Kurt Busch

Thermalization of isolated quantum systems is an intriguing phenomenon at the forefront of contemporary physics. In this work, we demonstrate that nonlinear multimode optical platforms can be harnessed to induce effective quantum interactions between photons. Through numerical experiments, we reveal the thermalization of quantum light, specifically single- and two-photon states, propagating alongside classical light within multimode nonlinear optical systems. This thermalization is clearly manifested by the emergence of Rayleigh-Jeans and Boltzmann statistical distributions. Beyond providing a deeper understanding of how classical nonlinearities can be used to investigate quantum many-body dynamics, our findings enable the exploration of a broader range of complex quantum phenomena, including aspects of quantum phase transitions, within readily accessible classical optical settings.

Casimir forces dominate the interaction between electrically neutral macroscopic objects at nanoscale separations, making their quantitative description highly relevant to modern technology and fundamental physics. In this work, we present a time-domain scheme for computing Casimir forces between vacuum-separated bodies based on the Maxwell stress tensor formalism. In particular, we provide a concrete implementation using the finite-element-based discontinuous Galerkin time-domain method. The approach enables accurate evaluation of finite-temperature Casimir–Lifshitz interactions in arbitrary geometries and for general material models. At the core of the method, the electromagnetic Green tensor is expressed as the system's response to dipolar excitations, thereby recasting the Maxwell stress tensor into a set of classical scattering problems driven by electric and magnetic dipoles. We validate the approach against reference calculations of the Casimir interaction between parallel half-spaces at both zero and finite temperature. We further demonstrate its applicability to the interaction between a finite cylindrical object and a half-space, for which semi-analytical expressions are not available, showing excellent agreement with both asymptotic predictions based on physical considerations and alternative numerical approaches. These findings illustrate the method's potential for studying Casimir interactions in micro- and nanoscale structures of relevance to nanodevice design and experimental settings.

The Casimir-Polder force acts on polarizable particles due to quantum fluctuations of the electromagnetic field that are modified by the presence of material bodies. We investigate the Casimir-Polder interaction for atoms near cylindrical fibers with hollow cores. This geometry represents one of the archetypal configurations encountered in numerous experimental setups designed to control and manipulate atoms in fundamental and quantum technological applications. Specifically, we analyze how the interplay of both geometrical and material-related length scales characterize the interaction, emphasizing the impact of the shell thickness. We develop a flexible and fast-converging numerical scheme for evaluating the interaction over a wide range of atom-cylinder separations at both zero and finite temperature. Furthermore, we provide a detailed analytical investigation of how various material properties modify the Casimir-Polder potential. Finally, we analyze and discuss a number of limiting cases and compare numerical computations with corresponding analytical asymptotic expressions. In particular, in this geometry the Casimir-Polder potential is able to distinguish between an Ohmic and non-Ohmic description of conductors. One of the most significant outcomes of our work is that the shell thickness emerges as a useful parameter for controlling the interaction, opening avenues for both fundamental physics and applications in quantum technologies.

We derive and classify properties of the power-spectrum tensor for systems in general steady-states, including stationary states not necessarily corresponding to equilibrium configurations. This allows us to establish a rigorous connection between the power-spectrum tensor and other quantities that characterize these systems, providing a systematic comparison with their equilibrium counterparts. As a physical application, we investigate the problem of quantum friction, describing the contactless quantum-electrodynamic drag acting on a particle moving in close proximity to material bodies at zero temperature. Specifically, we demonstrate how including additional information about the system’s physical properties facilitates the derivation of more precise constraints on the power spectrum and its functional dependencies.

The Casimir-Polder force is analyzed when an atom is moving at a constant velocity relative to a collection of translationally invariant macroscopic bodies with generic shapes and compositions. The interaction is described within an approach that accurately treats the atom-field coupling and accounts for the backaction from the environment onto the moving particle. Previously overlooked aspects are uncovered and linked to the nonequilibrium and nonconservative nature of the interaction. Specifically, we examine a behavior that can be understood by characterizing the underlying physical processes in terms of a motional-induced effective temperature. This phenomenon shares similarities with the Fulling-Davies-Unruh effect, opening new perspectives for the understanding of nonequilibrium physics at work in the system.

Electron microscopy techniques such as electron energy-loss spectroscopy (EELS) facilitate the spatiospectral characterization of plasmonic nanostructures. In this work, a time-dependent perspective is presented that significantly enhances the utility of EELS. In particular, this approach facilitates the analysis of the dynamics of plasmonic excitations that repeatedly interact with swift electrons in a STEM-EELS configuration. This includes the bulk plasmon mode, which can only be excited by penetrating electron beams, and the fundamental surface plasmon polariton modes propagating along the wire, which can be excited by both penetrating and aloof trajectories. In addition, the role of higher-order azimuthal surface plasmon polariton modes, often overlooked for very thin wires, is observed and analyzed in both the energy-loss spectrum and from the dynamical perspective. Such a complete understanding of the interaction of electrons and plasmonic excitations is key for the design of efficient plasmonic sensors, the study of hot electron dynamics in metals, and applications in the context of electron quantum optics, where full control of the spatial and temporal characteristics of the fields at the nanometer and femtosecond scales is highly desirable.

The Casimir-Polder force acts on polarizable particles due to quantum fluctuations of the electromagnetic field that are modified by the presence of material bodies. We investigate the Casimir-Polder interaction for atoms near cylindrical fibers with hollow cores. This geometry represents one of the archetypal configurations encountered in numerous experimental setups designed to control and manipulate atoms in fundamental and quantum technological applications. Specifically, we analyze how the interplay of both geometrical and material-related length scales characterize the interaction, emphasizing the impact of the shell thickness. We develop a flexible and fast-converging numerical scheme for evaluating the interaction over a wide range of atom-cylinder separations at both zero and finite temperature. Furthermore, we provide a detailed analytical investigation of how various material properties modify the Casimir-Polder potential. Finally, we analyze and discuss a number of limiting cases and compare numerical computations with corresponding analytical asymptotic expressions. In particular, in this geometry the Casimir-Polder potential is able to distinguish between an ohmic and non-ohmic description of conductors. One of the most significant outcomes of our work is that the shell thickness emerges as a useful parameter for controlling the interaction, opening avenues for both fundamental physics and applications in quantum technologies.