arXiv subjects

Bernd Bauerhenne

Publications and source records attributed to Bernd Bauerhenne.

7 recordsLinked to original sources

arXiv2026

Transition from Collective to Local Radial Motional Modes in a Tapered Paul Trap

With coupled detuned oscillators, either individual or collective oscillations are observable. The latter is used in quantum information processing in linear Paul traps. Here, we study the transition from collective radial modes at stronger axial confinements into individual radial oscillations at low axial confinements in a tapered Paul trap. The eigenmodes are experimentally studied in detail in the transition regime and compared with theoretical predictions. The features studied will enable investigation of modified heat transport phenomena and defect formation in trapped ions.

quant-ph↗

arXiv2025

Hydrodynamic Effects in Cryogenic Buffer Gas Cells: Design Insights from Hybrid Simulations

Cryogenic buffer gas beam sources have become an essential tool for experiments requiring cold molecular beams with low forward velocities. Although recent experimental advances have led to significant progress in source optimization, numerical studies remain limited due to the challenges posed by the large parameter ranges required to describe both the dense buffer gas and the dilute seed molecules. In this work, we report a numerical evaluation of cryogenic buffer gas beam cells operating in the hydrodynamic extraction regime. While most prior studies focused on box-like or cylindrical cells, we investigated hydrodynamic effects including vortex formation in a spherical cell and assessed whether these could be utilized to enhance the performance in molecule cooling and extraction. To achieve this, we performed steady-state slip-flow simulations for helium buffer gas and employed a direct-simulation Monte Carlo diffusion routine to track particle trajectories. We compared the performance of the source across different buffer gas throughputs and injection angles and identified parameter regimes where vortex formation enhances molecule extraction. From the simulations, we extracted experimental observables, which allow these effects to be verified through velocity or time-of-flight measurements on the molecular beam.

quant-ph↗

arXiv2025

Ensemble-Based Quantum Token Protocol Benchmarked on IBM Quantum Processors

Quantum tokens envision to store unclonable quantum states in a physical device, with the goal of being used for personal authentication protocols, as required by banks. Still, the experimental realization of such devices faces many technical challenges, which can be partially mitigated using ensembles instead of single qubits. In this work, we thus propose an ensemble-based quantum token protocol, describing it through a simple yet general model based on a quantum mechanical observable. The protocol is benchmarked on five IBM quantum processors and a general hacker attack scenario is analyzed, in which the attacker attempts to read the bank token and forge a fake one, based on the information gained from this measurement. We experimentally demonstrate that the probability that the bank erroneously accepts a forged coin composed of multiple tokens can reach values below $10^{-22}$, while the probability that the bank accepts its own coin is above 0.999. The overall security of the protocol is therefore demonstrated within a hardware-agnostic framework, confirming the practical viability of the protocol in arbitrary quantum systems and thus paving the way for future applications with different ensembles of qubits, such as color center defects in solids.

quant-ph↗

arXiv2025

Security Analysis of Ensemble-Based Quantum Token Protocol Under Advanced Attacks

We present and characterize advanced attacks on an ensemble-based quantum token protocol that allows for implementing non-clonable quantum coins. Multiple differently initialized tokens of identically prepared qubit ensembles are combined to a quantum coin that can be issued by a bank. A sophisticated attempt to copy tokens can assume that measurements on sub-ensembles can be carried through and that even individual qubits can be measured. Even though such an advanced attack might be perceived as technically unfeasible, we prove the security of the protocol under these conditions. We performed numerical simulations and verified our results by experiments on the IBM Quantum Platforms for different types of advanced attacks. Finally, we demonstrate that the security of the quantum coin can be made high by increasing the number of tokens. This paper in conjunction with provided numerical simulation tools verified against experimental data from the IBM Quantum Platforms allows for securely implementing our ensemble-based quantum token protocol with arbitrary quantum systems.

quant-ph↗

arXiv2024

Unified theoretical description of thermal and nonthermal laser-induced ultrafast structural changes in solids

The ultrafast dynamics of ions in solids following intense femtosecond laser excitation is governed by two fundamentally distinct yet interplaying effects. On one hand, the significant generation of hot electron-hole pairs by the light pulse alters the strength and nature of interatomic bonding, resulting in nonthermal ionic motion. On the other hand, incoherent electron-ion collisions drives an equilibration of the electrons and the ions, which reach a common temperature on a picosecond timescale. This letter introduces, for the first time, a unified theory that comprehensively accounts for both phenomena. Our approach is versatile, applicable to both ab-initio and large-scale molecular dynamics simulations, and leads to a generalization of the two-temperature model-molecular-dynamics equations of motion. To illustrate the effectiveness of our method, we apply it to describe the laser excitation of silicon thin films. Our simulations reproduce the time evolution of the Bragg peaks in excellent agreement with the experiments.

physics.comp-ph↗

arXiv2021

Correction of Density-Functional-Theory based polynomial interatomic potentials to reproduce experimental melting properties

Recently, we developed a method to construct polynomial interatomic potentials from ab-initio calculations in order to accurately describe laser excited solids [PRL 124, 085501 (2020)]. However, ab-initio methods, and therefore analytical potentials derived from them, commonly do not provide an accurate prediction of the melting temperature. In order to reproduce the experimental melting properties, but keeping the accuracy in the laser excited case, we present here an approach to modify few key coefficients of polynomial interatomic potentials constructed from ab-initio data. We show that, with the help of such corrections, the electronic-temperature dependent interatomic potential for silicon can, at the same time, describe nonthermal laser induced effects with ab-initio accuracy and also provide the correct experimental melting temperature and slope $dT/dp$.

cond-mat.mtrl-sci↗

arXiv2018

Self-learning analytical interatomic potential describing laser-excited silicon

We develop an electronic-temperature dependent interatomic potential $Φ(T_\text{e})$ for unexcited and laser-excited silicon. The potential is designed to reproduce ab initio molecular dynamics simulations by requiring force- and energy matching for each time step. $Φ(T_\text{e})$ has a simple and flexible analytical form, can describe all relevant interactions and is applicable for any kind of boundary conditions (bulk, thin films, clusters). Its overall shape is automatically adjusted by a self-learning procedure, which finally finds the global minimum in the parameter space. We show that $Φ(T_\text{e})$ can reproduce all thermal and nonthermal features provided by ab initio simulations. We apply the potential to simulate laser-excited Si nanoparticles and find critical damping of their breathing modes due to nonthermal melting.

cond-mat.mtrl-sci↗