arXiv subjects

Matthias Steffen

Publications and source records attributed to Matthias Steffen.

At least 19 recordsLinked to original sources

arXiv2026

Solar photospheric spectrum microvariability III. Radial velocities and line profiles in magnetic active-region granulation

Finding low-mass planets around solar-type stars requires to understand the physical variability of the host star, which greatly exceeds the planet-induced radial-velocity modulation. Different solar photospheric absorption lines have slightly disparate responses to stellar activity, which should permit to disentangle wavelength shifts induced by exoplanets from those originating in stellar atmospheres. Changing area coverage of magnetic active-region granulation (faculae and plage) causes radial-velocity fluctuations of the disk-integrated solar spectrum, whose precise modeling requires active-region spectral line profiles. Hydrodynamic 3D modeling of granulation in magnetic fields extends previous non-magnetic studies, revealing different line profiles and altered convective velocity shifts. Different types of lines in the visual and near infrared are examined in synthetic hyper-high resolution spectra (R~900,000), comparing non-magnetic areas with those with strongly magnetic (240 mT = 2400 G) granulation. Magnetic fields inhibit convective motions, decrease the energy flow, produce more symmetric lines, and remove the common blueshift with its familiar C-shape bisectors. Unexpectedly, magnetic granulation displays convective redshifts. Their origin is traced to contributions from small areas, where hot and bright down-moving elements are created through shocks and adiabatic compression when rising gas is forced over into magnetically channeled downflows. Understanding line formation in also stellar active regions is needed to simulate full-disk spectra toward exoEarth detections. Detailed shapes of spectral lines carry significant information, suggesting that hyper-high spectral resolution may ultimately be required

astro-ph.SR↗

arXiv2022

On the lithium abundance of the visual binary components $ξ$ Boo A (G8V) and $ξ$ Boo B (K5V)

A spectroscopic investigation of the lithium resonance doublet in $ξ$ Boo A and $ξ$ Boo B in terms of both abundance and isotopic ratio is presented. We obtained new $R$=130\,000 spectra with a signal-to-noise ratio (S/N) per pixel of up to 3200 using the 11.8m LBT and PEPSI. From fits with synthetic line profiles based on 1D-LTE MARCS model atmospheres and 3D-NLTE corrections, we determine the abundances of both isotopes. For $ξ$ Boo A, we find A(Li) = 2.40$\pm$0.03\,dex and $^6$Li/$^7$Li < 1.5$\pm$1.0\,\%\ in 1D-LTE, which increases to $\approx$2.45 for the 3D-NLTE case. For $ξ$ Boo B we obtain A(Li) = 0.37$\pm$0.09\,dex in 1D-LTE with an unspecified $^6$Li/$^7$Li level. Therefore, no $^6$Li is seen on any of the two stars. We consider a spot model for the Li fit for $ξ$ Boo B and find A(Li) = 0.45$\pm$0.09\,dex. The $^7$Li abundance is 23 times higher for $ξ$ Boo A than the Sun's, but three times lower than the Sun's for $ξ$ Boo B while both fit the trend of single stars in the similar-aged M35 open cluster. Effective temperatures are redetermined from the TiO band head strength. We note that the best-fit global metallicities are --0.13$\pm$0.01\,dex for $ξ$ Boo A but +0.13$\pm$0.02\,dex for $ξ$ Boo B. Lithium abundance for the K5V benchmark star 61\,Cyg\,A was obtained to A(Li)$\approx$0.53\,dex when including a spot model but to $\approx$0.15\,dex without a spot model.

astro-ph.SR↗

arXiv2021

Spectator Errors in Tunable Coupling Architectures

The addition of tunable couplers to superconducting quantum architectures offers significant advantages for scaling compared to fixed coupling approaches. In principle, tunable couplers allow for exact cancellation of qubit-qubit coupling through the interference of two parallel coupling pathways between qubits. However, stray microwave couplings can introduce additional pathways which complicate the interference effect. Here we investigate the primary spectator induced errors of the bus below qubit (BBQ) architecture in a six qubit device. We identify the key design parameters which inhibit ideal cancellation and demonstrate that dynamic cancellation pulses can further mitigate spectator errors.

quant-ph↗

arXiv2021

Tunable Coupling Architecture for Fixed-frequency Transmons

Implementation of high-fidelity two-qubit operations is a key ingredient for scalable quantum error correction. In superconducting qubit architectures tunable buses have been explored as a means to higher fidelity gates. However, these buses introduce new pathways for leakage. Here we present a modified tunable bus architecture appropriate for fixed-frequency qubits in which the adiabaticity restrictions on gate speed are reduced. We characterize this coupler on a range of two-qubit devices achieving a maximum gate fidelity of $99.85\%$. We further show the calibration is stable over one day.

quant-ph↗

arXiv2020

3D spectroscopic analysis of helium-line white dwarfs

In this paper, we present corrections to the spectroscopic parameters of DB and DBA white dwarfs with -10.0 < log(H/He) < -2.0, 7.5 < log(g) < 9.0 and 12000 K < T_eff < 34000 K, based on 282 3D atmospheric models calculated with the CO5BOLD radiation-hydrodynamics code. These corrections arise due to a better physical treatment of convective energy transport in 3D models when compared to the previously available 1D model atmospheres. By applying the corrections to an existing SDSS sample of DB and DBA white dwarfs, we find significant corrections both for the effective temperature and surface gravity. The 3D log(g) corrections are most significant for T_eff < 18000 K, reaching up to -0.20 dex at log(g) = 8.0. However, in this low effective temperature range, the surface gravity determined from the spectroscopic technique can also be significantly affected by the treatment of the neutral van der Waals line broadening of helium and by non-ideal effects due to the perturbation of helium by neutral atoms. Thus, by removing uncertainties due to 1D convection, our work showcases the need for improved description of microphysics for DB and DBA model atmospheres. Overall, we find that our 3D spectroscopic parameters for the SDSS sample are generally in agreement with Gaia DR2 absolute fluxes within 1-3σ for individual white dwarfs. By comparing our results to DA white dwarfs, we have determined that the precision and accuracy of DB/DBA atmospheric models are similar. For ease of user application of the correction functions, we provide an example Python code.

astro-ph.SR↗

arXiv2018

Hot bubbles of planetary nebulae with hydrogen-deficient winds - II. Analytical approximations with application to BD+30$^\circ$3639

The first high-resolution X-ray spectroscopy of the planetary nebula BD+30$^\circ$3639 allowed to study X-ray emitting "hot bubbles" (HBs) of planetary nebulae in unprecedented detail. We investigate (i) how diagnostic line ratios are affected by the HB thermal structure and chemical profile, (ii) if the HB chemical composition of BD+30$^\circ$3639 is consistent with the H-poor (H for hydrogen) composition of the stellar photosphere, and (iii) if H-rich nebular matter has been added to this HB by evaporation. We apply an analytical, 1D model for wind-blown HBs with temperature and density profiles based on self-similar solutions including thermal conduction. We construct heat-conduction HBs with chemical stratification. The X-ray emission is computed using the CHIANTI code. Our HB models are used to re-analyse the high-resolution X-ray spectrum of BD+30$^\circ$3639. Our models reproduce the observed line ratios much better than plasmas with single electron temperatures. All the temperature- and abundance-sensitive line ratios are consistent with BD+30$^\circ$3639 X-ray observations for (i) an intervening column density of neutral H, $N_{\rm H} = 0.20_{-0.10}^{+0.05}\times\!10^{22}\rm\ cm^{-2}$, (ii) a characteristic HB X-ray temperature of ${T_{\rm X} = 1.8\pm 0.1~ }$MK together with (iii) a very high neon mass fraction of about 0.05, virtually as high as that of oxygen. For lower values of $N_{\rm H}$, we cannot exclude that the HB of BD+30$^\circ$3639 contains a small amount of evaporated (or mixed) H-rich nebular matter. Given the possible range of $N_{\rm H}$, the fraction of evaporated H-rich matter cannot exceed 3% of the HB mass. The diffuse X-ray emission from BD+30$^\circ$3639 can well be explained by models of wind-blown HBs with thermal conduction and a chemical composition equal to that of the H-poor and carbon-, oxygen-, and neon-rich stellar surface.

astro-ph.SR↗

arXiv2018

Analytical determination of participation in superconducting coplanar architectures

Superconducting qubits are sensitive to a variety of loss mechanisms which include dielectric loss from interfaces. The calculation of participation near the key interfaces of planar designs can be accomplished through an analytical description of the electric field density based on conformal mapping. In this way, a two-dimensional approximation to coplanar waveguide and capacitor designs produces values of the participation as a function of depth from the top metallization layer as well as the volume participation within a given thickness from this surface by reducing the problem to a surface integration over the region of interest. These quantities are compared to finite element method numerical solutions, which validate the values at large distances from the coplanar metallization but diverge near the edges of the metallization features due to the singular nature of the electric fields. A simple approximation to the electric field energy at shallow depths (relative to the waveguide width) is also presented that closely replicates the numerical results based on conformal mapping and those reported in prior literature. These techniques are applied to the calculation of surface participation within a transmon qubit design, where the effects due to shunting capacitors can be easily integrated with those associated with metallization comprising the local environment of the qubit junction.

cond-mat.mes-hall↗

arXiv2017

Characterization of hidden modes in networks of superconducting qubits

We present a method for detecting electromagnetic (EM) modes that couple to a superconducting qubit in a circuit-QED architecture. Based on measurement-induced dephasing, this technique allows the measurement of modes that have a high quality factor (Q) and may be difficult to detect through standard transmission and reflection measurements at the device ports. In this scheme the qubit itself acts as a sensitive phase meter, revealing modes that couple to it through measurements of its coherence time. Such modes are indistinguishable from EM modes that do not couple to the qubit using a vector network analyzer. Moreover, this technique provides useful characterization parameters including the quality factor and the coupling strength of the unwanted resonances. We demonstrate the method for detecting both high-Q coupling resonators in planar devices as well as spurious modes produced by a 3D cavity.

quant-ph↗

arXiv2017

Analysis of the X-ray spectrum of the hot bubble of BD+30°3639

We developed a model for wind-blown bubbles with temperature and density profiles based on self-similar solutions including thermal conduction. We constructed also heat-conduction bubbles with chemical discontinuities. The X-ray emission is computed using the well-documented CHIANTI code (v6.0.1). These bubble models are used to (re)analyse the high-resolution X-ray spectrum of the hot bubble of BD+30°3639, and they appeared to be much superior to constant temperature approaches. We found for the X-ray emission of BD+30°3639 that temperature-sensitive and abundance-sensitive line ratios computed on the basis of heat-conducting wind-blown bubbles and with abundances as found in the stellar photosphere/wind can only be reconciled with the observations if the hot bubble of BD+30°3639 is chemically stratified, i.e. if it contains also a small mass fraction ($\simeq$ 3 %) of hydrogen-rich matter immediately behind the conduction front. Neon appears to be strongly enriched, with a mass fraction of at least about 0.06.

astro-ph.SR↗

arXiv2016

Hydrodynamical model atmospheres: Their impact on stellar spectroscopy and asteroseismology of late-type stars

Hydrodynamical, i.e. multi-dimensional and time-dependent, model atmospheres of late-type stars have reached a high level of realism. They are commonly applied in high-fidelity work on stellar abundances but also allow the study of processes that are not modelled in standard, one-dimensional hydrostatic model atmospheres. Here, we discuss two observational aspects that emerge from such processes, the photometric granulation background and the spectroscopic microturbulence. We use CO5BOLD hydrodynamical model atmospheres to characterize the total granular brightness fluctuations and characteristic time scale for FGK stars. Emphasis is put on the diagnostic potential of the granulation background for constraining the fundamental atmospheric parameters. We find a clear metallicity dependence of the granulation background. The comparison between the model predictions and available observational constraints at solar metallicity shows significant differences, that need further clarification. Concerning microturbulence, we report on the derivation of a theoretical calibration based on CO5BOLD models, which shows good correspondence with the measurements for stars in the Hyades. We emphasize the importance of a consistent procedure when determining the microturbulence, and point to limitations of the commonly applied description of microturbulence in hydrostatic model atmospheres.

astro-ph.SR↗

arXiv2015

Hot bubbles of planetary nebulae with hydrogen-deficient winds I. Heat conduction in a chemically stratified plasma

Heat conduction has been found a plausible solution to explain discrepancies between expected and measured temperatures in hot bubbles of planetary nebulae (PNe). While the heat conduction process depends on the chemical composition, to date it has been exclusively studied for pure hydrogen plasmas in PNe. A smaller population of PNe show hydrogen-deficient and helium- and carbon-enriched surfaces surrounded by bubbles of the same composition; considerable differences are expected in physical properties of these objects in comparison to the pure hydrogen case. The aim of this study is to explore how a chemistry-dependent formulation of the heat conduction affects physical properties and how it affects the X-ray emission from PN bubbles of hydrogen-deficient stars. We extend the description of heat conduction in our radiation hydrodynamics code to work with any chemical composition. We then compare the bubble-formation process with a representative PN model using both the new and the old descriptions. We also compare differences in the resulting X-ray temperature and luminosity observables of the two descriptions. The improved equations show that the heat conduction in our representative model of a hydrogen-deficient PN is nearly as efficient with the chemistry-dependent description; a lower value on the diffusion coefficient is compensated by a slightly steeper temperature gradient. The bubble becomes somewhat hotter with the improved equations, but differences are otherwise minute. The observable properties of the bubble in terms of the X-ray temperature and luminosity are seemingly unaffected.

astro-ph.SR↗

arXiv2015

Reducing Spontaneous Emission in Circuit Quantum Electrodynamics by a Combined Readout/Filter Technique

Physical implementations of qubits can be extremely sensitive to environmental coupling, which can result in decoherence. While efforts are made for protection, coupling to the environment is necessary to measure and manipulate the state of the qubit. As such, the goal of having long qubit energy relaxation times is in competition with that of achieving high-fidelity qubit control and measurement. Here we propose a method that integrates filtering techniques for preserving superconducting qubit lifetimes together with the dispersive coupling of the qubit to a microwave resonator for control and measurement. The result is a compact circuit that protects qubits from spontaneous loss to the environment, while also retaining the ability to perform fast, high-fidelity readout. Importantly, we show the device operates in a regime that is attainable with current experimental parameters and provide a specific example for superconducting qubits in circuit quantum electrodynamics.

quant-ph↗

arXiv2015

Building logical qubits in a superconducting quantum computing system

The technological world is in the midst of a quantum computing and quantum information revolution. Since Richard Feynman's famous "plenty of room at the bottom" lecture, hinting at the notion of novel devices employing quantum mechanics, the quantum information community has taken gigantic strides in understanding the potential applications of a quantum computer and laid the foundational requirements for building one. We believe that the next significant step will be to demonstrate a quantum memory, in which a system of interacting qubits stores an encoded logical qubit state longer than the incorporated parts. Here, we describe the important route towards a logical memory with superconducting qubits, employing a rotated version of the surface code. The current status of technology with regards to interconnected superconducting-qubit networks will be described and near-term areas of focus to improve devices will be identified. Overall, the progress in this exciting field has been astounding, but we are at an important turning point where it will be critical to incorporate engineering solutions with quantum architectural considerations, laying the foundation towards scalable fault-tolerant quantum computers in the near future.

quant-ph↗

arXiv2015

Consistent metallicity scale for cool dwarfs and giants. A benchmark test using the Hyades

In several instances chemical abundances of dwarf and giant stars are used simultaneously under the assumption that they share the same abundance scale. This assumption might have implications in different astrophysical contexts. We aim to ascertain a methodology capable of producing a consistent metallicity scale for giants and dwarfs. To achieve that, we analyzed giants and dwarfs in the Hyades open cluster. All these stars have archival high-resolution spectroscopic data obtained with HARPS and UVES. In addition, the giants have interferometric measurements of the angular diameters. We analyzed the sample with two methods. The first method constrains the atmospheric parameters independently from spectroscopy. For that we present a novel calibration of microturbulence based on 3D model atmospheres. The second method is the classical spectroscopic based on Fe lines. We also tested two line lists in an attempt to minimize possible non-LTE effects and to optimize the treatment of the giants. We show that it is possible to obtain a consistent metallicity scale between dwarfs and giants. The preferred method should constrain the three parameters $T_{\rm eff}$, $\log~g$, and $ξ$ independent of spectroscopy. In particular, the lines should be chosen to be free of blends in the spectra of giants. When attention is paid to the line list, the classical spectroscopic method can also produce consistent results. The metallicities derived with the well-constrained set of stellar parameters are consistent independent of the line list used. Therefore, for this cluster we favor the metallicity of +0.18$\pm$0.03 dex obtained with this method. The classical spectroscopic analysis, using the line list optimized for the giants, provides a metallicity of +0.14$\pm$0.03 dex, in agreement with previous works.

astro-ph.SR↗

arXiv2015

Bulk and surface loss in superconducting transmon qubits

Decoherence of superconducting transmon qubits is purported to be consistent with surface loss from two-level systems on the substrate surface. Here, we present a study of surface loss in transmon devices, explicitly designed to have varying sensitivities to different surface loss contributors. Our experiments also encompass two particular different sapphire substrates, which reveal the onset of a yet unknown additional loss mechanism outside of surface loss for one of the substrates. Tests across different wafers and devices demonstrate substantial variation, and we emphasize the importance of testing large numbers of devices for disentangling different sources of decoherence.

quant-ph↗

arXiv2015

Characterizing a Four-Qubit Planar Lattice for Arbitrary Error Detection

Quantum error correction will be a necessary component towards realizing scalable quantum computers with physical qubits. Theoretically, it is possible to perform arbitrarily long computations if the error rate is below a threshold value. The two-dimensional surface code permits relatively high fault-tolerant thresholds at the ~1% level, and only requires a latticed network of qubits with nearest-neighbor interactions. Superconducting qubits have continued to steadily improve in coherence, gate, and readout fidelities, to become a leading candidate for implementation into larger quantum networks. Here we describe characterization experiments and calibration of a system of four superconducting qubits arranged in a planar lattice, amenable to the surface code. Insights into the particular qubit design and comparison between simulated parameters and experimentally determined parameters are given. Single- and two-qubit gate tune-up procedures are described and results for simultaneously benchmarking pairs of two-qubit gates are given. All controls are eventually used for an arbitrary error detection protocol described in separate work [Corcoles et al., Nature Communications, 6, 2015]

quant-ph↗

arXiv2013

Molecular bands in extremely metal-poor stars. Granulation effects

The bands of diatomic molecules are important abundance indicators, especially in metal-poor stars, where they are still measurable in metallicity regimes where the atomic lines of their constituting metallic elements have become vanishingly small. In order to use them for abundance determinations it is imperative to understand the formation of these bands. In this contribution we report on our results obtained using CO5BOLD hydrodynamical simulations. Some effects that are qualitatively different from what found in 1D computations are highlighted. Due to the large number of lines that form the bands, their spectrum synthesis is computationally challenging. We discuss some of the computational strategies we employed to parallelise the computation and possible future developments.

astro-ph.SR↗

arXiv2013

The photospheric solar oxygen project: II. Non-concordance of the oxygen abundance derived from two forbidden lines

In the Sun, the two forbidden [OI] lines at 630 and 636 nm were previously found to provide discrepant oxygen abundances. aims: We investigate whether this discrepancy is peculiar to the Sun or whether it is also observed in other stars. method: We make use of high-resolution, high signal-to-noise ratio spectra of four dwarf to turn-off stars, five giant stars, and one sub-giant star observed with THEMIS, HARPS, and UVES to investigate the coherence of the two lines. results: The two lines provide oxygen abundances that are consistent, within observational errors, in all the giant stars examined by us. On the other hand, for the two dwarf stars for which a measurement was possible, for Procyon, and for the sub-giant star Capella, the 636 nm line provides systematically higher oxygen abundances, as already seen for the Sun. conclusions: The only two possible reasons for the discrepancy are a serious error in the oscillator strength of the NiI line blending the 630 nm line or the presence of an unknown blend in the 636 nm line, which makes the feature stronger. The CN lines blending the 636 nm line cannot be responsible for the discrepancy. The CaI autoionisation line, on the red wing of which the 636 nm line is formed, is not well modelled by our synthetic spectra. However, a better reproduction of this line would result in even higher abundances from the 636 nm, thus increasing the discrepancy.

astro-ph.SR↗