arXiv subjects

S. Kühn

Publications and source records attributed to S. Kühn.

13 recordsLinked to original sources

arXiv2025

Matching Lagrangian and Hamiltonian Simulations in (2+1)-dimensional U(1) Gauge Theory

At finite lattice spacing, Lagrangian and Hamiltonian predictions differ due to discretization effects. In the Hamiltonian limit, i.e. at vanishing temporal lattice spacing $a_t$, the path integral approach in the Lagrangian formalism reproduces the results of the Hamiltonian theory. In this work, we numerically calculate the Hamiltonian limit of a U$(1)$ gauge theory in $(2+1)$ dimensions. This is achieved by Monte Carlo simulations in the Lagrangian formalism with lattices that are anisotropic in the time direction. For each ensemble, we determine the ratio between the temporal and spatial scale with the static quark potential and extrapolate to $a_t \to 0$. Our results are compared with the data from Hamiltonian simulations at small volumes, showing agreement within $<2σ$. These results can be used to match the two formalisms.

hep-lat↗

arXiv2022

Hamiltonian limit of lattice QED in 2+1 dimensions

The Hamiltonian limit of lattice gauge theories can be found by extrapolating the results of anisotropic lattice computations, i.e., computations using lattice actions with different temporal and spatial lattice spacings ($a_t\neq a_s$), to the limit of $a_t\to 0$. In this work, we present a study of this Hamiltonian limit for a Euclidean $U(1)$ gauge theory in 2+1 dimensions (QED3), regularized on a toroidal lattice. The limit is found using the renormalized anisotropy $ξ_R=a_t/a_s$, by sending $ξ_R \to 0$ while keeping the spatial lattice spacing constant. We compute $ξ_R$ in $3$ different ways: using both the ``normal'' and the ``sideways'' static quark potential, as well as the gradient flow evolution of gauge fields. The latter approach will be particularly relevant for future investigations of combining quantum computations with classical Monte Carlo computations, which requires the matching of lattice results obtained in the Hamiltonian and Lagrangian formalisms.

hep-lat↗

arXiv2022

Two-XUV-photon double ionization of Neon studied at the Extreme Light Infrastructure (ELI-ALPS)

Two XUV-photon double ionization of Ne, induced by an intense few-pulse attosecond train with a ~ 4 fs envelope duration is investigated experimentally and theoretically. The experiment is performed at ELI-ALPS utilizing the recently constructed 10 Hz gas phase high-order harmonic generation SYLOS GHHG-COMPACT beamline. A total pulse energy up to ~1 μJ generated in Argon in conjunction with high reflectivity optics in the XUV region, allowed the observation of the doubly charged state of Ne induced by 40 eV central XUV photon energies. The interaction of the intense attosecond pulse train with Ne is also theoretically studied via a second-order time dependent perturbation theory equations-of-motion. The results of this work, combined with the feasibility of conducting XUV-pump-XUV-probe experiments, constitute a powerful tool for many potential applications. Those include attosecond pulse metrology as well as time resolved investigations of the dynamics underlying direct and sequential double ionization and their electron correlation effects.

physics.atom-ph↗

arXiv2022

A new benchmark of soft X-ray transition energies of Ne, CO$_2$, and SF$_6$: paving a pathway towards ppm accuracy

A key requirement for the correct interpretation of high-resolution X-ray spectra is that transition energies are known with high accuracy and precision. We investigate the K-shell features of Ne, CO$_2$, and SF$_6$ gases, by measuring their photo ion-yield spectra at the BESSY II synchrotron facility simultaneously with the 1s-np fluorescence emission of He-like ions produced in the Polar-X EBIT. Accurate ab initio calculations of transitions in these ions provide the basis of the calibration. While the CO$_2$ result agrees well with previous measurements, the SF$_6$ spectrum appears shifted by ~0.5 eV, about twice the uncertainty of the earlier results. Our result for Ne shows a large departure from earlier results, but may suffer from larger systematic effects than our other measurements. The molecular spectra agree well with our results of time-dependent density functional theory. We find that the statistical uncertainty allows calibrations in the desired range of 1-10 meV, however, systematic contributions still limit the uncertainty to ~40-100 meV, mainly due to the temporal stability of the monochromator energy scale. Combining our absolute calibration technique with a relative energy calibration technique such as photoelectron energy spectroscopy will be necessary to realize its full potential of achieving uncertainties as low as 1-10 meV.

astro-ph.IM↗

arXiv2021

High-order phase-dependent asymmetry in the above-threshold ionization plateau

Above-threshold ionization spectra from cesium are measured as a function of the carrier-envelope phase (CEP) using laser pulses centered at 3.1 $μ$m wavelength. The directional asymmetry in the energy spectra of backscattered electrons oscillates three times, rather than once, as the CEP is changed from $0$ to $2π$. Using the improved strong-field approximation, we show that the unusual behavior arises from the interference of few quantum orbits. We discuss the conditions for observing the high-order CEP dependence, and draw an analogy with time-domain holography with electron wave packets.

physics.atom-ph↗

arXiv2021

An ultralow-noise superconducting radio-frequency ion trap for frequency metrology with highly charged ions

We present a novel ultrastable superconducting radio-frequency (RF) ion trap realized as a combination of an RF cavity and a linear Paul trap. Its RF quadrupole mode at 34.52 MHz reaches a quality factor of $Q\approx2.3\times 10^5$ at a temperature of 4.1 K and is used to radially confine ions in an ultralow-noise pseudopotential. This concept is expected to strongly suppress motional heating rates and related frequency shifts which limit the ultimate accuracy achieved in advanced ion traps for frequency metrology. Running with its low-vibration cryogenic cooling system, electron beam ion trap and deceleration beamline supplying highly charged ions (HCI), the superconducting trap offers ideal conditions for optical frequency metrology with ionic species. We report its proof-of-principle operation as a quadrupole mass filter with HCI, and trapping of Doppler-cooled ${}^9\text{Be}^+$ Coulomb crystals.

physics.atom-ph↗

arXiv2020

High-Precision Determination of Oxygen-K$α$ Transition Energy Excludes Incongruent Motion of Interstellar Oxygen

We demonstrate a widely applicable technique to absolutely calibrate the energy scale of x-ray spectra with experimentally well-known and accurately calculable transitions of highly charged ions, allowing us to measure the K-shell Rydberg spectrum of molecular O$_2$ with 8 meV uncertainty. We reveal a systematic $\sim$450 meV shift from previous literature values, and settle an extraordinary discrepancy between astrophysical and laboratory measurements of neutral atomic oxygen, the latter being calibrated against the aforementioned O$_2$ literature values. Because of the widespread use of such, now deprecated, references, our method impacts on many branches of x-ray absorption spectroscopy. Moreover, it potentially reduces absolute uncertainties there to below the meV level.

physics.atom-ph↗

arXiv2020

The Heidelberg compact electron beam ion traps

Electron beam ion traps (EBIT) are ideal tools for both production and study of highly charged ions (HCI). In order to reduce their construction, maintenance, and operation costs we have developed a novel, compact, room-temperature design, the Heidelberg Compact EBIT (HC-EBIT). Four already commissioned devices operate at the strongest fields (up to 0.86 T) reported for such EBITs using permanent magnets, run electron beam currents up to 80 mA and energies up to 10 keV. They demonstrate HCI production, trapping, and extraction of pulsed Ar$^{16+}$ bunches and continuous 100 pA ion beams of highly charged Xe up to charge state 29+, already with a 4 mA, 2 keV electron beam. Moreover, HC-EBITs offer large solid-angle ports and thus high photon count rates, e. g., in x-ray spectroscopy of dielectronic recombination in HCIs up to Fe$^{24+}$, achieving an electron-energy resolving power of $E/ΔE > 1500$ at 5 keV. Besides traditional on-axis electron guns, we have also implemented a novel off-axis gun for laser, synchrotron, and free-electron laser applications, offering clear optical access along the trap axis. We report on its first operation at a synchrotron radiation facility demonstrating resonant photoexcitation of highly charged oxygen.

physics.atom-ph↗

arXiv2018

Gaussian states for the variational study of (1+1)-dimensional lattice gauge models

We introduce a variational ansatz based on Gaussian states for (1+1)-dimensional lattice gauge models. To this end we identify a set of unitary transformations which decouple the gauge degrees of freedom from the matter fields. Using our ansatz, we study static aspects as well as real-time dynamics of string breaking in two (1+1)-dimensional theories, namely QED and two-color QCD. We show that our ansatz captures the relevant features and is in excellent agreement with data from numerical calculations with tensor networks.

hep-lat↗

arXiv2018

Variational study of U(1) and SU(2) lattice gauge theories with Gaussian states in 1+1 dimensions

We introduce a method to investigate the static and dynamic properties of both Abelian and non-Abelian lattice gauge models in 1+1 dimensions. Specifically, we identify a set of transformations that disentangle different degrees of freedom, and apply a simple Gaussian variational ansatz to the resulting Hamiltonian. To demonstrate the suitability of the method, we analyze both static and dynamic aspects of string breaking for the U(1) and SU(2) gauge models. We benchmark our results against tensor network simulations and observe excellent agreement, although the number of variational parameters in the Gaussian ansatz is much smaller.

hep-lat↗

arXiv2018

Attosecond electronic recollision as field detector

We demonstrate the complete reconstruction of the electric field of visible-infrared pulses with energy as low as a few tens of nanojoules. The technique allows for the reconstruction of the instantaneous electric field vector direction and magnitude, thus giving access to the characterisation of pulses with an arbitrary time-dependent polarisation state. The technique combines extreme ultraviolet interferometry with the generation of isolated attosecond pulses.

physics.optics↗

arXiv2013

Transport of dipolar Bose-Einstein condensates in a one-dimensional optical lattice

We show that magnetic dipolar interactions can stabilize superfluidity in atomic gases but the dipole alignment direction required to achieve this varies, depending on whether the flow is oscillatory or continuous. If a condensate is made to oscillate through a lattice, damping of the oscillations can be reduced by aligning the dipoles perpendicular to the direction of motion. However, if a lattice is driven continuously through the condensate, superfluid behavior is best preserved when the dipoles are aligned parallel to the direction of motion. We explain these results in terms of the formation of topological excitations and tunnel barrier heights between lattice sites.

cond-mat.quant-gas↗

arXiv2008

Coupling of plasmonic nanoparticles to their environments in the context of van der Waals-Casimir interactions

We present experiments in which the interaction of a single gold nanoparticle with glass substrates or with another gold particle can be tuned by in-situ control of their separations using scanning probe technology. We record the plasmon resonances of the coupled systems as a function of the polarization of the incident field and the particle position. The distinct spectral changes of the scattered light from the particle pair are in good agreement with the outcome of finite difference time-domain (FDTD) calculations. We believe our experimental technique holds promise for the investigation of the van der Waals-Casimir type interactions between nanoscopic neutral bodies.

quant-ph↗