Ionization of Atoms in a Thermal Field

We study the stationary states of a quantum mechanical system describing an atom coupled to black-body radiation at positive temperature. The stationary states of the non-interacting system are given by product states, where the particle is in a bound state corresponding to an eigenvalue of the particle Hamiltonian, and the field is in its equilibrium state. We show that if Fermi's Golden Rule predicts that a stationary state disintegrates after coupling to the radiation field then it is unstable, provided the coupling constant is sufficiently small (depending on the temperature). The result is proven by analyzing the spectrum of the thermal Hamiltonian (Liouvillian) of the system within the framework of $W^*$-dynamical systems. A key element of our spectral analysis is the positive commutator method

Unemployed and their Caseworkers: Should they be Friends or Foes?

In many countries, caseworkers in a public employment office have the dual roles of counselling and monitoring unemployed persons. These roles often conflict with each other leading to important caseworker heterogeneity: Some consider providing services to their clients and satisfying their demands as their primary task. Others may however pursue their strategies even against the will of the unemployed person. They may assign job assignments and labour market programmes without consent of the unemployed person. Based on a very detailed linked jobseeker-caseworker dataset, we investigate the effects of caseworkers' cooperativeness on the employment probabilities of their clients. Modified statistical matching methods reveal that caseworkers who place less emphasis on a cooperative and harmonic relationship with their clients increase their employment chances in the short and medium term.Public employment services, unemployment, statistical matching methods

Usable cryptocurrency systems

Since the introduction of Bitcoin in 2008 cryptocurrency and blockchain technology have drawn increasing attention from research and industry alike. The probably most visible evidence of the growing adoption of cryptocurrencies is the combined market capitalization which had reached over USD 2.9 trillion in November 2021. While the market capitalization remains subject to high volatility and has fallen since, the field has been growing steadily behind the scenes. Developer activity has been growing over the last decade and multiple projects which had been started to improve over the original design have reached maturity in recent years. However, the introduction of new technologies is often accompanied by the emergence of equally new design challenges. Despite the technological progress over the past years, cryptocurrencies have earned a reputation of being hard to get started with and overall difficult to use. But what exactly are the aspects that make them difficult to use? How do users manage their cryptocurrency in practice? Which challenges do they need to overcome? And how can Human-Computer Interaction help overcome these challenges? In several studies, this dissertation addresses these questions and explores them through three different approaches: (1) Cryptocurrency in Human-Computer Interaction: By systematically reviewing published Human-Computer Interaction research since the inception of Bitcoin, we organize the existing research effort and juxtapose it with the changing landscape of emerging technologies from practice to identify avenues for future research. Our results show that existing research has overwhelmingly focused on Bitcoin and Ethereum, while not addressing novel cryptocurrencies. (2) Understanding User Behavior: By exploring user behavior through multiple lenses we shed light on real-world practices of users and the challenges they face. We explore security and privacy practices through a qualitative interview study and triangulate the results in a delphi-study with 25 experts. We conducted an interview study to understand a particularly relevant point for the adoption of cryptocurrency – we investigate challenges first-time users face. Our results show that many usability issues are not rooted in the technical aspects of blockchain technology and can be addressed through Human-Computer Interaction research. (3) Improving Application Usability: By evaluating different approaches on how to aid the development of cryptocurrency applications we translate the findings of our empirical work into artifacts and put them to the test. Our results show that onboarding in mobile apps can improve perceived usability for first-time users under the right conditions, that Bitcoin Lightning can serve as a usable settlement layer for everyday transactions, that education can support the next generation of developers in building more useful applications, and that systems for rapid interface prototyping may speed up development efforts. Collectively, the contribution of this dissertation centers around the ongoing discussion on how to build usable cryptocurrency systems. More precisely, this dissertation contributes (a) empirical studies that show how users manage their cryptocurrency in practice and which challenges they face in doing so and (b) constructive approaches attempting to support the development of cryptocurrency systems in the future. The work concludes by reflecting on the future role of Human-Computer Interaction research in the cryptocurrency and blockchain space

A Note on Polarization Vectors in Quantum Electrodynamics

A photon of momentum k can have only two polarization states, not three. Equivalently, one can say that the magnetic vector potential A must be divergence free in the Coulomb gauge. These facts are normally taken into account in QED by introducing two polarization vectors epsilon_\lambda(k) with lambda in {1,2}, which are orthogonal to the wave-vector k. These vectors must be very discontinuous functions of k and, consequently, their Fourier transforms have bad decay properties. Since these vectors have no physical significance there must be a way to eliminate them and their bad decay properties from the theory. We propose such a way here.Comment: 6 pages late