are necessary to push this field beyond its current status. In this work, we show that temporal filtering of single-photon pulses enables a performance op… ▽ More Quantum light sources emitting triggered single photons or entangled photon pairs have the potential to boost the performance of quantum key distribution (QKD) systems. Proof-of-principle experiments affirmed these prospects, but further efforts are necessary to push this field beyond its current status. In this work, we show that temporal filtering of single-photon pulses enables a performance optimization of QKD systems implemented with realistic quantum light sources, both in experiment and simulations. To this end, we analyze the influence of temporal filtering of sub-Poissonian single-photon pulses on the expected secret key fraction, the quantum bit error ratio, and the tolerable channel losses. For this purpose, we developed a basic QKD testbed comprising a triggered solid-state single-photon source and a receiver module designed for four-state polarization coding via the BB84 protocol. Furthermore, we demonstrate real-time security monitoring by analyzing the photon statistics, in terms of $g^{(2)}(0)$, inside the quantum channel by correlating the photon flux recorded at the four ports of our receiver. Our findings are useful for the certification of QKD and can be applied and further extended for the optimization of various implementations of quantum communication based on sub-Poissonian quantum light sources, including measurement-device-independent schemes of QKD as well as quantum repeaters. Our work represents an important contribution towards the development of QKD-secured communication networks based on quantum light sources. △ Less

Submitted 16 January, 2020; v1 submitted 7 August, 2019; originally announced August 2019.

Comments: 13 pages, 8 figures, including Supplementary Information; revised version with new results and extended discussions

Journal ref: npj Quantum Information 6, 29 (2020)

What Is the Value Added by Using Causal Machine Learning Methods in a Welfare Experiment Evaluation?

Authors: Anthony Strittmatter

Abstract: Recent studies have proposed causal machine learning (CML) methods to estimate conditional average treatment effects (CATEs). In this study, I investigate whether CML methods add value compared to conventional CATE estimators by re-evaluating Connecticut's Jobs First welfare experiment. This experiment entails a mix of positive and negative work incentives. Previous studies show that it is hard to… ▽ More Recent studies have proposed causal machine learning (CML) methods to estimate conditional average treatment effects (CATEs). In this study, I investigate whether CML methods add value compared to conventional CATE estimators by re-evaluating Connecticut's Jobs First welfare experiment. This experiment entails a mix of positive and negative work incentives. Previous studies show that it is hard to tackle the effect heterogeneity of Jobs First by means of CATEs. I report evidence that CML methods can provide support for the theoretical labor supply predictions. Furthermore, I document reasons why some conventional CATE estimators fail and discuss the limitations of CML methods. △ Less

Submitted 4 December, 2021; v1 submitted 16 December, 2018; originally announced December 2018.

Excitonic complexes in MOCVD-grown InGaAs/GaAs quantum dots emitting at telecom wavelengths

Authors: Paweł Mrowiński, Anna Musiał, Krzysztof Gawarecki, Łukasz Dusanowski, Tobias Heuser, Nicole Srocka, David Quandt, André Strittmatter, Sven Rodt, Stephan Reitzenstein, Grzegorz Sęk

Abstract: Hereby, we present a comprehensive experimental and theoretical study of the electronic structure and optical properties of excitonic complexes in strain-engineered InGaAs/GaAs quantum dots (QDs) grown by metal-organic chemical vapour deposition and emitting at the 1300 nm telecommunication window. Single QD properties have been determined experimentally for a number of nanostructures by means of… ▽ More Hereby, we present a comprehensive experimental and theoretical study of the electronic structure and optical properties of excitonic complexes in strain-engineered InGaAs/GaAs quantum dots (QDs) grown by metal-organic chemical vapour deposition and emitting at the 1300 nm telecommunication window. Single QD properties have been determined experimentally for a number of nanostructures by means of excitation-power-dependent and polarization-resolved microphotoluminescence and further compared with the results of confined states calculations employing the 8-band kp theory combined with the configuration interaction method. The origin of excitonic complexes has been exemplarily confirmed based on magnetooptical and correlation spectroscopy study. Understanding the influence of structural parameters and compositions (of QDs themselves as well as in the neighbouring strain reducing layer) allows to distinguish which of them are crucial to control the emission wavelength to achieve the telecommunication spectral range or to affect binding energies of the fundamental excitonic complexes. The obtained results provide deeper knowledge on control and on limitations of the investigated structures in terms of good spectral isolation of individual optical transitions and the spatial confinement that are crucial in view of QD applications in single-photon sources of high purity at telecom wavelengths. △ Less

Submitted 11 August, 2019; v1 submitted 4 November, 2018; originally announced November 2018.

Journal ref: Phys. Rev. B 100, 115310 (2019)

Machine Learning Estimation of Heterogeneous Causal Effects: Empirical Monte Carlo Evidence

Authors: Michael C. Knaus, Michael Lechner, Anthony Strittmatter

Abstract: We investigate the finite sample performance of causal machine learning estimators for heterogeneous causal effects at different aggregation levels. We employ an Empirical Monte Carlo Study that relies on arguably realistic data generation processes (DGPs) based on actual data. We consider 24 different DGPs, eleven different causal machine learning estimators, and three aggregation levels of the e… ▽ More We investigate the finite sample performance of causal machine learning estimators for heterogeneous causal effects at different aggregation levels. We employ an Empirical Monte Carlo Study that relies on arguably realistic data generation processes (DGPs) based on actual data. We consider 24 different DGPs, eleven different causal machine learning estimators, and three aggregation levels of the estimated effects. In the main DGPs, we allow for selection into treatment based on a rich set of observable covariates. We provide evidence that the estimators can be categorized into three groups. The first group performs consistently well across all DGPs and aggregation levels. These estimators have multiple steps to account for the selection into the treatment and the outcome process. The second group shows competitive performance only for particular DGPs. The third group is clearly outperformed by the other estimators. △ Less

Submitted 17 December, 2018; v1 submitted 31 October, 2018; originally announced October 2018.

Journal ref: Econometrics Journal (2021), volume 24, pp.134-161

Expropriations, Property Confiscations and New Offshore Entities: Evidence from the Panama Papers

Authors: Ralph-Christopher Bayer, Roland Hodler, Paul Raschky, Anthony Strittmatter

Abstract: Using the Panama Papers, we show that the beginning of media reporting on expropriations and property confiscations in a country increases the probability that offshore entities are incorporated by agents from the same country in the same month. This result is robust to the use of country-year fixed effects and the exclusion of tax havens. Further analysis shows that the effect is driven by countr… ▽ More Using the Panama Papers, we show that the beginning of media reporting on expropriations and property confiscations in a country increases the probability that offshore entities are incorporated by agents from the same country in the same month. This result is robust to the use of country-year fixed effects and the exclusion of tax havens. Further analysis shows that the effect is driven by countries with non-corrupt and effective governments, which supports the notion that offshore entities are incorporated when reasonably well-intended and well-functioning governments become more serious about fighting organized crime by confiscating proceeds of crime. △ Less

Submitted 23 October, 2018; originally announced October 2018.

Journal ref: Journal of Economic Behavior and Organization, 2020, vol. 171, pp. 132-152

Religion and Terrorism: Evidence from Ramadan Fasting

Authors: Roland Hodler, Paul Raschky, Anthony Strittmatter

Abstract: We study the effect of religion and intense religious experiences on terrorism by focusing on one of the five pillars of Islam: Ramadan fasting. For identification, we exploit two facts: First, daily fasting from dawn to sunset during Ramadan is considered mandatory for most Muslims. Second, the Islamic calendar is not synchronized with the solar cycle. We find a robust negative effect of more int… ▽ More We study the effect of religion and intense religious experiences on terrorism by focusing on one of the five pillars of Islam: Ramadan fasting. For identification, we exploit two facts: First, daily fasting from dawn to sunset during Ramadan is considered mandatory for most Muslims. Second, the Islamic calendar is not synchronized with the solar cycle. We find a robust negative effect of more intense Ramadan fasting on terrorist events within districts and country-years in predominantly Muslim countries. This effect seems to operate partly through decreases in public support for terrorism and the operational capabilities of terrorist groups. △ Less

Submitted 5 March, 2020; v1 submitted 23 October, 2018; originally announced October 2018.

Enhanced photon-extraction efficiency from InGaAs/GaAs quantum dots in deterministic photonic structures at 1.3 μm fabricated by in-situ electron-beam lithography

Authors: Nicole Srocka, Anna Musiał, Philipp-Immanuel Schneider, Paweł Mrowiński, Paweł Holewa, Sven Burger, David Quandt, André Strittmatter, Sven Rodt, Stephan Reitzenstein, Grzegorz Sęk

Abstract: The main challenge in the development of non-classical light sources remains their brightness that limits the data transmission and processing rates as well as the realization of practical devices operating in the telecommunication range. To overcome this issue, we propose to utilize universal and flexible in-situ electron-beam lithography and hereby, we demonstrate a successful technology transfe… ▽ More The main challenge in the development of non-classical light sources remains their brightness that limits the data transmission and processing rates as well as the realization of practical devices operating in the telecommunication range. To overcome this issue, we propose to utilize universal and flexible in-situ electron-beam lithography and hereby, we demonstrate a successful technology transfer to telecom wavelengths. As an example, we fabricate and characterize especially designed photonic structures with strain-engineered single InGaAs/GaAs quantum dots that are deterministically integrated into disc-shaped mesas. Utilizing this approach, an extraction efficiency into free-space (within a numerical aperture of 0.4) of (10${\pm}$2) % has been experimentally obtained in the 1.3 μm wavelength range in agreement with finite-element method calculations. △ Less

Submitted 2 May, 2018; originally announced May 2018.

Journal ref: AIP Adv. 8, 085205 (2018)

Generation of maximally entangled states and coherent control in quantum dot microlenses

Authors: Samir Bounouar, Christoph de la Haye, Max Strauß, Peter Schnauber, Alexander Thoma, Manuel Gschrey, Jan-Hindrik Schulze, André Strittmatter, Sven Rodt, Stephan Reitzenstein

Abstract: The integration of entangled photon emitters in nanophotonic structures designed for the broadband enhancement of photon extraction is a major challenge for quantum information technologies. We study the potential of quantum dot (QD) microlenses to act as efficient emitters of maximally entangled photons. For this purpose, we perform quantum tomography measurements on InGaAs QDs integrated determi… ▽ More The integration of entangled photon emitters in nanophotonic structures designed for the broadband enhancement of photon extraction is a major challenge for quantum information technologies. We study the potential of quantum dot (QD) microlenses to act as efficient emitters of maximally entangled photons. For this purpose, we perform quantum tomography measurements on InGaAs QDs integrated deterministically into microlenses. Even though the studied QDs show non-zero excitonic fine-structure splitting (FSS), polarization entanglement can be prepared with a fidelity close to unity. The quality of the measured entanglement is only dependent on the temporal resolution of the used single-photon detectors compared to the period of the excitonic phase precession imposed by the FSS. Interestingly, entanglement is kept along the full excitonic wave-packet and is not affected by decoherence. Furthermore, coherent control of the upper biexcitonic state is demonstrated. △ Less

Submitted 10 January, 2018; originally announced January 2018.

Journal ref: Appl. Phys. Lett. 112, 153107 (2018)

Heterogeneous Employment Effects of Job Search Programmes: A Machine Learning Approach

Authors: Michael Knaus, Michael Lechner, Anthony Strittmatter

Abstract: We systematically investigate the effect heterogeneity of job search programmes for unemployed workers. To investigate possibly heterogeneous employment effects, we combine non-experimental causal empirical models with Lasso-type estimators. The empirical analyses are based on rich administrative data from Swiss social security records. We find considerable heterogeneities only during the first si… ▽ More We systematically investigate the effect heterogeneity of job search programmes for unemployed workers. To investigate possibly heterogeneous employment effects, we combine non-experimental causal empirical models with Lasso-type estimators. The empirical analyses are based on rich administrative data from Swiss social security records. We find considerable heterogeneities only during the first six months after the start of training. Consistent with previous results of the literature, unemployed persons with fewer employment opportunities profit more from participating in these programmes. Furthermore, we also document heterogeneous employment effects by residence status. Finally, we show the potential of easy-to-implement programme participation rules for improving average employment effects of these active labour market programmes. △ Less

Submitted 11 May, 2018; v1 submitted 29 September, 2017; originally announced September 2017.

Journal ref: Journal of Human Resources, 2020, forthcoming

Enhancing the photon-extraction efficiency of site-controlled quantum dots by deterministically fabricated microlenses

Authors: Arsenty Kaganskiy, Sarah Fischbach, Andre Strittmatter, Tobias Heindel, Sven Rodt, Stephan Reitzenstein

Abstract: We report on the realization of scalable single-photon sources (SPSs) based on single site-controlled quantum dots (SCQDs) and deterministically fabricated microlenses. The fabrication process comprises the buried-stressor growth technique complemented with low-temperature in-situ electron-beam lithography for the integration of SCQDs into microlens structures with high yield and high alignment ac… ▽ More We report on the realization of scalable single-photon sources (SPSs) based on single site-controlled quantum dots (SCQDs) and deterministically fabricated microlenses. The fabrication process comprises the buried-stressor growth technique complemented with low-temperature in-situ electron-beam lithography for the integration of SCQDs into microlens structures with high yield and high alignment accuracy. The microlens-approach leads to a broadband enhancement of the photon-extraction efficiency of up to (21 $\pm$ 2) $\%$ and a high suppression of multi-photon events with g$^{(2)}$($τ$ = 0) $<$ 0.06 without background subtraction. The demonstrated combination of site-controlled growth of QDs and in-situ electron-beam lithography is relevant for arrays of efficient SPSs which can be applied in photonic quantum circuits and advanced quantum computation schemes. △ Less

Submitted 13 December, 2017; v1 submitted 11 August, 2017; originally announced August 2017.

Comments: 15 pages, 4 Figures

Journal ref: Optics Communications 413, 162-166 (2018)

Accessing the dark exciton spin in deterministic quantum-dot microlenses

Authors: Tobias Heindel, Alexander Thoma, Ido Schwartz, Emma R. Schmidgall, Liron Gantz, Dan Cogan, Max Strauß, Peter Schnauber, Manuel Gschrey, Jan-Hindrik Schulze, Andre Strittmatter, Sven Rodt, David Gershoni, Stephan Reitzenstein

Abstract: The dark exciton state in semiconductor quantum dots constitutes a long-lived solid-state qubit which has the potential to play an important role in implementations of solid-state based quantum information architectures. In this work, we exploit deterministically fabricated QD microlenses with enhanced photon extraction, to optically prepare and readout the dark exciton spin and observe its cohere… ▽ More The dark exciton state in semiconductor quantum dots constitutes a long-lived solid-state qubit which has the potential to play an important role in implementations of solid-state based quantum information architectures. In this work, we exploit deterministically fabricated QD microlenses with enhanced photon extraction, to optically prepare and readout the dark exciton spin and observe its coherent precession. The optical access to the dark exciton is provided via spin-blockaded metastable biexciton states acting as heralding state, which are identified deploying polarization-sensitive spectroscopy as well as time-resolved photon cross-correlation experiments. Our experiments reveal a spin-precession period of the dark exciton of $(0.82\pm0.01)\,$ns corresponding to a fine-structure splitting of $(5.0\pm0.7)\,μ$eV between its eigenstates $\left|\uparrow\Uparrow\pm\downarrow\Downarrow\right\rangle$. By exploiting microlenses deterministically fabricated above pre-selected QDs, our work demonstrates the possibility to scale up implementations of quantum information processing schemes using the QD-confined dark exciton spin qubit, such as the generation of photonic cluster states or the realization of a solid-state-based quantum memory. △ Less

Submitted 4 August, 2017; v1 submitted 16 June, 2017; originally announced June 2017.

Comments: 5 pages, 3 figures

Journal ref: APL Photonics 2, 121303 (2017)

A stand-alone fiber-coupled single-photon source

Authors: Alexander Schlehahn, Sarah Fischbach, Ronny Schmidt, Arsenty Kaganskiy, André Strittmatter, Sven Rodt, Tobias Heindel, Stephan Reitzenstein

Abstract: In this work, we present a stand-alone and fiber-coupled quantum-light source. The plug-and-play device is based on an optically driven quantum dot delivering single photons via an optical fiber. The quantum dot is deterministically integrated in a monolithic microlens which is precisely coupled to the core of an optical fiber via active optical alignment and epoxide adhesive bonding. The rigidly… ▽ More In this work, we present a stand-alone and fiber-coupled quantum-light source. The plug-and-play device is based on an optically driven quantum dot delivering single photons via an optical fiber. The quantum dot is deterministically integrated in a monolithic microlens which is precisely coupled to the core of an optical fiber via active optical alignment and epoxide adhesive bonding. The rigidly coupled fiber-emitter assembly is integrated in a compact Stirling cryocooler with a base temperature of 35 K. We benchmark our practical quantum device via photon auto-correlation measurements revealing $g^{(2)}(0)=0.07 \pm 0.05$ under continuous-wave excitation and we demonstrate triggered non-classical light at a repetition rate of 80 MHz. The long-term stability of our quantum light source is evaluated by endurance tests showing that the fiber-coupled quantum dot emission is stable within 4% over several successive cool-down/warm-up cycles. Additionally, we demonstrate non-classical photon emission for a user-intervention-free 100-hour test run and stable single-photon count rates up to 11.7 kHz with a standard deviation of 4%. △ Less

Submitted 1 December, 2017; v1 submitted 30 March, 2017; originally announced March 2017.

Comments: 8 pages, 4 figures, contains new experimental data

Journal ref: Scientific Reports 8, 1340 (2018)

Resonance fluorescence of a site-controlled quantum dot realized by the buried-stressor growth technique

Authors: Max Strauss, Arsenty Kaganskiy, Robert Voigt, Peter Schnauber, Jan-Hindrik Schulze, Sven Rodt, Andre Strittmatter, Stephan Reitzenstein

Abstract: Site-controlled growth of semiconductor quantum dots (QDs) represents a major advancement to achieve scalable quantum technology platforms. One immediate benefit is the deterministic integration of quantum emitters into optical microcavities. However, site-controlled growth of QDs is usually achieved at the cost of reduced optical quality. Here, we show that the buried-stressor growth technique en… ▽ More Site-controlled growth of semiconductor quantum dots (QDs) represents a major advancement to achieve scalable quantum technology platforms. One immediate benefit is the deterministic integration of quantum emitters into optical microcavities. However, site-controlled growth of QDs is usually achieved at the cost of reduced optical quality. Here, we show that the buried-stressor growth technique enables the realization of high-quality site-controlled QDs with attractive optical and quantum optical properties. This is evidenced by performing excitation power dependent resonance fluorescence experiments at cryogenic temperatures showing QD emission linewidths down to 10 $μ$eV. Resonant excitation leads to the observation of the Mollow triplet under CW excitation and enables coherent state preparation under pulsed excitation. Under resonant $π$-pulse excitation we observe clean single photon emission associated with $g^{(2)}(0)=0.12$ limited by non-ideal laser suppression. △ Less

Submitted 2 March, 2017; v1 submitted 23 December, 2016; originally announced December 2016.

Comments: 4 pages, 4 figures

Journal ref: Appl. Phys. Lett. 110, 111101 (2017)

Two-photon interference from remote deterministic quantum dot microlenses

Authors: Alexander Thoma, Peter Schnauber, Jonas Böhm, Manuel Gschrey, Jan-Hindrik Schulze, André Strittmatter, Sven Rodt, Tobias Heindel, Stephan Reitzenstein

Abstract: We report on two-photon interference (TPI) experiments using remote deterministic single-photon sources. Employing 3D in-situ electron-beam lithography, we fabricate quantum-light sources at specific target wavelengths by integrating pre-selected semiconductor quantum dots within monolithic microlenses. The individual single-photon sources show TPI visibilities of 49% and 22%, respectively, under… ▽ More We report on two-photon interference (TPI) experiments using remote deterministic single-photon sources. Employing 3D in-situ electron-beam lithography, we fabricate quantum-light sources at specific target wavelengths by integrating pre-selected semiconductor quantum dots within monolithic microlenses. The individual single-photon sources show TPI visibilities of 49% and 22%, respectively, under pulsed p-shell excitation at 80 MHz. For the mutual TPI of the remote sources, we observe an uncorrected visibility of 29%, in quantitative agreement with the pure dephasing of the individual sources. Due to its efficient photon extraction within a broad spectral range (> 20 nm), our microlens-based approach is predestinated for future entanglement swapping experiments utilizing entangled photon pairs emitted by distant biexciton-exciton radiative cascades. △ Less

Submitted 20 December, 2016; v1 submitted 21 November, 2016; originally announced November 2016.

Comments: 7 pages, 3 figures

Journal ref: Applied Physics Letters 110, 011104 (2017)

Impact of phonons on dephasing of individual excitons in deterministic quantum dot microlenses

Authors: T. Jakubczyk, V. Delmonte, S. Fischbach, D. Wigger, D. E. Reiter, Q. Mermillod, P. Schnauber, A. Kaganskiy, J. -H. Schulze, A. Strittmatter, S. Rodt, W. Langbein, T. Kuhn, S. Reitzenstein, J. Kasprzak

Abstract: Optimized light-matter coupling in semiconductor nanostructures is a key to understand their optical properties and can be enabled by advanced fabrication techniques. Using in-situ electron beam lithography combined with a low-temperature cathodoluminescence imaging, we deterministically fabricate microlenses above selected InAs quantum dots (QDs) achieving their efficient coupling to the external… ▽ More Optimized light-matter coupling in semiconductor nanostructures is a key to understand their optical properties and can be enabled by advanced fabrication techniques. Using in-situ electron beam lithography combined with a low-temperature cathodoluminescence imaging, we deterministically fabricate microlenses above selected InAs quantum dots (QDs) achieving their efficient coupling to the external light field. This enables to perform four-wave mixing micro-spectroscopy of single QD excitons, revealing the exciton population and coherence dynamics. We infer the temperature dependence of the dephasing in order to address the impact of phonons on the decoherence of confined excitons. The loss of the coherence over the first picoseconds is associated with the emission of a phonon wave packet, also governing the phonon background in photoluminescence (PL) spectra. Using theory based on the independent boson model, we consistently explain the initial coherence decay, the zero-phonon line fraction, and the lineshape of the phonon-assisted PL using realistic quantum dot geometries. △ Less

Submitted 31 October, 2016; originally announced October 2016.

Path-controlled time reordering of paired photons in a dressed three-level cascade

Authors: Samir Bounouar, Max Strauß, Alexander Carmele, Peter Schnauber, Alexander Thoma, Manuel Gschrey, Jan-Hindrik Schulze, André Strittmatter, Sven Rodt, Andreas Knorr, Stephan Reitzenstein

Abstract: The two-photon dressing of a "three-level ladder" system, here the ground state, the exciton and the biexciton of a semiconductor quantum dot, leads to new eigenstates and allows one to manipulate the time ordering of the paired photons without unitary post processing. We show that, after spectral post-selection of the single dressed states, the time ordering of the cascaded photons can be removed… ▽ More The two-photon dressing of a "three-level ladder" system, here the ground state, the exciton and the biexciton of a semiconductor quantum dot, leads to new eigenstates and allows one to manipulate the time ordering of the paired photons without unitary post processing. We show that, after spectral post-selection of the single dressed states, the time ordering of the cascaded photons can be removed or conserved. Our joint experimental and theoretical study demonstrates the high potential of a "ladder" system to be a versatile source of orthogonally polarized, bunched or antibunched pairs of photons. △ Less

Submitted 5 May, 2017; v1 submitted 26 October, 2016; originally announced October 2016.

Journal ref: Phys. Rev. Lett. 118, 233601 (2017)

A bright triggered twin-photon source in the solid state

Authors: Tobias Heindel, Alexander Thoma, Martin von Helversen, Marco Schmidt, Alexander Schlehahn, Manuel Gschrey, Peter Schnauber, Jan-Hindrik Schulze, André Strittmatter, Jörn Beyer, Sven Rodt, Alexander Carmele, Andreas Knorr, Stephan Reitzenstein

Abstract: A non-classical light source emitting pairs of identical photons represents a versatile resource of interdisciplinary importance with applications in quantum optics and quantum biology. Emerging research fields, which benefit from such type of quantum light source, include quantum-optical spectroscopy or experiments on photoreceptor cells sensitive to photon statistics. To date, photon twins have… ▽ More A non-classical light source emitting pairs of identical photons represents a versatile resource of interdisciplinary importance with applications in quantum optics and quantum biology. Emerging research fields, which benefit from such type of quantum light source, include quantum-optical spectroscopy or experiments on photoreceptor cells sensitive to photon statistics. To date, photon twins have mostly been generated using parametric downconversion sources, relying on Poissonian number distributions, or atoms, exhibiting low emission rates. Here, we propose and experimentally demonstrate the efficient, triggered generation of photon twins using the energy-degenerate biexciton-exciton radiative cascade of a single semiconductor quantum dot. Deterministically integrated within a microlens, this nanostructure emits highly-correlated photon pairs, degenerate in energy and polarization, at a rate of up to (234 $\pm$ 4) kHz. Furthermore, we verify a significant degree of photon-indistinguishability and directly observe twin-photon emission by employing photon-number-resolving detectors, which enables the reconstruction of the emitted photon number distribution. △ Less

Submitted 11 January, 2017; v1 submitted 9 August, 2016; originally announced August 2016.

Comments: 15 pages, 5 figures, including Supplementary Information

Journal ref: Nature Communications 8, 14870 (2017)

A charge-driven feedback loop in the resonance fluorescence of a single quantum dot

Authors: B. Merkel, A. Kurzmann, J. -H. Schulze, A. Strittmatter, M. Geller, A. Lorke

Abstract: Semiconductor quantum dots can emit antibunched, single photons on demand with narrow linewidths. However, the observed linewidths are broader than lifetime measurements predict, due to spin and charge noise in the environment. This noise randomly shifts the transition energy and destroys coherence and indistinguishability of the emitted photons. Fortunately, the fluctuations can be reduced by a s… ▽ More Semiconductor quantum dots can emit antibunched, single photons on demand with narrow linewidths. However, the observed linewidths are broader than lifetime measurements predict, due to spin and charge noise in the environment. This noise randomly shifts the transition energy and destroys coherence and indistinguishability of the emitted photons. Fortunately, the fluctuations can be reduced by a stabilization using a suitable feedback loop. In this work we demonstrate a fast feedback loop that manifests itself in a strong hysteresis and bistability of the exciton resonance fluorescence signal. Field ionization of photogenerated quantum dot excitons leads to the formation of a charged interface layer that drags the emission line along over a frequency range of more than 30 GHz. This internal charge-driven feedback loop could be used to reduce the spectral diffusion and stabilize the emission frequency within milliseconds, presently only limited by the sample structure, but already faster than nuclear spin feedback. △ Less

Submitted 10 June, 2016; originally announced June 2016.

Journal ref: Phys. Rev. B 95, 115305 (2017)

Generating single photons at GHz modulation-speed using electrically controlled quantum dot microlenses

Authors: A. Schlehahn, R. Schmidt, C. Hopfmann, J. -H. Schulze, A. Strittmatter, T. Heindel, L. Gantz, E. R. Schmidgall, D. Gershoni, S. Reitzenstein

Abstract: We report on the generation of single-photon pulse trains at a repetition rate of up to 1 GHz. We achieve this high speed by modulating the external voltage applied on an electrically contacted quantum dot microlens, which is optically excited by a continuous-wave laser. By modulating the photoluminescence of the quantum dot microlens using a square-wave voltage, single-photon emission is triggere… ▽ More We report on the generation of single-photon pulse trains at a repetition rate of up to 1 GHz. We achieve this high speed by modulating the external voltage applied on an electrically contacted quantum dot microlens, which is optically excited by a continuous-wave laser. By modulating the photoluminescence of the quantum dot microlens using a square-wave voltage, single-photon emission is triggered with a response time as short as 270 ps being 6.5 times faster than the radiative lifetime of 1.75 ns. This large reduction in the characteristic emission time is enabled by a rapid capacitive gating of emission from the quantum dot placed in the intrinsic region of a p-i-n-junction biased below the onset of electroluminescence. Here, the rising edge of the applied voltage pulses triggers the emission of single photons from the optically excited quantum dot. The non-classical nature of the photon pulse train generated at GHz-speed is proven by intensity autocorrelation measurements. Our results combine optical excitation with fast electrical gating and thus show promise for the generation of indistinguishable single photons at high rates, exceeding the limitations set by the intrinsic radiative lifetime. △ Less

Submitted 25 October, 2015; originally announced October 2015.

Comments: 7 pages, 3 figures

Journal ref: Applied Physics Letters 108, 021104 (2016)

Exploring Dephasing of a Solid-State Quantum Emitter via Time- and Temperature- Dependent Hong-Ou-Mandel Experiments

Authors: A. Thoma, P. Schnauber, M. Gschrey, M. Seifried, J. Wolters, J. -H. Schulze, A. Strittmatter, S. Rodt, A. Carmele, A. Knorr, T. Heindel, S. Reitzenstein

Abstract: We probe the indistinguishability of photons emitted by a semiconductor quantum dot (QD) via time- and temperature- dependent two-photon interference (TPI) experiments. An increase in temporal-separation between consecutive photon emission events, reveals a decrease in TPI visibility on a nanosecond timescale, theoretically described by a non-Markovian noise process in agreement with fluctuating c… ▽ More We probe the indistinguishability of photons emitted by a semiconductor quantum dot (QD) via time- and temperature- dependent two-photon interference (TPI) experiments. An increase in temporal-separation between consecutive photon emission events, reveals a decrease in TPI visibility on a nanosecond timescale, theoretically described by a non-Markovian noise process in agreement with fluctuating charge-traps in the QD's vicinity. Phonon-induced pure dephasing results in a decrease in TPI visibility from $(96\pm4)\,$\% at 10\,K to a vanishing visibility at 40\,K. In contrast to Michelson-type measurements, our experiments provide direct access to the time-dependent coherence of a quantum emitter at a nanosecond timescale. △ Less

Submitted 22 December, 2015; v1 submitted 21 July, 2015; originally announced July 2015.

Comments: 11 pages, 7 figures

Journal ref: Phys. Rev. Lett. 116, 033601 (2016)

Enhanced photon-extraction efficiency from deterministic quantum-dot microlenses

Authors: M. Gschrey, M. Seifried, L. Krüuger, R. Schmidt, J. -H. Schulze, T. Heindel, S. Burger, S. Rodt, F. Schmidt, A. Strittmatter, S. Reitzenstein

Abstract: The prospect of realizing building blocks for long-distance quantum communication is a major driving force for the development of advanced nanophotonic devices. Significant progress has been achieved in this field with respect to the fabrication of efficient quantum-dot-based single-photon sources. More recently, even spin-photon entanglement and quantum teleportation have been demonstrated in sem… ▽ More The prospect of realizing building blocks for long-distance quantum communication is a major driving force for the development of advanced nanophotonic devices. Significant progress has been achieved in this field with respect to the fabrication of efficient quantum-dot-based single-photon sources. More recently, even spin-photon entanglement and quantum teleportation have been demonstrated in semiconductor systems. These results are considered as crucial steps towards the realization of a quantum repeater. The related work has almost exclusively been performed on self-assembled quantum dots (QDs) and random device technology. At this point it is clear that further progress in this field towards real applications will rely crucially on deterministic device technologies which will, for instance, enable the processing of bright quantum light sources with pre-defined emission energy. Here we report on enhanced photon-extraction efficiency from monolithically integrated microlenses which are coupled deterministically to single QDs. The microlenses with diameters down to 800 nm were aligned to single QDs by in-situ electron-beam lithography using a low-temperature cathodoluminescence setup. This deterministic device technology allowed us to obtain an enhancement of photon extraction efficiency for QDs integrated into microlenses as compared to QDs in unstructured surfaces. The excellent optical quality of the structures is demonstrated by cathodoluminescence and micro-photoluminescence spectroscopy. A Hong-Ou-Mandel experiment states the emission of single indistinguishable photons. △ Less

Submitted 21 December, 2013; originally announced December 2013.

Comments: 18 pages, 5 figures

Journal ref: Nat. Commun. 6, 7662 (2015)

In-situ electron-beam lithography of deterministic single-quantum-dot mesa-structures using low-temperature cathodoluminescence spectroscopy

Authors: M. Gschrey, F. Gericke, A. Schüßler, R. Schmidt, J. -H. Schulze, T. Heindel, S. Rodt, A. Strittmatter, S. Reitzenstein

Abstract: We report on the deterministic fabrication of sub-um mesa structures containing single quantum dots by in-situ electron-beam lithography. The fabrication method is based on a two-step lithography process using a low-temperature cathodoluminescence (CL) spectroscopy setup. In the first step the position and spectral features of single InGaAs quantum dots (QDs) are detected by CL. Then circular sub-… ▽ More We report on the deterministic fabrication of sub-um mesa structures containing single quantum dots by in-situ electron-beam lithography. The fabrication method is based on a two-step lithography process using a low-temperature cathodoluminescence (CL) spectroscopy setup. In the first step the position and spectral features of single InGaAs quantum dots (QDs) are detected by CL. Then circular sub-um mesa-structures are exactly defined by high-resolution electron-beam lithography and subsequent etching in the second step. CL spectroscopy and micro-photoluminscence spectroscopy demonstrate the high optical quality of the single-QD mesa-structures with emission linewidths below 15 ueV and g(2)(0) = 0.04. Our lithography method allows for an alignment precision better than 100 nm which paves the way for a fully-deterministic device technology using in-situ CL lithography. △ Less

Submitted 27 June, 2013; v1 submitted 12 April, 2013; originally announced April 2013.

Comments: 4 pages, 4 figures

Journal ref: Applied Physics Letters 102, 251113 (2013)

Origin of the Broad Lifetime Distribution of Localized Excitons in InGaN/GaN Quantum Dots

Authors: M. Winkelnkemper, M. Dworzak, T. P. Bartel, A. Strittmatter, A. Hoffmann, D. Bimberg

Abstract: We derive an energy-dependent decay-time distribution function from the multi-exponential decay of the ensemble photoluminescence (PL) of InGaN/GaN quantum dots (QDs), which agrees well with recently published single-QD time-resolved PL measurements. Using eight-band k.p modelling, we show that the built-in piezo- and pyroelectric fields within the QDs cause a sensitive dependence of the radiati… ▽ More We derive an energy-dependent decay-time distribution function from the multi-exponential decay of the ensemble photoluminescence (PL) of InGaN/GaN quantum dots (QDs), which agrees well with recently published single-QD time-resolved PL measurements. Using eight-band k.p modelling, we show that the built-in piezo- and pyroelectric fields within the QDs cause a sensitive dependence of the radiative lifetimes on the exact QD geometry and composition. Moreover, the radiative lifetimes also depend heavily on the composition of the direct surrounding of the QDs. A broad lifetime distribution occurs even for moderate variations of the QD structure. Thus, for unscreened fields a multi-exponential decay of the ensemble PL is generally expected in this material system. △ Less

Submitted 31 July, 2008; originally announced July 2008.

Comments: 5 pages, 4 figures. accepted at Physica Status Solidi

Polarized Emission Lines from Single InGaN/GaN Quantum Dots: Role of the Valence-band Structure of Wurtzite Group-III Nitrides

Authors: M. Winkelnkemper, R. Seguin, S. Rodt, A. Schliwa, L. Reissmann, A. Strittmatter, A. Hoffmann, D. Bimberg

Abstract: We present a study of the polarization properties of emission lines from single InGaN/GaN quantum dots (QDs). The QDs, formed by spinodal decomposition within ultra-thin InGaN quantum wells, are investigated using single-QD cathodoluminescence (CL). The emission lines exhibit a systematic linear polarization in the orthogonal crystal directions [1 1 -2 0] and [-1 1 0 0]--a symmetry that is non-n… ▽ More We present a study of the polarization properties of emission lines from single InGaN/GaN quantum dots (QDs). The QDs, formed by spinodal decomposition within ultra-thin InGaN quantum wells, are investigated using single-QD cathodoluminescence (CL). The emission lines exhibit a systematic linear polarization in the orthogonal crystal directions [1 1 -2 0] and [-1 1 0 0]--a symmetry that is non-native to hexagonal crystals. Eight-band k.p calculations reveal a mechanism that can explain the observed polarizations: The character of the hole(s) in an excitonic complex determines the polarization direction of the respective emission if the QD is slightly elongated. Transitions involving A-band holes are polarized parallel to the elongation; transitions involving B-type holes are polarized in the orthogonal direction. The energetic separation of both hole states is smaller than 10 meV. The mechanism leading to the linear polarizations is not restricted to InGaN QDs, but should occur in other wurtzite-nitride QDs and in materials with similar valence band structure. △ Less

Submitted 21 November, 2007; originally announced November 2007.

Comments: Conf. Proc. of the MSS-13 in Genova 2007, accepted at Physica E

Journal ref: Physica E 40, 2217 (2008)

Polarized emission lines from A- and B-type excitonic complexes in single InGaN/GaN quantum dots

Authors: M. Winkelnkemper, R. Seguin, S. Rodt, A. Schliwa, L. Reissmann, A. Strittmatter, A. Hoffmann, D. Bimberg

Abstract: Cathodoluminescence measurements on single InGaN/GaN quantum dots (QDs) are reported. Complex spectra with up to five emission lines per QD are observed. The lines are polarized along the orthogonal crystal directions [1 1 -2 0] and [-1 1 0 0]. Realistic eight-band k.p electronic structure calculations show that the polarization of the lines can be explained by excitonic recombinations involving… ▽ More Cathodoluminescence measurements on single InGaN/GaN quantum dots (QDs) are reported. Complex spectra with up to five emission lines per QD are observed. The lines are polarized along the orthogonal crystal directions [1 1 -2 0] and [-1 1 0 0]. Realistic eight-band k.p electronic structure calculations show that the polarization of the lines can be explained by excitonic recombinations involving hole states which are either formed by the A or the B valence band. △ Less

Submitted 24 May, 2007; originally announced May 2007.

Comments: accepted at Journal of Applied Physics

Journal ref: J. Appl. Phys. 101, 113708 (2007)

Control of fine-structure splitting and excitonic binding energies in selected individual InAs/GaAs quantum dots

Authors: R. Seguin, A. Schliwa, T. D. Germann, S. Rodt, M. Winkelnkemper, K. Pötschke, A. Strittmatter, U. W. Pohl, T. Hammerschmidt, P. Kratzer, D. Bimberg

Abstract: A systematic study of the impact of annealing on the electronic properties of single InAs/GaAs quantum dots (QDs) is presented. Single QD cathodoluminescence spectra are recorded to trace the evolution of one and the same QD over several steps of annealing. A substantial reduction of the excitonic fine-structure splitting upon annealing is observed. In addition, the binding energies of different… ▽ More A systematic study of the impact of annealing on the electronic properties of single InAs/GaAs quantum dots (QDs) is presented. Single QD cathodoluminescence spectra are recorded to trace the evolution of one and the same QD over several steps of annealing. A substantial reduction of the excitonic fine-structure splitting upon annealing is observed. In addition, the binding energies of different excitonic complexes change dramatically. The results are compared to model calculations within eight-band k.p theory and the configuration interaction method, suggesting a change of electron and hole wave function shape and relative position. △ Less

Submitted 2 January, 2007; originally announced January 2007.

Comments: 4 pages, 4 figures

Journal ref: Appl. Phys. Lett. 89, 263109 (2006)

Multi-excitonic complexes in single InGaN quantum dots

Authors: R. Seguin, S. Rodt, A. Strittmatter, L. Reißmann, T. Bartel, A. Hoffmann, D. Bimberg

Abstract: Cathodoluminescence spectra employing a shadow mask technique of InGaN layers grown by metal organic chemical vapor deposition on Si(111) substrates are reported. Sharp lines originating from InGaN quantum dots are observed. Temperature dependent measurements reveal thermally induced carrier redistribution between the quantum dots. Spectral diffusion is observed and was used as a tool to correla… ▽ More Cathodoluminescence spectra employing a shadow mask technique of InGaN layers grown by metal organic chemical vapor deposition on Si(111) substrates are reported. Sharp lines originating from InGaN quantum dots are observed. Temperature dependent measurements reveal thermally induced carrier redistribution between the quantum dots. Spectral diffusion is observed and was used as a tool to correlate up to three lines that originate from the same quantum dot. Variation of excitation density leads to identification of exciton and biexciton. Binding and anti-binding complexes are discovered. △ Less

Submitted 16 October, 2006; originally announced October 2006.

Comments: 3 pages, 4 figures

Journal ref: Appl. Phys. Lett. 84, 4023 (2004)

The Cosmogony of Super-Massive Black Holes

Authors: Wolfgang J. Duschl, Peter A. Strittmatter

Abstract: We report results of a project investigating the growth of super-massive black holes (BHs) by disk accretion. We find that the BH mass growth is quick enough to account for the inferred masses in the highest-redshift quasars, and the growth time is an inverse function of the final BH mass as seems to be required by recent X-ray surveys. We report results of a project investigating the growth of super-massive black holes (BHs) by disk accretion. We find that the BH mass growth is quick enough to account for the inferred masses in the highest-redshift quasars, and the growth time is an inverse function of the final BH mass as seems to be required by recent X-ray surveys. △ Less

Submitted 1 February, 2006; originally announced February 2006.

Comments: 5 pages, 2 figures, to appear in the proceedings of "Relativistic Astrophysics and Cosmology - Einstein's Legacy" (Eds.: B. Aschenbach, V. Burwitz, G. Hasinger, and B. Leibundgut), 7 - 11 November 2005, Munich, Bavaria, Germany

Formation of Super-massive Black Holes

Authors: Wolfgang J. Duschl, Peter A. Strittmatter

Abstract: We show that the rapid formation of super-massive black holes in quasars can indeed be understood in terms of major galaxy mergers followed by disk accretion. The necessary short disk evolution time can be achieved provided the disk viscosity is sufficiently large, which, for instance, is the case for hydrodynamic turbulence, unlimited by shock dissipation. We present numerical calculations for… ▽ More We show that the rapid formation of super-massive black holes in quasars can indeed be understood in terms of major galaxy mergers followed by disk accretion. The necessary short disk evolution time can be achieved provided the disk viscosity is sufficiently large, which, for instance, is the case for hydrodynamic turbulence, unlimited by shock dissipation. We present numerical calculations for a representative case. This general picture can account for (a) the presence of highly luminous quasars at redshifts z > 6; (b) for the peak in quasar activity at z ~ 2; and (c) for a subsequent rapid disappearance of quasars at later epochs. △ Less

Submitted 2 January, 2004; originally announced January 2004.

Comments: 9 pages, 1 figure, to appear in the proceedings of the Xth Marcel Grossman Meeting, Rio de Janeiro, 2003

A Note on Hydrodynamic Viscosity and Selfgravitation in Accretion Disks

Authors: Wolfgang J. Duschl, Peter A. Strittmatter, Peter L. Biermann

Abstract: We propose a generalized accretion disk viscosity prescription based on hydrodynamically driven turbulence at the critical effective Reynolds number. This approach is consistent with recent re-analysis by Richard & Zahn (1999) of experimental results on turbulent Couette-Taylor flows. This new $β$-viscosity formulation is applied to both selfgravitating and non-selfgravitating disks and is shown… ▽ More We propose a generalized accretion disk viscosity prescription based on hydrodynamically driven turbulence at the critical effective Reynolds number. This approach is consistent with recent re-analysis by Richard & Zahn (1999) of experimental results on turbulent Couette-Taylor flows. This new $β$-viscosity formulation is applied to both selfgravitating and non-selfgravitating disks and is shown to yield the standard $α$-disk prescription in the case of shock dissipation limited, non-selfgravitating disks. A specific case of fully selfgravitating $β$-disks is analyzed. We suggest that such disks may explain the observed spectra of protoplanetary disks and yield a natural explanation for the radial motions inferred from the observed metallicity gradients in disk galaxies. The $β$-mechanism may also account for the rapid mass transport required to power ultra luminous infrared galaxies. △ Less

Submitted 6 April, 2000; originally announced April 2000.

Comments: 10 pages, no figures, accepted by Astronomy & Astrophysics

Viscosity in Selfgravitating Accretion Disks

Authors: W. J. Duschl, Peter A. Strittmatter, Peter L. Biermann

Abstract: We show that the standard model for geometrically thin accretion disks (alpha-disks) leads to inconsistencies if selfgravity plays a role. This problem arises from the parametrization of viscosity in terms of local sound velocity and vertical disk scale height. A viscosity prescription based on turbulent flows at the critical effective Reynolds number allows for consistent models of thin selfgra… ▽ More We show that the standard model for geometrically thin accretion disks (alpha-disks) leads to inconsistencies if selfgravity plays a role. This problem arises from the parametrization of viscosity in terms of local sound velocity and vertical disk scale height. A viscosity prescription based on turbulent flows at the critical effective Reynolds number allows for consistent models of thin selfgravitating disks, and recovers the alpha-disk solution as the limiting case of negligible selfgravity. We suggest that such selfgravitating disks may explain the observed spectra of protoplanetary disks and yield a natural explanation for the radial motions inferred from the observed metallicity gradients in disk galaxies. △ Less

Submitted 14 February, 1997; originally announced February 1997.

Comments: 10 pages, LaTeX, AAS 2 column preprint style (aas2pp4.sty), submitted to ApJ

Report number: Steward Observatory Preprint No. 1357