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In the process of interatomic Coulombic electron capture, an incident free electron is captured at an atomic center $A$ and the transition energy is transferred radiationlessly over a rather large distance to a neighboring atom $B$ of different species, upon which the latter is ionized. We consider two-step cascade processes where the electron emitted from atom $B$ is subsequently captured as well, either at center $A$ or yet another atomic center $C$, leading to emission of a second electron from one of the centers. We derive formulas for the cross section of this double interatomic Coulombic electron capture and discuss the relevance of this process, which leads to a substantial rearrangement of the electronic configuration, in various two- and three-center atomic systems.

Near-future experiments intend to detect strong-field Breit-Wheeler pair creation from the collision between bremsstrahlung bursts containing GeV-$\gamma$ quanta and high-intensity laser pulses. In this theoretical study, we investigate the influence of laser focusing, radiation reaction and a broad bremsstrahlung $\gamma$ spectrum on the energy and angular distributions of created pairs. Understanding the role of these inherent reaction attributes provides relevant insights for experimental detection strategies and data interpretation. We show that the inclusion of radiation reaction leads to a narrow energy spectrum of the yielded particles, whose maximum is shifted to substantially lower energies as compared to the case in which radiative energy losses are ignored. The broad bremsstrahlung spectrum also has distinct influence on the particle distributions, whereas the impact of laser focusing turns out to be rather moderate in the considered parameter regime., Comment: 11 pages, 7 figures

3C84 is a nearby radio source with a complex total intensity structure, showing linear polarisation and spectral patterns. A detailed investigation of the central engine region necessitates the use of VLBI above the hitherto available maximum frequency of 86GHz. Using ultrahigh resolution VLBI observations at the highest available frequency of 228GHz, we aim to directly detect compact structures and understand the physical conditions in the compact region of 3C84. We used EHT 228GHz observations and, given the limited (u,v)-coverage, applied geometric model fitting to the data. We also employed quasi-simultaneously observed, multi-frequency VLBI data for the source in order to carry out a comprehensive analysis of the core structure. We report the detection of a highly ordered, strong magnetic field around the central, SMBH of 3C84. The brightness temperature analysis suggests that the system is in equipartition. We determined a turnover frequency of $\nu_m=(113\pm4)$GHz, a corresponding synchrotron self-absorbed magnetic field of $B_{SSA}=(2.9\pm1.6)$G, and an equipartition magnetic field of $B_{eq}=(5.2\pm0.6)$G. Three components are resolved with the highest fractional polarisation detected for this object ($m_\textrm{net}=(17.0\pm3.9)$%). The positions of the components are compatible with those seen in low-frequency VLBI observations since 2017-2018. We report a steeply negative slope of the spectrum at 228GHz. We used these findings to test models of jet formation, propagation, and Faraday rotation in 3C84. The findings of our investigation into different flow geometries and black hole spins support an advection-dominated accretion flow in a magnetically arrested state around a rapidly rotating supermassive black hole as a model of the jet-launching system in the core of 3C84. However, systematic uncertainties due to the limited (u,v)-coverage, however, cannot be ignored., Comment: 15 pages, 6 figures, published in A&A

It is generally assumed that ionization in slow collisions of light atomic particles, whose constituents (electrons and nuclei) move with velocities orders of magnitude smaller than the speed of light, is driven solely by the Coulomb force. Here we show, however, that the Breit interaction -- a relativistic correction to the Coulomb interaction between electrons -- can become the main actor when the colliding system couples resonantly to the quantum radiation field. Our results demonstrate that this ionization mechanism can be very efficient in various not too dense physical environments, including stellar plasmas and atomic beams propagating in gases., Comment: 6 pages, 2 figures

Ionization-excitation of an atom induced by the absorption of a single photon in the presence of a neighbouring atom is studied. The latter is, first, resonantly photoexcited and, afterwards, transfers the excitation energy radiationlessly to the other atom, leading to its ionization with simultaneous excitation. The process relies on the combined effects of interatomic and intraatomic electron correlations. Under suitable conditions, it can dominate by several orders of magnitude over direct photoionization-excitation and even over direct photoionization. In addition, we briefly discuss another kind of two-center resonant photoionization with excitation where the ionization and residual excitation in the final state are located at different atomic sites., Comment: 6 pages, 2 figures. arXiv admin note: text overlap with arXiv:1906.08123

Production of electron-positron pairs by a high-energy $\gamma$ photon and a bichromatic laser wave is considered where the latter is composed of a strong low-frequency and a weak high-frequency component, both with circular polarization. An integral expression for the production rate is derived that accounts for the strong laser mode to all orders and for the weak laser mode to first order. The structure of this formula resembles the well-known expression for the nonlinear Breit-Wheeler process in a strong laser field, but includes the dynamical assistance from the weak laser mode. We analyze the dependence of the dynamical rate enhancement on the applied field parameters and show, in particular, that it is substantially higher when the two laser modes have opposite helicity., Comment: 11 pages, 7 figures

Production of electron-positron pairs in two oscillating strong electric field pulses with largely different frequencies and durations is considered. In a first scenario, the influence of a low-frequency background field on pair production by a short main pulse of high frequency is analyzed. The background field is shown to cause characteristic modifications of the momentum spectra of created particles which, in turn, may be used for imaging of the background pulse. In a second scenario, an ultrashort, relatively weak assisting pulse is superimposed onto a strong main pulse. By studying the dependence of the pair production on the field parameters it is shown that duration and relative position of the ultrashort pulse modify the momentum spectra of produced particles in a distinctive way. Both scenarios enable, moreover, to extract partial information about the time periods when pairs with certain momenta are produced predominantly., Comment: 10 pages, 9 figures

In interatomic Coulombic electron capture, the capture of a free electron at an atomic center is accompanied by the radiationless transfer of the excess energy to a neighboring atom of different species, leading to ionization of the latter. We show that this interatomic process can be strongly enhanced by the presence of an additional third atom, provided the energy of the free-bound capture transition in the first atom is resonant to a dipole-allowed excitation energy in this assisting atom. The relation of the resonantly enhanced three-center electron capture with other processes is discussed, and its dependencies on the incident electron energy and the spatial geometry of the triatomic system are illustrated., Comment: 8 pages, 5 figures

Relativistic strong-field ionization of hydrogen-like atoms or ions in a constant crossed electromagnetic field is studied. The transition amplitude is formulated within the strong-field approximation in G\"oppert-Mayer gauge, with initial and final electron states being described by the corresponding Dirac-Coulomb and Dirac-Volkov wave functions, respectively. Coulomb corrections to the electron motion during tunneling are taken into account by adjusting an established method to the present situation. Total and energy-differential ionization rates are calculated and compared with predictions from other theories in a wide range of atomic numbers and applied field strengths., Comment: 9 pages, 4 figures

KAGRA, the underground and cryogenic gravitational-wave detector, was operated for its solo observation from February 25th to March 10th, 2020, and its first joint observation with the GEO 600 detector from April 7th -- 21st, 2020 (O3GK). This study presents an overview of the input optics systems of the KAGRA detector, which consist of various optical systems, such as a laser source, its intensity and frequency stabilization systems, modulators, a Faraday isolator, mode-matching telescopes, and a high-power beam dump. These optics were successfully delivered to the KAGRA interferometer and operated stably during the observations. The laser frequency noise was observed to limit the detector sensitivity above a few kHz, whereas the laser intensity did not significantly limit the detector sensitivity.

It is generally assumed that for ionization processes, which occur in slow atomic collisions, the coupling of the colliding system to the quantum radiation field is irrelevant. Here we show, however, that -- contrary to expectations -- such a coupling can strongly influence ionization of a beam of atomic species A slowly traversing a gas of atomic species B excited by a weak laser field. Moreover, this coupling becomes even more important when the collision velocity increases, getting comparable or exceeding the typical electron orbiting velocities in A and B. Our results imply furthermore that the Breit interaction can, in fact, dominate over the Coulomb interaction at very low energies., Comment: 6 pages, 1 figure

We report a detailed Shubnikov-de Haas (SdH) study on the Weyl type-II semimetal WTe2 in magnetic fields up to 29 T. By using the SdH results to guide our density functional theory calculations, we are able to accurately determine its Fermi surface by employing a moderate Hubbard U term, which is an essential step in explaining the unusual electronic properties of this much studied material. In addition to the fundamental orbits, we observe magnetic breakdown, which can consistently be explained within the model of a Russian-doll-nested Fermi surface of electron and hole pockets. The onset of magnetic breakdown in WTe2 is solely determined by impurity damping in contrast to magnetic breakdown scenarios in other metallic systems., Comment: 5 pages, 4 figures

Photoproduction of electron-positron pairs by bremsstrahlung gamma-rays propagating through a high-intensity optical laser wave is studied. By calculating total pair production yields as well as energy and angular distributions of the produced particles for a range of laser intensities, we analyze the influence that the broad frequency spectrum of bremsstrahlung exerts on the properties of the process in the transition region between the perturbative and non-perturbative interaction regimes. In particular, we demonstrate a continuous shifting of the mainly contributing interval of bremsstrahlung frequencies during the course of the transition and indicate characteristic signatures of the laser-dressed particle mass., Comment: 10 pages, 4 figures, to appear in Phys. Rev. A

The formation of positive ions of antihydrogen $\bar{\rm{H}}^+$ via the three body reaction (i) $\rm{e}^+ + \rm{e}^- + \bar{\rm{H}} \rightarrow \rm{e}^- + \bar{\rm{H}}^+$ is considered. In reaction (i), free positrons $\rm{e}^+$ are incident on antihydrogen $\bar{\rm{H}}$ embedded in a gas of low-energy ($\sim $ meV) electrons and, due to the positron-electron interaction, a positron is attached to $\bar{\rm{H}}$ whereas an electron carries away the energy excess. We compare reaction (i) with two radiative attachment mechanisms. One of them is (ii) spontaneous radiative attachment, in which the ion is formed due to spontaneous emission of a photon by a positron incident on $\bar{\rm{H}}$. The other is (iii) two-center dileptonic attachment which takes place in the presence of a neighboring atom B and in which an incident positron is attached to $\bar{\rm{H}}$ via resonant transfer of energy to B with its subsequent relaxation through spontaneous radiative decay. It is shown that reaction (i) can strongly dominate over mechanisms (ii) and (iii) for positron energies below $0.1$ eV. It is also shown that at the energies considered reaction (i) will not be influenced by annihilation and that the reaction $\rm{e}^+ + \rm{e}^+ + \bar{\rm{H}} \rightarrow \rm{e}^+ + \bar{\rm{H}}^+$ has a vanishingly small rate compared to reaction (i)., Comment: 7 pages, 2 figures

One of the main limitations in state-of-the art solid-state quantum processors are qubit decoherence and relaxation due to noise in their local environment. For the field to advance towards full fault-tolerant quantum computing, a better understanding of the underlying microscopic noise sources is therefore needed. Adsorbates on surfaces, impurities at interfaces and material defects have been identified as sources of noise and dissipation in solid-state quantum devices. Here, we use an ultra-high vacuum package to study the impact of vacuum loading, UV-light exposure and ion irradiation treatments on coherence and slow parameter fluctuations of flux tunable superconducting transmon qubits. We analyse the effects of each of these surface treatments by comparing averages over many individual qubits and measurements before and after treatment. The treatments studied do not significantly impact the relaxation rate $\Gamma_1$ and the echo dephasing rate $\Gamma_2^\textrm{e}$, except for Ne ion bombardment which reduces $\Gamma_1$. In contrast, flux noise parameters are improved by removing magnetic adsorbates from the chip surfaces with UV-light and NH$_3$ treatments. Additionally, we demonstrate that SF$_6$ ion bombardment can be used to adjust qubit frequencies in-situ and post fabrication without affecting qubit coherence at the sweet spot.

We describe design, implementation and performance of an ultra-high vacuum (UHV) package for superconducting qubit chips or other surface sensitive quantum devices. The UHV loading procedure allows for annealing, ultra-violet light irradiation, ion milling and surface passivation of quantum devices before sealing them into a measurement package. The package retains vacuum during the transfer to cryogenic temperatures by active pumping with a titanium getter layer. We characterize the treatment capabilities of the system and present measurements of flux tunable qubits with an average T$_1=84~\mu$s and T$^{echo}_2=134~\mu$s after vacuum-loading these samples into a bottom loading dilution refrigerator in the UHV-package., Comment: 5 pages, 6 figures

Following our study of the radio and high-energy properties of $\gamma$-ray-emitting radio galaxies, here we investigate the kinematic and spectral properties of the parsec-scale jets of radio galaxies that have not yet been detected by Fermi-LAT. We take advantage of the regular VLBI observations provided by the TANAMI monitoring program, and explore the kinematic properties of six $\gamma$-ray-faint radio galaxies. We include publicly available VLBI kinematics of $\gamma$-ray-quiet radio galaxies monitored by the MOJAVE program and perform a Fermi-LAT analysis, deriving upper limits. We combine these results with those from our previous paper to construct the largest sample of radio galaxies with combined VLBI and $\gamma$-ray measurements to date. We find superluminal motion up to $\beta_\mathrm{app}=3.6$ in the jet of PKS 2153$-$69. We find a clear trend of higher apparent speed as a function of distance from the jet core on scales of $\sim10^5\,R_s$, corresponding to the end of the collimation and acceleration zone in nearby radio galaxies. We find evidence of subluminal apparent motion in the jets of PKS 1258$-$321 and IC 4296, and no measurable motion for PKS 1549$-$79, PKS 1733$-$565 and PKS 2027$-$308. We compare the VLBI properties of $\gamma$-ray-detected and undetected radio galaxies, and find significantly different distributions of median core flux density, and, possibly, of median core brightness temperature. We find a significant correlation between median core flux density and $\gamma$-ray flux, but no correlation with typical Doppler boosting indicators such as median core brightness temperature and core dominance. Our study suggests that high-energy emission from radio galaxies is related to parsec-scale radio emission from the inner jet, but is not driven by Doppler boosting effects, in contrast to the situation in their blazar counterparts., Comment: Main text: 15 pages, 12 figures, 6 tables. Appendix: 14 pages, 20 figures, 13 tables. Accepted for publication in Astronomy and Astrophysics

Majorana fermions act as their own antiparticle, and they have long been thought to be confined to the realm of pure theory. However, interest in them has recently resurfaced, as it was realized through the work of Kitaev that some experimentally accessible condensed matter systems can host these exotic excitations as bound states on the boundaries of 1D chains, and that their topological and non-abelian nature holds promise for quantum computation. Unambiguously detecting the experimental signatures of Majorana bound states has turned out to be challenging, as many other phenomena lead to similar experimental behaviour. Here, we computationally study a ring comprised of two Kitaev model chains with tunnel coupling between them, where an applied magnetic field allows for Aharonov-Bohm interference in transport through the resulting ring structure. We use a non-equilibrium Green's function technique to analyse the transport properties of the ring in both the presence and absence of Majorana zero modes. Further, we show that these results are robust against weak disorder in the presence of an applied magnetic field. This computational model suggests another signature for the presence of these topologically protected bound states can be found in the magnetic field dependence of devices with loop geometries., Comment: 12 pages, 10 figures. Included new section about disorder in the nanowires and the effect of this on our results

The possibility to utilize different types of two-qubit gates on a single quantum computing platform adds flexibility in the decomposition of quantum algorithms. A larger hardware-native gate set may decrease the number of required gates, provided that all gates are realized with high fidelity. Here, we benchmark both controlled-Z (CZ) and exchange-type (iSWAP) gates using a parametrically driven tunable coupler that mediates the interaction between two superconducting qubits. Using randomized benchmarking protocols we estimate an error per gate of $0.9\pm0.03\%$ and $1.3\pm0.4\%$ fidelity for the CZ and the iSWAP gate, respectively. We argue that spurious $ZZ$-type couplings are the dominant error source for the iSWAP gate, and that phase stability of all microwave drives is of utmost importance. Such differences in the achievable fidelities for different two-qubit gates have to be taken into account when mapping quantum algorithms to real hardware., Comment: 24 pages, including supplementary information

It is well known that interatomic or intermolecular interactions driven by two-center electronic dipole-dipole correlations fall off rapidly with the inter-site distance. We show, however, that the effective strength of interatomic reaction channels, which are triggered by a resonant field, can exhibit a nonmonotonous distance dependence, being strongly reduced when the atoms come closer. This surprising result is demonstrated by considering resonant two-center photoionization as an example. Our findings are supported by available experimental data., Comment: 5 pages, 1 figure

Photoionization of an atom in the presence of a neighboring atom of different species is studied. The latter first undergoes resonant photoexcitation, leading to an autoionizing state of the diatomic system. Afterwards, the excitation energy is transferred radiationlessly via a two-center Auger process to the other atom, causing its ionization. Assuming a fixed internuclear distance, it has been predicted theoretically that this indirect ionization pathway can strongly dominate over the direct photoionization. Here, we extend the theory of resonant two-center photoionization by including the nuclear motion in van-der-Waals molecules. An analytical formula is derived reflecting the influence of molecular vibrational dynamics on the relative strength of the two-center channel. For the specific example of Li-He dimers we show that the two-center autoionizing resonances are split by the nuclear motion into multiplets, with the resonance lines reaching a comparable level of enhancement over direct photoionization as is obtained in a model based on spatially fixed nuclei.

Realistic synthetic observations of theoretical source models are essential for our understanding of real observational data. In using synthetic data, one can verify the extent to which source parameters can be recovered and evaluate how various data corruption effects can be calibrated. These studies are important when proposing observations of new sources, in the characterization of the capabilities of new or upgraded instruments, and when verifying model-based theoretical predictions in a comparison with observational data. We present the SYnthetic Measurement creator for long Baseline Arrays (SYMBA), a novel synthetic data generation pipeline for Very Long Baseline Interferometry (VLBI) observations. SYMBA takes into account several realistic atmospheric, instrumental, and calibration effects. We used SYMBA to create synthetic observations for the Event Horizon Telescope (EHT), a mm VLBI array, which has recently captured the first image of a black hole shadow. After testing SYMBA with simple source and corruption models, we study the importance of including all corruption and calibration effects. Based on two example general relativistic magnetohydrodynamics (GRMHD) model images of M87, we performed case studies to assess the attainable image quality with the current and future EHT array for different weather conditions. The results show that the effects of atmospheric and instrumental corruptions on the measured visibilities are significant. Despite these effects, we demonstrate how the overall structure of the input models can be recovered robustly after performing calibration steps. With the planned addition of new stations to the EHT array, images could be reconstructed with higher angular resolution and dynamic range. In our case study, these improvements allowed for a distinction between a thermal and a non-thermal GRMHD model based on salient features in reconstructed images., Comment: 20 pages, 15 figures, accepted for publication in A&A

Unambiguous and complete determination of the Fermi surface is a primary step in understanding the electronic properties of topical metals and semi-metals, but only in a relatively few cases has this goal been realized. In this work, we present a systematic high-field quantum oscillation study up to 35 T on ZrSiS, a textbook example of a nodal-line semimetal with only linearly dispersive bands crossing the Fermi energy. The topology of the Fermi surface is determined with unprecedented precision and all pockets are identified by comparing the measured angle dependence of the quantum oscillations to density functional theory calculations. Comparison of the Shubnikov-de Haas and de Haas-van Alphen oscillations at low temperatures and analysis of the respective Dingle plots reveal the presence of significantly enhanced scattering on the electron pocket. Above a threshold field that is aligned along the c-axis of the crystal, the specific cage-like Fermi surface of ZrSiS allows for electron-hole tunneling to occur across finite gaps in momentum space leading to quantum oscillations with a complex frequency spectrum. Additional high-frequency quantum oscillations signify magnetic breakdown orbits that encircle the entire Dirac nodal loop. We suggest that the persistence of quantum oscillations in the resistivity to high temperatures is caused by Stark interference between orbits of nearly equal masses., Comment: 14 pages, 13 figures

Generation of quasi-particle--hole pairs in gapped graphene monolayers in the combined field of two counterpropagating light waves is studied. The process represents an analogue of electron-positron pair production from the QED vacuum by the Breit-Wheeler effect. We show, however, that the two-dimensional structure of graphene causes some striking differences between both scenarios. In particular, contrary to the QED case, it allows for non-zero pair production rates at the energy threshold when the Breit-Wheeler reaction proceeds nonlinearly with absorption of three photons., Comment: 6 pages, 2 figures

Production of electron-positron pairs from vacuum in strong bichromatic electric fields, oscillating in time with a fundamental frequency and its second harmonic, is studied. Strong-field processes occuring in such field configurations are generally known to be sensitive to the relative phase between the field modes. Phase-of-the-phase spectroscopy has recently been introduced in the context of strong-field photoionization as a systematic means to analyze these coherence effects. We apply this method to field-induced pair production by calculating the phase dependence of the momentum-resolved particle yields. We show that asymmetric checkerboard patterns arise in the phase-of-the-phase spectra, similarly to those found in strong-field photoionization. The physical origin of these characteristic structures, which differ between the created electron and positron, are discussed., Comment: 8 pages, 6 figures

The full statistical distribution of the superfluid fraction characterizing one-dimensional Bose gases in random potentials is discussed. Rare configurations with extreme fluctuations of the disorder potential can fragment the condensate and reduce the superfluid fraction to zero. The resulting bimodal probability distribution for the superfluid fraction is calculated numerically in the quasi-1D mean-field regime of ultracold atoms in laser speckle potentials. Using extreme-value statistics, an analytical scaling of the zero-superfluid probability as function of disorder strength, disorder correlation length and system size is presented. It is argued that similar results can be expected for point-like impurities, and that these findings are in reach for present-day experiments., Comment: 7 pages, 5 figures

We improve on the strong uniqueness results of [GLM+17], which deal with the following system of SDE. \begin{align*} dX_t&=Y_tdt \\ dY_t&=|X_{t}|^{\alpha}dB_t \end{align*} and $X_0=x_0,Y_0=y_0$. For $(x_0,y_0)\ne(0,0)$, we show that short-time uniqueness holds for $\alpha>-1/2$., Comment: 8 pages

Double ionization of an atom by single-photon absorption in the presence of a neighbouring atom is studied. The latter is, first, resonantly photoexcited and, afterwards, transfers the excitation energy radiationlessly to the other atom, leading to its double ionization. The process relies on the combined effect of interatomic and intraatomic electron correlations. It can dominate over the direct double photoionization by several orders of magnitude at interatomic distances up to few nanometers. The relative position of the neighbouring atom is shown to exert a characteristic influence on the angular distribution of emitted electrons., Comment: 8 pages, 4 figures

In the framework of the TANAMI multi-wavelength and VLBI monitoring, we study the evolution of the parsec-scale radio emission in radio galaxies in the southern hemisphere and their relationship to the $\gamma$-ray properties. In this first paper, we focus on Fermi-LAT-detected sources. We perform a kinematic analysis for five $\gamma$-ray detected radio galaxies using multi-epoch 8.4 GHz VLBI images, deriving limits on intrinsic jet parameters. We analyzed Fermi-LAT data in order to study possible connections between the $\gamma$-ray properties and the pc-scale jets of Fermi-LAT-detected radio galaxies, both in terms of variability and average properties. We discuss the individual source results and draw preliminary conclusions on sample properties including published VLBI results from the MOJAVE survey, with a total of fifteen sources. We find that the first $\gamma$-ray detection of Pictor A might be associated with the passage of a new VLBI component through the radio core. For the peculiar AGN PKS 0521-36, we detect subluminal parsec-scale jet motions, and we confirm the presence of fast $\gamma$-ray variability in the source down to timescales of 6 hours. We robustly confirm the presence of significant superluminal motion, up to $\beta_{app}\sim$3, in the jet of the TeV radio galaxy PKS 0625-35. Finally, we place a lower limit on the age of the Compact Symmetric Object (CSO) PKS 1718-649. We draw some preliminary conclusions on the relationship between pc-scale jets and $\gamma$-ray emission in radio galaxies. We find that the VLBI core flux density correlates with the $\gamma$-ray flux, as seen in blazars. On the other hand, the $\gamma$-ray luminosity does not show any dependence on the core brightness temperature and core dominance, two indicators of Doppler boosting, suggesting that $\gamma$-ray emission in radio galaxies is not driven by orientation-dependent effects., Comment: Main text: 22 pages, 22 figures, 11 tables. Appendix: 19 pages, 10 figures, 9 tables. Accepted for publication in Astronomy and Astrophysics

Electron-impact ionization of an atom or ion in the presence of a neighboring atom is studied. The latter is first collisionally excited by the incident electron, whose energy is assumed to be high but nonrelativistic. Afterwards, the excitation energy is transferred radiationlessly via a two-center Auger process to the other atom or ion, leading to its ionization. We show that the participation of the neighboring atom manifests in a very pronounced resonance peak in the energy-differential cross section and can substantially enhance the total cross section of electron-impact ionization. We also discuss the influence of the neighbouring atom on the angular distribution of the ejected electron.

A free electron can form a bound state with an atomic center $A$ upon photo emission (radiative recombination). In the presence of a neighboring atom $B$, such a bound state can, under certain conditions, be also formed via resonant transfer of energy to $B$, with its subsequent relaxation through radiative decay (two-center dielectronic recombination). This two-center process is very efficient in the 'static' case where $A$ and $B$ form a weakly bound system, dominating over single-center radiative recombination up to internuclear distances as large as several nanometers. Here we study its dynamic variant in which recombination occurs when a beam of species $A$ collides with a gas of atoms $B$ and show that, even though the average distance between $A$ and $B$ in collisions is orders of magnitude larger than the typical size of a bound system, the two-center recombination can still outperform the single-center radiative recombination., Comment: 8 pages, 2 figures

Spin qubits and superconducting qubits are among the promising candidates for a solid state quantum computer. For the implementation of a hybrid architecture which can profit from the advantages of either world, a coherent long-distance link is necessary that integrates and couples both qubit types on the same chip. We realize such a link with a frequency-tunable high impedance SQUID array resonator. The spin qubit is a resonant exchange qubit hosted in a GaAs triple quantum dot. It can be operated at zero magnetic field, allowing it to coexist with superconducting qubits on the same chip. We find a working point for the spin qubit, where the ratio between its coupling strength and decoherence rate is optimized. We observe coherent interaction between the resonant exchange qubit and a transmon qubit in both resonant and dispersive regimes, where the interaction is mediated either by real or virtual resonator photons.

Electron-positron pair production from vacuum in a strong electric field oscillating in time is studied. The field is assumed to consist of two consecutive pulses, with variable time delay in between. Pair production probabilities are obtained by numerical solution of the corresponding time-dependent Dirac equation. Considering symmetric field configurations comprising two identical pulses, we show that the pulse distance strongly affects the momentum spectra of produced particles and even the total production probability. Conversely, in a highly asymmetric situation when the field contains low-intensity prepulse and high-intensity main pulse, we identify a range of field parameters where the prepulse despite its weakness can leave visible traces in the particle spectra., Comment: 6 pages, 7 figures

The Galactic Center supermassive black hole Sagittarius A* (Sgr A*) is one of the most promising targets to study the dynamics of black hole accretion and outflow via direct imaging with very long baseline interferometry (VLBI). At 3.5 mm (86 GHz), the emission from Sgr A* is resolvable with the Global Millimeter VLBI Array (GMVA). We present the first observations of Sgr A* with the phased Atacama Large Millimeter/submillimeter Array (ALMA) joining the GMVA. Our observations achieve an angular resolution of ~87{\mu}as, improving upon previous experiments by a factor of two. We reconstruct a first image of the unscattered source structure of Sgr A* at 3.5 mm, mitigating effects of interstellar scattering. The unscattered source has a major axis size of 120 $\pm$ 34{\mu}as (12 $\pm$ 3.4 Schwarzschild radii), and a symmetrical morphology (axial ratio of 1.2$^{+0.3}_{-0.2}$), which is further supported by closure phases consistent with zero within 3{\sigma}. We show that multiple disk-dominated models of Sgr A* match our observational constraints, while the two jet-dominated models considered are constrained to small viewing angles. Our long-baseline detections to ALMA also provide new constraints on the scattering of Sgr A*, and we show that refractive scattering effects are likely to be weak for images of Sgr A* at 1.3 mm with the Event Horizon Telescope. Our results provide the most stringent constraints to date for the intrinsic morphology and refractive scattering of Sgr A*, demonstrating the exceptional contribution of ALMA to millimeter VLBI., Comment: Accepted for publication in ApJ

An additional channel opens for photoionization of atom $A$ by an electromagnetic field if it traverses a gas of atoms $B$ resonantly coupled to this field. We show that this channel, in which $A$ is ionized via resonant photoexcitation of $B$ with subsequent energy transfer to $A$ through two-center electron correlations and which is very efficient when $A$ and $B$ constitute a bound system, can strongly dominate the ionization of $A$ also in collisions where the average distance between $A$ and $B$ exceeds the typical size of a bound system by orders of magnitude., Comment: 6 pages, 1 figure

Photoionization of an atom $A$ by a strong laser field in the presence of a spatially well-separated neighboring atom $B$ is considered. The laser field frequency is assumed to lie below the ionization potential of atom $A$ and be resonant with a dipole-allowed transition in atom $B$. In this situation, the ionization may occur either directly by multiphoton absorption from the laser field at the first atomic center. Or via an indirect pathway involving two-center electron-electron correlations, where the neighbor atom $B$ is first photoexcited and, afterwards, transfers its energy upon deexcitation radiationlessly to atom $A$. Considering monochromatic as well as bichromatic laser fields, we study various coupling regimes of the photoionization process and identify experimentally accessible parameter domains where the two-center channel is dominant., Comment: 10 pages, 5 figures

We report a study of quantum oscillations in the high-field magneto-resistance of the nodal-line semimetal HfSiS. In the presence of a magnetic field up to 31 T parallel to the c-axis, we observe quantum oscillations originating both from orbits of individual electron and hole pockets, and from magnetic breakdown between these pockets. In particular, we find an oscillation associated with a breakdown orbit enclosing one electron and one hole pocket in the form of a `figure of eight'. This observation represents an experimental confirmation of the momentum space analog of Klein tunneling. When the c-axis and the magnetic field are misaligned with respect to one another, this oscillation rapidly decreases in intensity. Finally, we extract the cyclotron masses from the temperature dependence of the oscillations, and find that the mass of the 'figure of eight' orbit corresponds to the sum of the individual pockets, consistent with theoretical predictions for Klein tunneling in topological semimetals., Comment: 6 pages, 4 figures

Electron scattering and dielectronic recombination with an ion in the presence of a neighboring atom is studied. The incident electron is assumed to be captured by the ion, leading to resonant excitation of the atom which afterwards may stabilize either by electron re-emission or radiative decay. We show that the participation of the atom can strongly affect, both quantitatively and qualitatively, the corresponding processes of electron scattering and recombination. Various ion-atom systems and electronic transitions are considered. In particular we show that electron scattering under backward angles may be strongly enhanced and derive the scaling behavior of two-center dielectronic recombination with the principal quantum numbers of the participating atomic states., Comment: 12 pages, 6 figures