arXiv subjects

H. Fehske

Publications and source records attributed to H. Fehske.

At least 19 recordsLinked to original sources

arXiv2025

Cross-talk in superconducting qubit lattices with tunable couplers -- comparing transmon and fluxonium architectures

Cross-talk between qubits is one of the main challenges for scaling superconducting quantum processors. Here, we use the density-matrix renormalization-group to numerically analyze lattices of superconducting qubits from a perspective of many-body localization. Specifically, we compare different architectures that include tunable couplers designed to decouple qubits in the idle state, and calculate the residual ZZ interactions as well as the inverse participation ratio in the computational basis states. For transmon qubits outside of the straddling regime, the results confirm that tunable C-shunt flux couplers are significantly more efficient in mitigating the ZZ interactions than tunable transmons. A recently proposed fluxonium architecture with tunable transmon couplers is demonstrated to also maintain its strong suppression of the ZZ interactions in larger systems, while having a higher inverse participation ratio in the computational basis states than lattices of transmon qubits. Our results thus suggest that fluxonium architectures may feature lower cross talk than transmon lattices when designed to achieve similar gate speeds and fidelities.

quant-ph↗

arXiv2022

Charge kinetics across a negatively biased semiconducting plasma-solid interface

An investigation of the selfconsistent ambipolar charge kinetics across a negatively biased semiconducting plasma-solid interface is presented. For the specific case of a thin germanium layer with nonpolar electron-phonon scattering, sandwiched between an Ohmic contact and a collisionless argon plasma, we calculate the current-voltage characteristic and show that it is affected by the electron microphysics of the semiconductor. We also obtain the spatially and energetically resolved fluxes and charge distributions inside the layer, visualizing thereby the behavior of the charge carriers responsible for the charge transport. Albeit not quantitative, because of the crude model for the germanium band structure and the neglect of particle-nonconserving scattering processes, such as impact ionization and electron-hole recombination, which at the energies involved cannot be neglected, our results clearly indicate (i) the current through the interface is carried by rather hot carriers and (ii) the perfect absorber model, often used for the description of charge transport across plasma-solid interfaces, cannot be maintained for semiconducting interfaces.

physics.plasm-ph↗

arXiv2021

Invariant embedding approach to secondary electron emission from metals

Based on an invariant embedding principle for the backscattering function we calculate the electron emission yield for metal surfaces at very low electron impact energies. Solving the embedding equation within a quasi-isotropic approximation and using the effective mass model for the solid, experimental data are fairly well reproduced provided (i) incoherent scattering on ion cores is allowed to contribute to the scattering cascades inside the solid and (ii) the transmission through the surface potential takes into account Bragg gaps due to coherent scattering on crystal planes parallel to the surface as well as randomization of the electron's lateral momentum due to elastic scattering on surface defects. Our results suggest that in order to get secondary electrons out of metals, the large energy loss due to inelastic electron-electron scattering has to be compensated for by incoherent elastic electron-ion core scattering, irrespective of the crystallinity of the sample.

cond-mat.mtrl-sci↗

arXiv2021

Infrared spectroscopy of surface charges in plasma-facing dielectrics

We propose to measure the surface charge accumulating at the interface between a plasma and a dielectric by infrared spectroscopy using the dielectric as a multi-internal reflection element. The surplus charge leads to an attenuation of the transmitted signal from which the magnitude of the charge can be inferred. Calculating the optical response perturbatively in first order from the Boltzmann equation for the electron-hole plasma inside the solid, we can show that in the parameter range of interest a classical Drude term results. Only the integrated surface charge enters, opening up thereby a very efficient analysis of measured data.

physics.plasm-ph↗

arXiv2020

Electron microphysics at plasma-solid interfaces

The most fundamental response of a solid to a plasma and vice versa is electric. An electric double layer forms with a solid-bound electron-rich region-the wall charge-and a plasma-bound electron-depleted region-the plasma sheath. But it is only the plasma sheath which has been studied extensively ever since the beginning of plasma physics. The wall charge received much less attention. Especially little is known about the in-operando electronic structure of plasma-facing solids and how it affects the spatio-temporal scales of the wall charge. The purpose of this perspective is to encourage investigations of this terra incognito by techniques of modern surface physics. Using our own theoretical explorations of the electron microphysics at plasma-solid interfaces and a proposal for measuring the wall charge by infrared reflectivity to couch the discussion, we hope to put together enough convincing reasons for getting such efforts started. They would open up-at the intersection of plasma and surface physics-a new arena for applied as well as fundamental research.

physics.plasm-ph↗

arXiv2020

Kinetic modeling of the electric double layer at a dielectric plasma-solid interface

For a collisionless plasma in contact with a dielectric surface, where with unit probability electrons and ions are, respectively, absorbed and neutralized, thereby injecting electrons and holes into the conduction and valence band, we study the kinetics of plasma loss by nonradiative electron-hole recombination inside the dielectric. We obtain a self-consistently embedded electric double layer, merging with the quasi-neutral, field-free regions inside the plasma and the solid. After a description of the numerical scheme for solving the two sets of Boltzmann equations, one for the electrons and ions of the plasma and one for the electrons and holes of the solid, to which this transport problem gives rise to, we present numerical results for a p-doped dielectric. Besides potential, density, and flux profiles, plasma-induced changes in the electron and hole distribution functions are discussed, from which a microscopic view on plasma loss inside the dielectric emerges.

physics.plasm-ph↗

arXiv2020

Block-Lanczos density-matrix renormalization-group approach to spin transport in Heisenberg chains coupled to leads

We adapt the block-Lanczos density-matrix renormalization-group technique to study the spin transport in a spin chain coupled to two non-interacting fermionic leads. As an example, we consider leads described by two-dimensional tight-binding models on a square lattice. Although the simulations are carried out using a chain representation of the leads, observables in the original two-dimensional lattice can be calculated by reversing the block-Lanczos transformation. This is demonstrated for leads with Rashba spin-orbit coupling.

cond-mat.str-el↗

arXiv2019

Electron energy loss spectroscopy of wall charges in plasma-facing dielectrics

We propose a setup enabling electron energy loss spectroscopy to determine the density of the electrons accumulated by an electro-positive dielectric in contact with a plasma. It is based on a two-layer structure inserted into a recess of the wall. Consisting of a plasma-facing film made out of the dielectric of interest and a substrate layer the structure is designed to confine the plasma-induced surplus electrons to the region of the film. The charge fluctuations they give rise to can then be read out from the backside of the substrate by near specular electron reflection. To obtain in this scattering geometry a strong charge-sensitive reflection maximum due to the surplus electrons the film has to be most probably pre-n-doped and sufficiently thin with the mechanical stability maintained by the substrate. We demonstrate the feasibility of the proposal by calculating the loss spectrum for an sapphire film on top of a CaO layer. We find a reflection maximum strongly shifting with the density of the surplus electrons and suggest to use it for its diagnostics.

physics.app-ph↗

arXiv2018

Floquet scattering of light and sound in Dirac optomechanics

The inelastic scattering and conversion process between photons and phonons by laser-driven quantum dots is analyzed for a honeycomb array of optomechanical cells. Using Floquet theory for an effective two-level system, we solve the related time-dependent scattering problem, beyond the standard rotating-wave approximation approach, for a plane Dirac-photon wave hitting a cylindrical oscillating barrier that couples the radiation field to the vibrational degrees of freedom. We demonstrate different scattering regimes and discuss the formation of polaritonic quasiparticles. We show that sideband-scattering becomes important when the energies of the sidebands are located in the vicinity of avoided crossings of the quasienergy bands. The interference of Floquet states belonging to different sidebands causes a mixing of long-wavelength (quantum) and short-wavelength (quasiclassical) behavior, making it possible to use the oscillating quantum dot as a kind of transistor for light and sound. We comment under which conditions the setup can be utilized to observe zitterbewegung.

cond-mat.mes-hall↗

arXiv2018

Measuring the plasma-wall charge by infrared spectroscopy

We show that the charge accumulated by a dielectric plasma-facing solid can be measured by infrared spectroscopy. The approach utilizes a stack of materials supporting a surface plasmon resonance in the infrared. For frequencies near the Berreman resonance of the layer facing the plasma the reflectivity dip--measured from the back of the stack, not in contact with the plasma--depends strongly on the angle of incidence making it an ideal sensor for the changes of the layer's dielectric function due to the polarizability of the trapped surplus charges. The charge-induced shifts of the dip, both as a function of the angle and the frequency of the incident infrared light, are large enough to be measurable by attenuated total reflection setups.

physics.plasm-ph↗

arXiv2018

Ion-induced secondary electron emission from metal surfaces

Using a helium ion hitting various metal surfaces as a model system, we describe a general quantum-kinetic approach for calculating ion-induced secondary electron emission spectra at impact energies where the emission is driven by the internal potential energy of the ion. It is based on an effective model of the Anderson-Newns-type for the subset of electronic states of the ion-surface system most strongly affected by the collision. Central to our approach is a pseudo-particle representation for the electronic configurations of the projectile which enables us, by combining it with two additional auxiliary bosons, to describe in a single Hamiltonian emission channels involving electronic configurations with different internal potential energies. It is thus possible to treat Auger neutralization of the ion on an equal footing with Auger de-excitation of temporarily formed radicals and/or negative ions. From the Dyson equations for the projectile propagators and an approximate evaluation of the self-energies, rate equations are obtained for the probabilities with which the projectile configurations occur and an electron is emitted in the course of the collision. Encouraging numerical results, especially for the helium-tungsten system, indicate the potential of the approach.

physics.plasm-ph↗

arXiv2018

Electron kinetics at the plasma interface

The most fundamental response of an ionized gas to a macroscopic object is the formation of the plasma sheath. It is an electron depleted space charge region, adjacent to the object, which screens the object's negative charge arising from the accumulation of electrons from the plasma. The plasma sheath is thus the positively charged part of an electric double layer whose negatively charged part is inside the wall. In the course of the Transregional Collaborative Research Center SFB/TRR24 we investigated, from a microscopic point of view, the elementary charge transfer processes responsible for the electric double layer at a floating plasma-wall interface and made first steps towards a description of the negative part of the layer inside the wall. Below we review our work in a colloquial manner, describe possible extensions, and identify key issues which need to be resolved to make further progress in the understanding of the electron kinetics across plasma-wall interfaces.

physics.plasm-ph↗

arXiv2018

Plasma flow around and charge distribution of a dust cluster in a rf discharge

We employ a particle-in-cell Monte Carlo collision/particle-particle particle-mesh (PIC-MCC/PPPM) simulation to study the plasma flow around and the charge distribution of a three-dimensional dust cluster in the sheath of a low-pressure rf argon discharge. The geometry of the cluster and its position in the sheath are fixed to the experimental values, prohibiting a mechanical response of the cluster. Electrically, however, the cluster and the plasma environment, mimicking also the experimental situation, are coupled self-consistently. We find a broad distribution of the charges collected by the grains. The ion flux shows on the scale of the Debye length strong focusing and shadowing inside and outside the cluster due to the attraction of the ions to the negatively charged grains whereas the electron flux is characterized on this scale only by a weak spatial modulation of its magnitude depending on the rf phase. On the scale of the individual dust potentials, however, the electron flux deviates in the vicinity of the cluster strongly from the laminar flow associated with the plasma sheath. It develops convection patterns to compensate for the depletion of electrons inside the dust cluster.

physics.plasm-ph↗

arXiv2017

Generation, dynamical buildup and detection of bi- and mulipartite entangled states in cavity systems

We inspect different quantum optical setups from the viewpoint of entanglement generation and detection. As a first step we consider a planar semiconductor microcavity and optimize the Bell-type correlations and their robustness against dephasing to create strong bipartite entanglement between polariton branches, which subsequently can be transfered to the emitted photons. In a second step, in order to create multipartite entangled light, we place the microcavity in an optical resonator driven by pump pulses with a frequency comb spectrum. For this system we show how phase matching of all comb modes can be achieved and will lead to indistinguishable scattering processes causing entanglement among every mode. Finally we demonstrate the buildup of entanglement in the dissipative dynamics of emitters coupled to a single cavity photon mode driven by an external laser. From a Floquet master equation approach we find that entanglement production predominates during the first few laser oscillation periods.

quant-ph↗

arXiv2017

Non-Markovian dynamics of few emitters in a laser-driven cavity

We study the laser-driven Dicke model beyond the rotating-wave approximation. For weak coupling of the system to environmental degrees of freedom the dissipative dynamics of the emitter-cavity system is described by the Floquet master equation. Projection of the system evolution onto the emitter degrees of freedom results in non-Markovian behavior. We quantify the non-Markovianity of the resulting emitter dynamics and show that this quantity can be used as an indicator of the dissipative quantum phase transition occurring at high driving amplitudes.

quant-ph↗

arXiv2017

Optical signatures of exciton-polarons from diagrammatic Monte Carlo

We study the interplay of electron-electron and electron-phonon interactions in the course of electron-hole bound state formation for gapped solid state systems. Adapting the essentially approximation-free diagrammatic Monte Carlo method for the calculation of the optical response, we discuss the absorption of light in correlated electron-phonon systems for the whole interaction and phonon frequency regimes. The spectral function obtained by analytical continuation from the imaginary-time current-current correlation function demonstrates the dressing of excitons by a phonon cloud when the coupling the lattice degrees of freedom becomes increasingly important, where notable differences show up between the adiabatic and anti-adiabatic cases.

cond-mat.str-el↗

arXiv2017

Efficient computation of the $W_3$ topological invariant and application to Floquet-Bloch systems

We introduce an efficient algorithm for the computation of the $W_3$ invariant of general unitary maps, which converges rapidly even on coarse discretization grids. The algorithm does not require extensive manipulation of the unitary maps, identification of the precise positions of degeneracy points, or fixing the gauge of eigenvectors. After construction of the general algorithm, we explain its application to the $2+1$ dimensional maps that arise in the Floquet-Bloch theory of periodically driven two-dimensional quantum systems. We demonstrate this application by computing the $W_3$ invariant for an irradiated graphene model with a continuously modulated Hamilton operator, where it predicts the number of anomalous edge states in each gap.

quant-ph↗

arXiv2017

Dynamic Stark effect, light emission, and entanglement generation in a laser-driven quantum optical system

We calculate the emission spectra, the Glauber $g^{(2)}$ function, and the entanglement of formation for a few two-level emitters coupled to a single cavity mode and subject to an external laser-excitation. To evaluate these quantities we couple the system to environmental degrees of freedom which leads to dissipative dynamics. Because of the periodic time-dependence of the system Hamiltonian, the coefficients of the (Markovian) master equation are constant if Floquet states are used as the computational basis. Studying the emission spectra we show that the dynamic Stark effect, i.e., the shift of spectral lines, first appears in the second order of the laser intensity. For the Glauber function, we find clearly distinguished parameter regimes of super- and sub-Poissonian light emission and explain the additional features appearing for finite laser intensity in terms of the quasienergy spectrum of the driven emitter-cavity system. Finally, we analyze the temperature and emitter-cavity coupling regimes where entanglement among the emitters is generated, and show that the laser-excitation leads to a decrease of entanglement.

quant-ph↗