Publications
Involved institute: Institute of Theoretical Physics (from 2018)
Year 20202025
Public type of publication: Articles ref. in Journals
"Online First" included
Study of QED singular properties for variable gyromagnetic ratio g≃2
Rafelski, J.; Evans, S.; Labun, L.
Using the external field method, {\it i.e.\/} evaluating the effective action $V_{\mathrm{eff}}$ for an arbitrarily strong constant and homogeneous field, we explore nonperturbative properties of QED allowing arbitrary gyromagnetic ratio $g$. We find a cusp at $g = 2$ in: a) The QED $b_0$renormalization group coefficient, and in the infinite wavelength limit in b) a subclass containing the pseudoscalar ${\cal P}^{2n}= (\vec E\cdot\vec B)^{2n} $ of lightlight scattering coefficients. Properties of $b_0$ imply for certain domains of $g$ asymptotic freedom in an Abelian theory.
Keywords: Schwinger effect; vacuum stability; magnetic moment

Contribution to WWW
https://arxiv.org/abs/2212.13165
DOI: 10.48550/arXiv.2212.13165
arXiv: 2212.13165 
Physical Review D 107(2023), 076002107600212
DOI: 10.1103/PhysRevD.107.076002
Cited 1 times in Scopus
Improving EulerHeisenbergSchwinger Effective Action with Dressed Photons
We implement a longstanding proposal by Weisskopf to apply virtual polarization corrections to
the in/out external fields in study of the EulerHeisenbergSchwinger effective action. Our approach
requires distinguishing the electromagnetic and polarization fields based on mathematical tools
developed by Bia lynickiBirula, originally for the BornInfeld action. Our solution is expressed
as a differential equation where the oneloop effective action serves as input. As a first result of
our approach, we recover the higherorder onecut reducible loop diagrams discovered by Gies and
Karbstein.
Keywords: Euler–Heisenberg–Schwinger (EHS); nonperturbative vacuum structure; quantum electrodynamics (QED)

Contribution to WWW
https://arxiv.org/abs/2306.07887
DOI: 10.12693/APhysPolA.143.S13
arXiv: 2306.07887 
Acta Physica Polonica A 6(2023)143, S13S17
DOI: 10.12693/APhysPolA.143.S13
Dimer Coupling Energies of the Si(001) Surface
Schützhold, R.; Brand, C.; Hucht, A.; Jnawali, G.; Fortmann, J. D.; Sothmann, B.; Mehdipour, H.; Kratzer, P.; HornVon Hoegen, M.
The coupling energies between the buckled dimers of the Si(001) surface were determined through analysis of the anisotropic critical behavior of its orderdisorder phase transition. Spot profiles in highresolution lowenergy electron diffraction as a function of temperature were analyzed within the framework of the anisotropic twodimensional Ising model. The validity of this approach is justified by the large ratio of correlation lengths, ζ +/ζ∥+=5.2 of the fluctuating c(4×2) Domains above the critical temperature Tc=(190.6±10) K. We obtain effective couplings J∥=(24.9±1.3) meV along the dimer rows and J⊥=(0.8±0.1) meV across the dimer rows, i.e., antiferromagneticlike coupling of the dimers with c(4×2) symmetry.
Keywords: Dimers; Electrons; Ising model; Correlation lengths; Coupling energies; Critical behaviour; Critical temperatures; Effective coupling; High resolution; Lowenergy electron diffraction; Order/disorder phase transition; Spot profile; Twodimensional; Anisotropy

Physical Review Letters 130(2023), 126203
DOI: 10.1103/PhysRevLett.130.126203 
Contribution to WWW
arXiv:2302.01715 [condmat.meshall]: https://arxiv.org/abs/2302.01715
DOI: 10.48550/arXiv.2302.01715
Fresh look at experimental evidence for odderon exchange
Schmidt, S. M.; Cui, Z.F.; Binosi, D.; Roberts, C. D.; Triantafyllopoulos, D. N.
Theory suggests that in highenergy elastic hadron+hadron scattering, tchannel exchange of a family of colourless crossingodd
states – the odderon – may generate differences between pp¯ and pp crosssections in the neighbourhood of the
diffractive minimum. Using a mathematical approach based on interpolation via continued fractions enhanced by statistical
sampling, we develop robust comparisons between pp¯ elastic differential crosssections measured at √s=1.96 TeV by the
D0 Collaboration at the Tevatron and functionformunbiased extrapolations to this energy of kindred pp measurements at
√s/TeV=2.76,7,8,13 by the TOTEM Collaboration at the LHC and a combination of these data with earlier crosssection
measurements at √s/GeV=23.5,30.7,44.7,52.8,62.5 made at the intersecting storage rings. Focusing on a domain that
straddles the diffractive minimum in the pp¯ and pp crosssections, we find that these two crosssections differ at the
(2.2−2.6)σ level; hence, supply evidence with this level of significance for the existence of the odderon. If combined with
evidence obtained through different experimenttheory comparisons, whose significance is reported to lie in the range
(3.4−4.6)σ, one arrives at a (4.0−5.2)σ signal for the odderon.
Keywords: Diffractive processes; Highenergy hadron; Odderon; Regge phenomenology; Schlessinger point; Strong interactions in the standard model of particle physics; hadron interactions

Physics Letters B B(2023)839, 137826
DOI: 10.1016/j.physletb.2023.137826
Cited 1 times in Scopus 
Contribution to WWW
https://arxiv.org/abs/2205.15438
DOI: 10.48550/arXiv.2205.15438
arXiv: Fresh look at experimental evidence for odderon exchange
Generalized GelfandDikii equation and solitonic electric fields for fermionic Schwinger pair production
Ahmadiniaz, N.; Fedotov, A. M.; Gelfer, E. G.; Pyo Kim, S.; Schubert, C.
Schwinger pair creation in a purely timedependent electric field can be reduced to an effective quantum mechanical problem using a variety of formalisms. Here we develop an approach based on the GelfandDikii equation for scalar QED, and extent it to spinor QED. We discuss some solvable special cases from this point of view. It was previously shown how to use the wellknown solitonic solutions of the KdV equation to construct “solitonic” electric fields that do not create scalar pairs with an arbitrary fixed momentum. We show that this construction can be adapted to the fermionic case in two inequivalent ways, both leading to the vanishing of the paircreation rate at certain values of the P ̈oschlTeller like index of the associated Schr ̈odinger equation. Thus for any given momentum, we can construct electric fields that create scalar particles but not spinor particles, and also the other way round. Therefore, while often spin is even neglected in Schwingerpair creation, in such cases it becomes decisive.
Keywords: Schwinger mechanism; Pair production; GelfandDikii equation

Contribution to WWW
2205.159 [hepth]: https://arxiv.org/pdf/2205.15946.pdf
arXiv: https://arxiv.org/pdf/2205.15946.pdf 
Physical Review D 108(2023)3, 036019
DOI: 10.1103/PhysRevD.108.036019
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Precision Storage Rings for Electric Dipole Moment Searches: A Tool En Route to Physics BeyondtheStandardModel
Ströher, H.; Schmidt, S. M.; Lenisa, P.; Pretz, J.
Electric Dipole Moments (EDM) of particles (leptons, nucleons and light nuclei) are currently deemed one of the best indicators for new physics, i.e. phenomena, which lie outside the Standard Model (SM) of elementary particle physics – so called physics “BeyondtheStandardModel” (BSM). Since EDMs of the SM are vanishingly small, a finite permanent EDM would indicate chargeparity symmetry (CP) violation in addition to the wellknown sources of the SM and could explain the baryon asymmetry of the Universe, while an oscillating EDM would hint at a possible Dark Matter (DM) field comprising axions or axionlike particles (ALPs). A new approach exploiting polarized charged particles (proton, deuteron, 3He) in precision storage rings offers the prospect to push current experimental EDM upper limits significantly further including the possibility of an EDM discovery. In this paper, we describe the scientific background and the steps towards the realization of a precision storage ring, which will make such measurements possible.
Keywords: Baryon Asymmetry; Dark Matter; Electric Dipole Moments; Storage Rings; Polarized Beams

Particles 6(2023)1, 385398
DOI: 10.3390/particles6010020
Detection schemes for quantum vacuum diffraction and birefringence
Ahmadiniaz, N.; Cowan, T.; Grenzer, J.; FranchinoVinas, S.; Laso García, A.; Smid, M.; Toncian, T.; Trejo Espinosa, M. A.; Schützhold, R.
Motivated by recent experimental initiatives, such as at the
Helmholtz International Beamline for Extreme Fields (HIBEF)
at the European Xray Free Electron Laser (XFEL), we calculate
the birefringent scattering of xrays at the combined field of
two optical (or nearoptical) lasers and compare various scenarios.
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In order to facilitate an experimental detection of quantum vacuum diffraction and
birefringence, special emphasis is placed on scenarios where the initial
and final xray photons differ not just in polarization, but also in
propagation direction (corresponding to scattering angles in the mrad regime)
and possibly energy.
Keywords: Strong Field QED; Vacuum qirefringence; Quantum vacuum diffraction; EulerHeisenberg Lagrangian; XFEL; Laser; Lightbylight scattering

Contribution to WWW
https://arxiv.org/abs/2208.14215 
Physical Review D 108(2023)7, 076005
DOI: 10.1103/PhysRevD.108.076005
Cited 1 times in Scopus
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Resummed heatkernel and form factors for surface contributions: Dirichlet semitransparent boundary conditions
FranchinoVinas, S.
In this article we consider resummed expressions for the heatkernel's
trace of a Laplace operator, the latter including a potential and imposing Dirichlet semitransparent boundary conditions on a surface of codimension one in flat space.
We obtain resummed expressions that correspond to the first and second order expansion of the heatkernel in powers of the potential.
We show how to apply these results to obtain the bulk and surface form factors of a scalar quantum field theory in $d=4$ with a Yukawa coupling to a background.
A characterization of the form factors in terms of pseudodifferential operators is given.

Contribution to WWW
https://arxiv.org/abs/2208.11979 
Journal of Physics A 56(2023), 115202
Online First (2023) DOI: 10.1088/17518121/acbd26
Cited 1 times in Scopus
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Geometrizing the Klein–Gordon and Dirac equations in Doubly Special Relativity
FranchinoVinas, S.; Relancio, J. J.
In this work we discuss the deformed relativistic wave equations, namely the KleinGordon and Dirac equations in a Doubly Special Relativity scenario.
We employ what we call a geometric approach, based on the geometry of a curved momentum space, which should be seen as complementary to the more spread algebraic one.
In this frame we are able to rederive wellknown algebraic expressions, as well as to treat yet unresolved issues, to wit, the explicit relation between both equations, the discrete symmetries for Dirac particles, the fate of covariance, and the formal definition of a Hilbert space for the KleinGordon case.
Keywords: Doubly Special Relativity; KleinGordon equation; Dirac equation; Curved momentum space

Contribution to WWW
https://arxiv.org/abs/2203.12286 
Classical and Quantum Gravity 40(2023), 054001
Online First (2023) DOI: 10.1088/13616382/acb4d4
Cited 6 times in Scopus
Downloads
 Open Access Version from arxiv.org
 Secondary publication expected from 02.02.2024
Scalable quantum control and nonAbelian anyon creation in the Kitaev honeycomb model
Raii, O.; Mintert, F.; Burgarth, D.
The Kitaev honeycomb model is a system allowing for experimentally realizable quantum computation with
topological protection of quantum information. Practical implementation of quantum information processing
typically relies on adiabatic, i.e., slow dynamics. Here we show that the restriction to adiabatic dynamics can be
overcome with optimal control theory, enabled by an extension of the fermionization of the Kitaev honeycomb
model to the timedependent case. Moreover, we present a quantum control method that is applicable to large
lattice models due to subexponential scaling.

Physical Review A 106(2022), 062401062401
DOI: 10.1103/PhysRevA.106.062401
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 Open Access Version from arxiv.org
 Secondary publication expected
Local Neumann semitransparent layers: Resummation, pair production, and duality
Ahmadiniaz, N.; FranchinoVinas, S.; Manzo, L.; Mazzitelli, F. D.
We consider local semitransparent Neumann boundary conditions for a quantum scalar field as imposed by a quadratic coupling to a source localized on a flat codimensionone surface. Upon a proper regularization to give meaning to the interaction, we interpret the effective action as a theory in a firstquantized phase space. We compute the relevant heatkernel to all order in a homogeneous background and quadratic order in perturbations, giving a closed expression for the corresponding effective action in $D=4$. In the dynamical case, we analyze the pair production caused by a harmonic perturbation and a Sauter pulse. Notably, we prove the existence of a strong/weak duality that links this Neumann field theory to the analogue Dirichlet one.
Keywords: Neumann boundary conditions; Dynamical Casimir effect; Quantum field theory in background fields; Effective action; Pair creation; Duality; Dirichlet semitransparent boundary conditions

Physical Review D 106(2022), 105022
DOI: 10.1103/PhysRevD.106.105022
arXiv: 2208.07383
Cited 1 times in Scopus
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Suppression of Interband Heating for Random Driving
Zhao, H.; Knolle, J.; Moessner, R.; Mintert, F.
Heating to highlying states strongly limits the experimental observation of driving induced nonequilibrium phenomena, particularly when the drive has a broad spectrum. Here we show that, for entire
families of structured random drives known as random multipolar drives, particle excitation to higher bands
can be well controlled even away from a highfrequency driving regime. This opens a window for
observing driveinduced phenomena in a longlived prethermal regime in the lowest band.

Physical Review Letters 129(2022), 120605120605
DOI: 10.1103/PhysRevLett.129.120605
Cited 1 times in Scopus
Iterative quantum optimization with an adaptive problem Hamiltonian for the shortest vector problem
Zhu, Y. R.; Joseph, D.; Ling, C.; Mintert, F.
Quantum optimization algorithms hold the promise of solving classically hard, discrete optimization problems
in practice. The requirement of encoding such problems in a Hamiltonian realized with a finite (and currently
small) number of qubits, however, poses the risk of finding only the optimum within the restricted space
supported by this Hamiltonian. We describe an iterative algorithm in which a solution obtained with such
a restricted problem Hamiltonian is used to define a new problem Hamiltonian that is better suited than the
previous one. In numerical examples of the shortest vector problem, we show that the algorithm with a sequence
of improved problem Hamiltonians converges to the desired solution.

Physical Review A 106(2022), 2243522435
DOI: 10.1103/PhysRevA.106.022435
Cited 1 times in Scopus
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 Open Access Version from arxiv.org
 Secondary publication expected
Identifying Time Scales in Particle Production from Fields
Diez, M.; Alkofer, R.; Kohlfürst, C.
Particle production through ultrastrong electric fields is a wellstudied research field. Nevertheless, despite repeated attempts to relate the production rate within the field to the formation time of a particle, the latter is still shrouded in mystery. We provide an interpretation of a particle
distribution at finite times enabling us to isolate and, therefore, identify the relevant time scales
regarding particle formation in quantum physics within and beyond perturbation theory.
Keywords: Nonequilibrium Quantum Field Theory; StrongField Quantum Electrodynamics; Schwinger effect; Time scales

Contribution to WWW
arXiv:2211.07510 [hepph]: https://arxiv.org/pdf/2211.07510.pdf
DOI: 10.48550/arXiv.2211.07510 
Physics Letters B 844(2023), 138063
Online First (2023) DOI: 10.1016/j.physletb.2023.138063
Cited 2 times in Scopus
Emergence of Hadron Mass and Structure
Ding, M.; Roberts, C. D.; Schmidt, S. M.
Visible matter is characterised by a single mass scale; namely, the proton mass. The proton’s existence and structure are supposed to be described by quantum chromodynamics (QCD); yet, absent Higgs boson couplings, chromodynamics is scale invariant. Thus, if the Standard Model is truly a part of the theory of Nature, then the proton mass is an emergent feature of QCD; and emergent hadron mass (EHM) must provide the basic link between theory and observation. Nonperturbative tools are necessary if such connections are to be made; and in this context, we sketch recent progress in the application of continuum Schwinger function methods to an array of related problems in hadron and particle physics. Special emphasis is given to the three pillars of EHM – namely, the running gluon mass, processindependent effective charge, and running quark mass; their role in stabilising QCD; and their measurable expressions in a diverse array of observables.
Keywords: confinement of gluons and quarks; continuum Schwinger function methods; Dyson Schwinger equations

Particles 1(2023)6, 50120
DOI: 10.3390/particles6010004
Cited 20 times in Scopus
Performance boost of a collective qutrit refrigerator
A single qutrit with transitions selectively driven by weaklycoupled reservoirs can implement one of the world's smallest refrigerators. We analyze the performance of N such fridges that are collectively coupled to the reservoirs. We observe a quantum boost, manifest in a quadratic scaling of the steadystate cooling current with N. As N grows further, the scaling reduces to linear, since the transitions responsible for the quantum boost become energetically unfavorable. Finetuned interqutrit interactions may be used to maintain the quantum boost for all N and also for notperfectly collective scenarios.
Keywords: open quantum systems; Lindblad equation; Redfield equation; collective effects; quantum absorption refrigerator; qutrits; HolsteinPrimakoff transform; quadratic boost; nonequilibrium steady state

Contribution to WWW
arXiv:2210.07844 [quantph]: https://arxiv.org/abs/2210.07844
DOI: 10.48550/arXiv.2210.07844 
Physical Review Applied 19(2023), 034023
Online First (2023) DOI: 10.1103/PhysRevApplied.19.034023
Cited 4 times in Scopus
Downloads
 Open Access Version from arxiv.org
 Secondary publication expected from 08.03.2024
Resummation for quantum propagators in bounded spaces
Edwards, J. P.; GonzálezDomínguez, V. A.; Huet, I.; Trejo Espinosa, M. A.
We outline an approach to calculating the quantum mechanical propagator in the presence of geometrically nontrivial Dirichlet boundary conditions. The method is based on a generalization of an integral transform of the propagator studied in previous work (the socalled “hit function”) and a convergent sequence of Padé approximants that exposes the limit of perfectly reflecting boundaries. In this paper the generalized hit function is defined as a manypoint propagator, and we describe its relation to the sum over trajectories in the Feynman path integral. We then show how it can be used to calculate the Feynman propagator. We calculate analytically all such hit functions in D = 1 and D = 3 dimensions, giving recursion relations between them in the same or different dimensions and apply the results to the simple cases of propagation in the presence of perfectly conducting planar and spherical plates. We use these results to conjecture a general analytical formula for the propagator when Dirichlet boundary conditions are present in a given geometry, also explaining how it can be extended for application for more general, nonlocalized potentials. Our work has resonance with previous results obtained by Grosche in the study of path integrals in the presence of delta potentials. We indicate the eventual application in a relativistic context to determining Casimir energies using this technique.
Keywords: Integral equations; Quantum theory; Bounded spaces; Convergent sequences; Dirichlet boundary conditions; Feynman path integrals; Generalisation; Integral transform; Pade approximants; Quantum mechanical; Reflecting boundary; Resummation

Physical Review E 105(2022)6, 064132
Online First (2022) DOI: 10.1103/PhysRevE.105.064132
Cited 1 times in Scopus
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 Open Access Version from arxiv.org
 Secondary publication expected
Kaon and pion parton distributions
Cui, Z.F.; Ding, M.; Gao, F.; Raya, K.; Binosi, D.; Chang, L.; Roberts, C. D.; RodriguezQuintero, J.; Schmidt, S. M.
Beginning with results for the leadingtwist twoparticle distribution amplitudes of π and Kmesons, each of which exhibits dilation driven by the mechanism responsible for the emergence of hadronic mass, we develop parameterfree predictions for the pointwise behaviour of all π and K distribution functions (DFs), including glue and sea. The largex behaviour of each DF meets expectations based on quantum chromodynamics; the valencequark distributions match extractions from available data, including the pion case when threshold resummation effects are included; and at ζ5=5.2GeV, the scale of existing measurements, the lightfront momentum of these hadrons is shared as follows: ⟨xvalence⟩π=0.41(4), ⟨xglue⟩π=0.45(2), ⟨xsea⟩π=0.14(2); and ⟨xvalence⟩K=0.42(3), ⟨xglue⟩K=0.44(2), ⟨xsea⟩K=0.14(2). The kaon’s glue and sea distributions are similar to those in the pion, although the inclusion of massdependent splitting functions introduces some differences on the valencequark domain. This study should stimulate improved analyses of existing data and motivate new experiments sensitive to all π and K DFs. With little known empirically about the structure of the Standard Model’s (pseudo) NambuGoldstone modes and analyses of existing, limited data being controversial, it is likely that new generation experiments at upgraded and anticipated facilities will provide the information needed to resolve the puzzles and complete the picture of these complex bound states.
Keywords: parton; Kaon

European Physical Journal C 80(2020)1064
DOI: 10.1140/epjc/s10052020085784
Cited 54 times in Scopus
Reflections upon the Emergence of Hadronic Mass
Roberts, C. D.; Schmidt, S. M.
With discovery of the Higgs boson, science has located the source for ≲2% of the mass of visible matter. The focus of attention can now shift to the search for the origin of the remaining ≳98%. The instruments at work here must be capable of simultaneously generating the 1 GeV massscale associated with the nucleon and ensuring that this massscale is completely hidden in the chirallimit pion. This hunt for an understanding of the emergence of hadronic mass (EHM) has actually been underway for many years. What is changing are the impacts of QCDrelated theory, through the elucidation of clear signals for EHM in hadron observables, and the ability of modern and planned experimental facilities to access these observables. These developments are exemplified in a discussion of the evolving understanding of pion and kaon parton distributions.
Keywords: hadronic; Phenomenology; physics

European Physical Journal  Special Topics 229(2020), 33193340
DOI: 10.1140/epjst/e20200000646
Cited 39 times in Scopus 
Contribution to WWW
arXiv:2006.08782: https://arxiv.org/abs/2006.08782
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Nucleon elastic form factors at accessible large spacelike momenta
Schmidt, S. M.; Cui, Z.F.; Roberts, C. D.; Chen, C.; Binosi, D.; de Soto, F.; RodríguezQuintero, J.; Segovia, J.
A Poincarécovariant quark+diquark Faddeev equation is used to compute nucleon elastic form factors on 0≤Q2≤18m2N (mN is the nucleon mass) and elucidate their role as probes of emergent hadronic mass in the Standard Model. The calculations expose features of the form factors that can be tested in new generation experiments at existing facilities, e.g. a zero in GpE/GpM; a maximum in GnE/GnM; and a zero in the proton's dquark Dirac form factor, Fd1. Additionally, examination of the associated lightfronttransverse number and anomalous magnetisation densities reveals, inter alia: a marked excess of valence uquarks in the neighbourhood of the proton's centre of transverse momentum; and that the valence dquark is markedly more active magnetically than either of the valence uquarks. The calculations and analysis also reveal other aspects of nucleon structure that could be tested with a highluminosity accelerator capable of delivering higher beam energies than are currently available.
Keywords: nucleon; quark

Physical Review D 102(2020)1, 014043
DOI: 10.1103/PhysRevD.102.014043
Cited 28 times in Scopus 
Contribution to WWW
arXiv:2003.11655: https://arxiv.org/abs/2003.11655
A correspondence between the free and interacting field theories
Gao, F.; Ding, M.; Liu, Y.; Schmidt, S. M.
We discover a correspondence between the free field and the interacting states. This correspondence is firstly given from the fact that the free propagator can be converted into a tower of propagators for massive states, when expanded with the Hermite function basis. The equivalence of propagators reveals that in this particular case the duality can naturally be regarded as the equivalence of one theory on the plane wave basis to the other on the Hermite function basis. More generally, the Hermite function basis provides an alternative quantization process with the creation/annihilation operators that correspond directly to the interacting fields. Moreover, the Hermite function basis defines an exact way of dimensional reduction. As an illustration, we apply this basis on 3+1 dimensional YangMills theory with three dimensional space being reduced through the Hermite function basis, and if with only the lowest order Hermite function, the equivalent action becomes the BanksFischlerShenkerSusskind (BFSS) matrix model.

Contribution to WWW
https://arxiv.org/abs/2202.03191 
European Physical Journal C 83(2023), 144
DOI: 10.1140/epjc/s10052023112784
Quantum Zeno Manipulation of Quantum Dots
Ahmadiniaz, N.; Paul Geller, M.; König, J.; Kratzer, P.; Lorke, A.; Schaller, G.; Schützhold, R.
We investigate whether and how the quantum Zeno effect, i.e., the inhibition of quantum evolution by frequent measurements, can be employed to isolate a quantum dot from its surrounding electron reservoir. In contrast to the often studied case of tunneling between discrete levels, we consider the tunnelling of an electron from a continuum reservoir to a discrete level in the dot. Realizing the quantum Zeno effect in this scenario can be much harder because the measurements should be repeated before the wave packet of the hole left behind in the reservoir moves away from the vicinity of the dot. Thus, the required repetition rate could be lowered by having a flat band (with a slow group velocity) in resonance with the dot or a sufficiently small Fermi velocity or a strong external magnetic field.
Keywords: Quantum Zeno effect; quantum dots; wave packet decay; measurement problem; perturbation theory

Contribution to WWW
arXiv:2201.11718 [condmat.meshall]: https://arxiv.org/abs/2201.11718 
Physical Review Research 4(2022), L032045
Online First (2022) DOI: 10.1103/PhysRevResearch.4.L032045
ISSN: 26431564
Cited 2 times in Scopus
Decay of quantum sensitivity due to threebody loss in BoseEinstein condensates
In view of the coherent properties of a large number of atoms, BoseEinstein condensates (BECs) have a high potential for sensing applications. Several proposals have been put forward to use collective excitations such as phonons in BECs for quantumenhanced sensing in quantum metrology. However, the associated highly nonclassical states tend to be very vulnerable to decoherence. In this article, we investigate the effect of decoherence due to the omnipresent process of threebody loss in BECs.We find strong restrictions for a wide range of parameters, and we discuss possibilities to limit these restrictions.
Keywords: quantum; BoseEinstein

Physical Review A 103(2021)6, 063321
DOI: 10.1103/PhysRevA.103.063321
Cited 5 times in Scopus 
Contribution to WWW
arXiv:2101.05312: https://arxiv.org/abs/2101.05312
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Electron spin and photon polarizationresolved probabilities of strongfield QED processes
Chen, Y.Y.; Hatsagortsyan, K. Z.; Keitel, C. H.; Shaisultanov, R.
A derivation of fully polarizationresolved probabilities is provided for highenergy photon emission and electronpositron pair production in ultrastrong laser fields. The probabilities resolved in both electron spin and photon polarization of incoming and outgoing particles are indispensable for developing QED Monte Carlo and QEDParticleinCell codes, aimed at the investigation of polarization effects in nonlinear QED processes in ultraintense laserplasma and laserelectron beam interactions, and other nonlinear QED processes in external ultrastrong fields, which involve multiple elementary processes of a photon emission and pair production. The quantum operator method introduced by Baier and Katkov is employed for the calculation of probabilities within the quasiclassical approach and the local constant field approximation. The probabilities for the ultrarelativistic regime are given in a compact form and are suitable to describe polarization effects in strong laser fields of arbitrary configuration, rendering them very well suited for applications.

Contribution to WWW
https://arxiv.org/abs/2201.10863
DOI: 10.1103/PhysRevD.105.116013
Cited 12 times in Scopus 
Physical Review D 105(2022)11, e116013
DOI: 10.1103/PhysRevD.105.116013
Cited 12 times in Scopus
Colorkinematics duality from the BernKosower formalism
Ahmadiniaz, N.; Balli, F. M.; LopezArcos, C.; Quintero Velez, A.; Schubert, C.
BerendsGiele currents are fundamental building blocks for onshell amplitudes in nonabelian gauge theory. We present a novel procedure to construct them using the BernKosower formalism foroneloop gluon amplitudes. Applying the pinch procedure of that formalism to a suitable special casethe currents are naturally obtained in terms of multiparticle fields and obeying colorkinematics duality. As a feedback to the BernKosower formalism, we outline how the
multiparticle polarisations and fieldstrength tensors can be used to significantly streamline the pinch procedure
Keywords: BerendsGiele currents; onshell amplitudes; BernKosower formalism

Physical Review D 104(2021), L041702
DOI: 10.1103/PhysRevD.104.L041702
arXiv: https://arxiv.org/pdf/2105.06745.pdf
Cited 10 times in Scopus
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Manifest colourkinematics duality and doublecopy in the stringbased formalism
Ahmadiniaz, N.; Balli, F. M.; Corradini, O.; LopezArcos, C.; Quintero Velez, A.; Schubert, C.
The relation for the gravity polarisation tensor as the tensor product of two gluon polarisation vectors has been wellknown for a long time, but a version of this relation for multiparticle fields is presently still not known. Here we show that in order for this to happen we first have to ensure that the multiparticle polarisations satisfy colorkinematics duality. In previous work, it has been show that this arises naturally from the BernKosower formalism for oneloop gluon amplitudes, and here we show that the tensor product for multiparticle fields arise naturally in the BernDunbarShimada formalism for oneloop gravity amplitudes. This allows us to formulate a new prescription for doublecopy gravity BerendsGiele currents,and to obtain both the colourdressed YangMills BerendsGiele currents in the BernCarrascoJohanssongauge and the gravitational BerendsGiele currents explicitly. An attractive feature of our formalism is that it never becomes necessary to determine gauge transformation terms. Our doublecopy prescription can also be applied to other cases, and to make this point we derive the doublecopy perturbers for α′deformed gravity and the biadjoint scalar model
Keywords: BernDunbarShimada formalism; BerendsGiele currents; colorkinematics duality; gravity amplitudes

Contribution to WWW
arXiv:2110.04853 [hepth]: https://arxiv.org/abs/2110.04853 
Nuclear Physics B 975(2022), 115690
DOI: 10.1016/j.nuclphysb.2022.115690
Cited 6 times in Scopus
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Worldline master formulas for the dressed electron propagato, part 2: onshell amplitudes
Ahmadiniaz, N.; Banda Guzman, V. M.; Bastianelli, F.; Corradini, O.; Edwards, J. P.; Schubert, C.
In the first part of this series, we employed the secondorder formalism and the“symbol” map to construct a particle pathintegral representation of the electron propagator in a background electromagnetic field, suitable for open fermionline calculations. Its main advantages are the avoidance of long products of Dirac matrices, and its ability to unify whole sets of Feynman diagrams related by permutation of photon legs along the fermion lines. We obtained a BernKosower type master formula for the fermion propagator, dressedwithNphotons, in terms of the “Nphoton kernel,” where this kernel appears also in“subleading” terms involving only N−1 of the Nphotons.In this sequel, we focus on the application of the formalism to the calculation of onshell amplitudes and crosssections. Universal formulas are obtained for the fully polarised matrix elements of the fermion propagator dressed with an arbitrary number of photons, as well as for the corresponding spinaveraged crosssections. A major simplification of the onshell case is that the subleading terms drop out, but we also pinpoint other, less obvious simplifications. We use integration by parts to achieve manifest transversality of these amplitudes at the integrand level and exploit this property using the spinor helicity technique. We give a simple proof of the vanishing of the matrix element for all “+” photon helicities in the massless case and find a novel relation between the scalar and spinor spinaveraged crosssections in the massive case. Testing the formalism on the standard linear Comptonscattering process, we find that it reproduces the known results with remarkable efficiency. Further applications and generalisations are pointed out.
Keywords: Onshell amplitudes; Dressed fermion propagator; Worldline formalism

Journal of High Energy Physics 01(2022), 50
DOI: 10.1007/JHEP01(2022)050
Cited 7 times in Scopus
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SauterSchwinger effect for colliding laser pulses
Kohlfürst, C.; Ahmadiniaz, N.; Oertel, J.; Schützhold, R.
We study electronpositron pair creation by the electromagnetic field of two colliding laser pulses as described by the vector potential
A(t,r) = [f(ct−x) +f(ct+x)]ey. Employing the worldline instanton technique as well as a generalized WKB approach, we find that the pair creation rate along the symmetry axisx= 0(where one would expect the maximum contribution) displays the same exponential dependence as for a purely timedependent electric field A(t) = 2f(ct)ey. The prefactor in front of this exponential does also contain corrections due to focusing or defocusing effects induced by the spatially inhomogeneous magnetic field. We compare our analytical results to numerical simulations using the DiracHeisenbergWigner method and find good agreement.
Keywords: SauterSchwinger effect; WKB; Worldline instanton; Numerical analysis

Contribution to WWW
arXiv:2107.08741 [hepph]: https://arxiv.org/abs/2107.08741 
Physical Review Letters 129(2022)24, 241801
DOI: 10.1103/PhysRevLett.129.241801
Cited 9 times in Scopus
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Generation of arbitrarily polarized GeV lepton beams via nonlinear BreitWheeler process
Xue, K.; Guo, R.T.; Wan, F.; Shaisultanov, R.; Chen, Y.Y.; Xu, Z.F.; Ren, X.G.; Hatsagortsyan, K. Z.; Keitel, C. H.; Li, J.X.
Generation of arbitrarily spinpolarized lepton (here refer in particular to electron and positron) beams has been investigated in the singleshot interaction of highenergy polarized γ photons with an ultraintense asymmetric laser pulse via nonlinear BreitWheeler (BW) pair production. We develop a fully spinresolved semiclassical Monte Carlo method to describe the pair creation and polarization in the local constant field approximation. In nonlinear BW process the polarization of created pairs is simultaneously determined by the polarization of parent γ photons, the polarization and asymmetry of scattering laser field, due to the spin angular momentum transfer and the asymmetric spindependent pair production probabilities, respectively. In considered alloptical method, dense GeV lepton beams with average polarization degree up to about 80% (adjustable between the transverse and longitudinal components) can be obtained with currently achievable laser facilities, which could be used as injectors of the polarized e+e− collider to search for new physics beyond the Standard Model.

Contribution to WWW
arXiv:2104.14864 [physics.plasmph]: https://arxiv.org/abs/2104.14864 
Fundamental Research 2(2022)4, 539545
Online First (2021) DOI: 10.1016/j.fmre.2021.11.022
Cited 9 times in Scopus
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Photon polarization effects in polarized electronpositron pair production in a strong laser field
Dai, Y.N.; Shen, B.F.; Li, J.X.; Shaisultanov, R.; Hatsagortsyan, K. Z.; Keitel, C. H.; Chen, Y.Y.
Deep understanding of the impact of photon polarization on pair production is essential for the efficient generation of laserdriven polarized positron beams and demands a complete description of polarization effects in strongfield QED processes. Employing fully polarizationresolved Monte Carlo simulations, we investigate correlated photon and electron (positron) polarization effects in the multiphoton Breit–Wheeler pair production process during the interaction of an ultrarelativistic electron beam with a counterpropagating elliptically polarized laser pulse. We show that the polarization of e−e+ pairs is degraded by 35% when the polarization of the intermediate photon is resolved, accompanied by an ∼13% decrease in the pair yield. Moreover, in this case, the polarization direction of energetic positrons at small deflection angles can even be reversed when highenergy photons with polarization parallel to the laser electric field are involved.

Matter and Radiation at Extremes 7(2022), 014401
Online First (2021) DOI: 10.1063/5.0063633
Cited 10 times in Scopus
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Emergence of pion parton distributions
Cui, Z.F.; Ding, M.; Morgado, J. M.; Raya, K.; Binosi, D.; Chang, L.; de Soto, F.; Roberts, C. D.; RodriguezQuintero, J.; Schmidt, S. M.
Supposing only that there is an effective charge which defines an evolution scheme for parton distribution functions (DFs) that is allorders exact, strict lower and upper bounds on all Mellin moments of the valencequark DFs of pionlike systems are derived. Exploiting contemporary results from numerical simulations of latticeregularised quantum chromodynamics (QCD) that are consistent with these bounds, parameterfree predictions for pion valence, glue, and sea DFs are obtained. The form of the valencequark DF at large values of the lightfront momentum fraction is consistent with predictions derived using the QCDprescribed behaviour of the pion wave function.

Contribution to WWW
https://arxiv.org/abs/2201.00884 
Physical Review D 105(2022), L091502
DOI: 10.1103/PhysRevD.105.L091502
Cited 17 times in Scopus
Concerning pion parton distributions
Cui, Z.F.; Ding, M.; Morgado, J. M.; Raya, K.; Binosi, D.; Chang, L.; Papavassiliou, J.; Roberts, C. D.; RodriguezQuintero, J.; Schmidt, S. M.
Analyses of the pion valencequark distribution function (DF), ${\mathpzc u}^\pi(x;\zeta)$, which explicitly incorporate the behaviour of the pion wave function prescribed by quantum chromodynamics (QCD), predict ${\mathpzc u}^\pi(x\simeq 1;\zeta) \sim (1x)^{\beta(\zeta)}$, $\beta(\zeta \gtrsim m_p)>2$, where $m_p$ is the proton mass. Nevertheless, more than forty years after the first experiment to collect data suitable for extracting the $x\simeq 1$ behaviour of ${\mathpzc u}^\pi$, the empirical status remains uncertain because some methods used to fit existing data return a result for ${\mathpzc u}^\pi$ that violates this constraint. Such disagreement entails one of the following conclusions: the analysis concerned is incomplete; not all data being considered are a true expression of qualities intrinsic to the pion; or QCD, as it is currently understood, is not the theory of strong interactions. New, precise data are necessary before a final conclusion is possible. In developing these positions, we exploit a single proposition, \emph{viz}.\ there is an effective charge which defines an evolution scheme for parton DFs that is allorders exact. This proposition has numerous corollaries, which can be used to test the character of any DF, whether fitted or calculated.

Contribution to WWW
https://arxiv.org/abs/2112.09210 
European Physical Journal A 58(2022), 10
DOI: 10.1140/epja/s10050021006587
Cited 21 times in Scopus
Optical absorption and carrier multiplication at graphene edges in a magnetic field
Queisser, F.; Lang, S.; Schützhold, R.
We study optical absorption at graphene edges in a transversal magnetic field. The magnetic field bends the trajectories of particle and hole excitations into antipodal direction which generates a directed current. We find a rather strong amplification of the edge current by impact ionization processes. More concretely, the primary absorption and the subsequent carrier multiplication is analyzed for a graphene fold and a zigzag edge. We identify exact and approximate selection rules and discuss the dependence of the decay rates on the initial state.
Keywords: Graphene; Auger processes; optical absorption

Contribution to WWW
arXiv:2107.14524 [condmat.meshall]: https://arxiv.org/abs/2107.14524 
Physical Review B 108(2023)045403
DOI: 10.1103/PhysRevB.108.045403
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Dressed Dirac Propagator from a Locally Supersymmetric N=1 Spinning Particle
Degli Esposti, G.; Corradini, O.
We study the Dirac propagator dressed by an arbitrary number N of photons by means of a worldline approach, which makes use of a supersymmetric N=1 spinning particle model on the line, coupled to an external Abelian vector field. We obtain a compact offshell master formula for the tree level scattering amplitudes associated to the dressed Dirac propagator. In particular, unlike in other approaches, we express the particle fermionic degrees of freedom using a coherent state basis, and consider the gauging of the supersymmetry, which ultimately amounts to integrating over a worldline gravitino modulus, other than the usual worldline einbein modulus which corresponds to the Schwinger time integral. The path integral over the gravitino reproduces the numerator of the dressed Dirac propagator.

Nuclear Physics B 970(2021), 115498
DOI: 10.1016/j.nuclphysb.2021.115498
Cited 4 times in Scopus
Environmentinduced decay dynamics of antiferromagnetic order in the MottHubbard system
Schaller, G.; Queisser, F.; Szpak, N.; König, J.; Schützhold, R.
We study the dissipative FermiHubbard model in the limit of weak tunneling and strong repulsive interactions, where each lattice site is tunnelcoupled to a Markovian fermionic bath. For cold baths at intermediate chemical potentials, the Mott insulator property remains stable and we find a fast relaxation of the particle number towards half filling. On longer time scales, we find that the antiferromagnetic order of the MottNéel ground state on bipartite lattices decays, even at zero temperature. For zero and nonzero temperatures, we quantify the different relaxation time scales by means of waiting time distributions which can be derived from an effective (nonHermitian) Hamiltonian and obtain fully analytic expressions for the FermiHubbard model on a tetramer ring.
Keywords: FermiHubbard model; local master equation; magnetic order; waitingtime distributions

Contribution to WWW
arXiv:2112.04956 [condmat.strel]: https://arxiv.org/abs/2112.04956 
Physical Review B 105(2022), 115139
Online First (2022) DOI: 10.1103/PhysRevB.105.115139
Cited 3 times in Scopus
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Dynamically assisted tunneling in the impulse regime
Kohlfürst, C.; Queißer, F.; Schützhold, R.
We study the enhancement of tunneling through a potential barrier V(x) by a timedependent electric field with special emphasis on pulseshaped vector potentials such as A(t)=A0/cosh^2(ωt). In addition to the known effects of preacceleration and potential deformation already present in the adiabatic regime, as well as energy mixing in analogy to the FranzKeldysh effect in the nonadiabatic (impulse) regime, the pulse A(t) can enhance tunneling by ``pushing'' part of the wavefunction out of the rear end of the barrier. Besides the natural applications in condensed matter and atomic physics, these findings could be relevant for nuclear fusion, where pulses A(t) with ω=1 keV and peak field strengths of 10^16 V/m might enhance tunneling rates significantly.
Keywords: Tunneling & traversal time; Nuclear fusion; Schroedinger equation

Physical Review Research 3(2021), 033153
DOI: 10.1103/PhysRevResearch.3.033153
Cited 12 times in Scopus
Trident process in laser pulses
Dinu, V.; Torgrimsson, G.
We study the trident process in laser pulses. We provide exact numerical results for all contributions, including the difficult exchange term. We show that all terms are in general important for a short pulse. For a long pulse, we identify a term that gives the dominant contribution even if the intensity is only moderately high, a0≳1, which is an experimentally important regime where the standard locally constant field (LCF) approximation cannot be used. We show that the spectrum has a richer structure at a0∼1, compared to the LCF regime a0≫1. We study the convergence to LCF as a0 increases and how this convergence depends on the momentum of the initial electron. We also identify the terms that dominate at high energy.

Physical Review D 101(2020), 056017
DOI: 10.1103/PhysRevD.101.056017
Cited 20 times in Scopus
Approximating higherorder nonlinear QED processes with firstorder building blocks
Dinu, V.; Torgrimsson, G.
Higherorder treelevel processes in strong laser fields, i.e., cascades, are in general extremely difficult to calculate, but in some regimes the dominant contribution comes from a sequence of firstorder processes, i.e., nonlinear Compton scattering and nonlinear BreitWheeler pair production. At high intensity the field can be treated as locally constant, which is the basis for standard particleincell codes. However, the locallyconstantfield (LCF) approximation and these particleincell codes cannot be used when the intensity is only moderately high, which is a regime that is experimentally relevant. We have shown that one can still use a sequence of firstorder processes to estimate higher orders at moderate intensities provided the field is sufficiently long. An important aspect of our new “gluing” approach is the role of the spin and polarization of intermediate particles, which is more nontrivial compared to the LCF regime.

Physical Review D 102(2020), 016018
DOI: 10.1103/PhysRevD.102.016018
Cited 25 times in Scopus
Nonlinear trident in the highenergy limit: Nonlocality, Coulomb field and resummations
Torgrimsson, G.
We study nonlinear trident in laser pulses in the highenergy limit, where the initial electron experiences, in its rest frame, an electromagnetic field strength above Schwinger’s critical field. At lower energies the dominant contribution comes from the “twostep” part, but in the highenergy limit the dominant contribution comes instead from the onestep term. We obtain new approximations that explain the relation between the highenergy limit of trident and pair production by a Coulomb field, as well as the role of the WeizsäckerWilliams approximation and why it does not agree with the highχ limit of the locallyconstantfield approximation. We also show that the nexttoleading order in the largea0 expansion is, in the highenergy limit, nonlocal and is numerically very important even for quite large a0. We show that the smalla0 perturbation series has a finite radius of convergence, but using Padéconformal methods we obtain resummations that go beyond the radius of convergence and have a large numerical overlap with the largea0 approximation. We use BorelPadéconformal methods to resum the smallχ expansion and obtain a high precision up to very large χ. We also use newer resummation methods based on hypergeometric/MeijerG and confluent hypergeometric functions.

Physical Review D 102(2020), 096008
DOI: 10.1103/PhysRevD.102.096008
Cited 19 times in Scopus
Nonlinear photon trident versus double Compton scattering and resummation of onestep terms
Torgrimsson, G.
We study the photon trident process, where an initial photon turns into an electronpositron pair and a final photon under a nonlinear interaction with a strong planewave background field. We show that this process is very similar to double Compton scattering, where an electron interacts with the background field and emits two photons. We also show how the onestep terms can be obtained by resumming the small and large\chiχ expansions. We consider a couple of different resummation methods, and also propose new resummations (involving MeijerG functions) which have the correct type of expansions at both small and large \chiχ. These new resummations require relatively few terms to give good precision.

Physical Review D 102(2020), 116008
DOI: 10.1103/PhysRevD.102.116008
Cited 9 times in Scopus
Loops and polarization in strongfield QED
Torgrimsson, G.
In a previous paper we showed how higherorder strongfieldQED processes in long laser pulses can be approximated by multiplying sequences of ‘strongfield Mueller matrices’. We obtained expressions that are valid for arbitrary field shape and polarization. In this paper we derive practical approximations of these Mueller matrices in the locallyconstant and the locallymonochromaticfield regimes. The spin and polarization can also change due to loop contributions (the mass operator for electrons and the polarization operator for photons). We derive Mueller matrices for these as well, for arbitrary laser polarization and arbitrarily polarized initial and final particles.

New Journal of Physics 23(2021), 065001
DOI: 10.1088/13672630/abf274
Cited 28 times in Scopus
Resummation of Quantum Radiation Reaction in Plane Waves
Torgrimsson, G.
We propose a new approach to obtain the momentum expectation value of an electron in a highintensity laser, including multiple photon emissions and loops. We find a recursive formula that allows us to obtain the O(αn) term from O(αn1), which can also be expressed as an integrodifferential equation. In the classical limit we obtain the solution to the LandauLifshitz equation to all orders. We show how spindependent quantum radiation reaction can be obtained by resumming both the energy expansion as well as the α expansion.

Physical Review Letters 127(2021), 111602
DOI: 10.1103/PhysRevLett.127.111602
Cited 19 times in Scopus
Resummation of quantum radiation reaction and induced polarization
Torgrimsson, G.
In a previous paper we proposed a new method based on resummations for studying radiation reaction of an electron in a planewave electromagnetic field. In this paper we use this method to study the electron momentum expectation value for a circularly polarized monochromatic field with a0=1, for which standard locally constantfield methods cannot be used. We also find that radiation reaction has a significant effect on the induced polarization, as compared to the results without radiation reaction, i.e., the SokolovTernov formula for a constant field, or the zero result for a circularly monochromatic field. We also study the AbrahamLorentzDirac equation using BorelPadé resummations.

Physical Review D 104(2021), 056016
DOI: 10.1103/PhysRevD.104.056016
Cited 9 times in Scopus
Ginzburg effect in a dielectric medium with dispersion and dissipation
Lang, S.; Sauerbrey, R.; Schützhold, R.; Unruh, W.
As a quantum analog of Cherenkov radiation, an inertial photon detector moving through a medium with constant refractive index n may perceive the electromagnetic quantum fluctuations as real photons if its velocity v exceeds the medium speed of light c/n. For dispersive Hopfield type media, we find this Ginzburg effect to extend to much lower v because the phase velocity of light is very small near the medium resonance. In this regime, however, dissipation effects become important. Via an extended Hopfield model, we present a consistent treatment of quantum fluctuations in dispersive and dissipative media and derive the Ginzburg effect in such systems. Finally, we propose an experimental test.
Keywords: dielectric media; dispersion and dissipation; quantum vacuum phenomena; Ginzburg effect; inertial atom; spontaneous excitation

Contribution to WWW
arXiv:2108.10693 [quantph]: https://arxiv.org/abs/2108.10693 
Physical Review Research 4(2022)3, 033074
DOI: 10.1103/PhysRevResearch.4.033074
ISSN: 26431564
Cited 1 times in Scopus
Generalized LandauKhalatnikovFradkin transformations for arbitrary Npoint fermion correlators
Ahmadiniaz, N.; Edwards, J. P.; Nicasio, J.; Schubert, C.
We examine the nonperturbative gauge dependence of arbitrary configuration space fermion correlators in quantum electrodynamics (QED). First, we study the dressed electron propagator (allowing for emission or absorption of any number of photons along a fermion line) using the first quantized approach to quantum field theory and analyze its gauge transformation properties induced by virtual photon exchange. This is then extended to the Npoint functions where we derive an exact, generalized version of the fully nonperturbative LandauKhalatnikovFradkin (LKF) transformation for these correlators. We discuss some general aspects of the application in perturbation theory and investigate the structure of the LKF factor aboutD¼2dimensions
Keywords: LKFT; Worldline formalism; Nonperturbative QED

Physical Review D 104(2021), 025014
Online First (2021) DOI: 10.1103/PhysRevD.104.025014
Cited 4 times in Scopus
Observability of Coulombassisted quantum vacuum birefringence
Ahmadiniaz, N.; Bussmann, M.; Cowan, T.; Debus, A.; Kluge, T.; Schützhold, R.
We consider the scattering of an xray freeelectron laser (XFEL) beam on the superposition of
a strong magnetic field $\bf{B}_{\rm ext}$ with the Coulomb field $\bf{E}_{\rm ext}$
of a nucleus with charge number $Z$. In contrast to Delbr\"uck scattering
(Coulomb field only), the magnetic field $\bf{B}_{\rm ext}$
introduces an asymmetry (i.e., polarization dependence) and renders the effective interaction volume quite
large, while the nuclear Coulomb field facilitates a significant momentum transfer $\Delta\bf k$.
For a field strength of $B_{\rm ext}=10^6 T$ (corresponding to an intensity of order $10^{22}~\rm W/cm^2$)
and an XFEL frequency of 24~keV, we find a differential cross section
$d\sigma/d\Omega\sim10^{25}~Z^2/(\Delta{\bf k})^2$ in forward direction for one nucleus.
Thus, this effect might be observable in the near future at facilities such as the
Helmholtz International Beamline for Extreme Fields (HIBEF) at the European XFEL.

Physical Review D 104(2021), L011902
Online First (2021) DOI: 10.1103/PhysRevD.104.L011902
Cited 8 times in Scopus
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Superradiant manyqubit absorption refrigerator
Kloc, M.; Meier, K.; Hadjikyriakos, K.; Schaller, G.
We show that the lower levels of a largespin network with a collective antiferromagnetic interaction and collective couplings to three reservoirs may function as a quantum absorption refrigerator. In appropriate regimes, the steadystate cooling current of this refrigerator scales quadratically with the size of the working medium, i.e., the number of spins. The same scaling is observed for the noise and the entropy production rate.
Keywords: open quantum systems; collective interactions; quantum heat engine; quantum absorbtion refrigerator; Dicke superradiance

Contribution to WWW
https://arxiv.org/abs/2106.04164 
Physical Review Applied 16(2021), 044061104406115
Online First (2021) DOI: 10.1103/PhysRevApplied.16.044061
Cited 11 times in Scopus
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Nonequilibrium boundary driven quantum systems: models, methods and properties
Gabriel, T. L.; Dario, P.; Gernot, S.
Recent years have seen tremendous progress in the theoretical understanding of quantum systems driven dissipatively by coupling them to different baths at their edges. This was possible because of the concurrent advances in the models used to represent these systems, the methods employed, and the analysis of the emerging phenomenology. Here we aim to give a comprehensive review of these three integrated research directions. We first provide an overarching view of the models of boundary driven open quantum systems, both in the weak and strong coupling regimes. This is followed by a review of stateoftheart analytical and numerical methods, both exact, perturbative and approximate. Finally, we discuss the transport properties of some paradigmatic onedimensional chains, with an emphasis on disordered and quasiperiodic systems, the emergence of rectification and negative differential conductance, and the role of phase transitions.
Keywords: open quantum systems; quantum transport; phase transitions; local and global master equations; chain models

Contribution to WWW
https://arxiv.org/abs/2104.14350 
Reviews of Modern Physics 94(2022)4, 045006
DOI: 10.1103/RevModPhys.94.045006
Cited 29 times in Scopus
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Broadband frequency filters with quantum dot chains
Ehrlich, T.; Schaller, G.
Twoterminal electronic transport systems with a rectangular transmission can violate standard thermodynamic uncertainty relations. This is possible beyond the linear response regime and for parameters that are not accessible with rate equations obeying detailedbalance. Looser bounds originating from fluctuation theorem symmetries alone remain respected. We demonstrate that optimal finitesized quantum dot chains can implement rectangular transmission functions with high accuracy and discuss the resulting violations of standard thermodynamic uncertainty relations as well as heat engine performance.
Keywords: fluctuation theorems; thermodynamic uncertainty relation; LevitovLesovik formula; transmission; reactioncoordinate mapping
Related publications

Data publication: Broadband frequency filters with quantum dot chains
ROBIS: 33004 HZDRprimary research data are used by this (Id 32439) publication

Contribution to WWW
https://arxiv.org/abs/2103.04322 
Physical Review B 104(2021), 045424
Online First (2021) DOI: 10.1103/PhysRevB.104.045424
ISSN: 24699950
Cited 8 times in Scopus
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Coarsegraining master equation for periodically driven systems
We analyze LindbladGoriniKossakowskiSudarshantype generators for selected periodically driven open quantum systems. All these generators can be obtained by temporal coarsegraining procedures, and we compare different coarsegraining schemes. Similar to for undriven systems, we find that a dynamically adapted coarsegraining time, effectively yielding nonMarkovian dynamics by interpolating through a series of different but invididually Markovian solutions, yields the best results among the different coarsegraining schemes, albeit at highest computational cost.
Keywords: open quantum systems; Floquet theory; periodic driving; Lindblad master equation

Contribution to WWW
arXiv:2102.03063 [quantph]: https://arxiv.org/abs/2102.03063 
Physical Review A 104(2021), 052219
Online First (2021) DOI: 10.1103/PhysRevA.104.052219
Cited 2 times in Scopus
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Autonomous implementation of thermodynamic cycles at the nanoscale
Strasberg, P.; Wächtler, C. W.; Schaller, G.
There are two paradigms to study nanoscale engines in stochastic and quantum thermodynamics.
Autonomous models, which do not rely on any external timedependence, and models that make use of timedependent control fields, often combined with dividing the control protocol into idealized strokes of a thermodynamic cycle. While the latter paradigm offers theoretical simplifications, its utility in practice has been questioned due to the involved approximations. Here, we bridge the two paradigms by constructing an autonomous model, which implements a thermodynamic cycle in a certain parameter regime. This effect is made possible by selfoscillations, realized in our model by the well studied electron shuttling mechanism. Based on experimentally realistic values, we find that a thermodynamic cycle analysis for a singleelectron working fluid is unrealistic, but already a fewelectron working fluid could suffice to justify it. We also briefly discuss additional open challenges to autonomously implement the more studied Carnot and Otto cycles.
Keywords: thermodynamic cycle; selfoscillation; autonomous control; electron shuttle

Contribution to WWW
https://arxiv.org/abs/2101.05027 
Physical Review Letters 126(2021), 180605
Online First (2021) DOI: 10.1103/PhysRevLett.126.180605
ISSN: 00319007
Cited 15 times in Scopus
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Worldline master formulas for the dressed electron propagator, part 1: Offshell amplitudes
Ahmadiniaz, N.; Guzman, V. M. B.; Bastianelli, F.; Corradini, O.; Edwards, J. P.; Schubert, C.
In the firrstquantised worldline approach to quantum field theory, a longstanding problem has been to extend this formalism to amplitudes involving open fermion lines while maintaining the efficiency of the welltested closedloop case. In the present series of papers, we develop a suitable formalism for the case of quantum electrodynamics (QED) in vacuum (part one and two) and in a constant external electromagnetic field (part three), based on secondorder fermions and the symbol map. We derive this formalism from standard field theory, but also give an alternative derivation intrinsic to the worldline theory. In this first part, we use it to obtain a BernKosower type master formula for the fermion propagator, dressed with N photons in configuration as well as in momentum space.
Keywords: Worlline formalism; QED; Scattering Amplitudes; Gauge Symmetry

Journal of High Energy Physics 08(2020)018
Online First (2020) DOI: 10.1007/JHEP08(2020)018
ISSN: 10298479
Cited 18 times in Scopus
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Trappedion toolkit for studies of quantum harmonic oscillators under extreme conditions
Wittemer, M.; Schröder, J.P.; Hakelberg, F.; Kiefer, P.; Fey, C.; Schützhold, R.; Warring, U.; Schaetz, T.
Many phenomena described in relativistic quantum field theory are inaccessible to direct observations, but analogue processes studied under welldefined laboratory conditions can present an alternative perspective. Recently, we demonstrated an analogy of particle creation using an intrinsically robust motional mode of two trapped atomic ions. Here, we substantially extend our classical control techniques by implementing machinelearning strategies in our platform and, consequently, increase the accessible parameter regime. As a proof of methodology, we present experimental results of multiple quenches and parametric modulation of an unprotected motional mode of a single ion, demonstrating the increased level of realtime control. In combination with previous results, we enable future experiments that may yield entanglement generation using a process in analogy to Hawking radiation. This article is part of a discussion meeting issue 'The next generation of analogue gravity experiments'.
Keywords: Trapped Ions; Qubits; Ion Traps (Instrumentation)

Philosophical Transactions of the Royal Society A 378(2020)2177, 20190230
DOI: 10.1098/rsta.2019.0230
ISSN: 14712962
Cited 3 times in Scopus
Heisenberg limit for detecting vacuum birefringence
Ahmadiniaz, N.; Cowan, T.; Sauerbrey, R.; Schramm, U.; Schlenvoigt, H.P.; Schützhold, R.
Quantum electrodynamics predicts the vacuum to behave as a nonlinear medium, including effects such as birefringence. However, for experimentally available field strengths, this vacuum polarizability is extremely small and thus very hard to measure. In analogy to the Heisenberg limit in quantum metrology, we study the minimum requirements for such a detection in a given strong field (the pump field). Using a laser pulse as the probe field, we find that its energy must exceed a certain threshold depending on the interaction time. However, a detection at that threshold, i.e., the Heisenberg limit, requires highly nonlinear measurement schemeswhile for ordinary linearoptics schemes, the required energy (Poisson or shot noise limit) is much larger. Finally, we discuss several currently considered experimental scenarios from this point of view.
Keywords: Quantum Electrodynamics; Vacuum birefringence; Heisenberg limit

Physical Review D 101(2020), 116019
Online First (2020) DOI: 10.1103/PhysRevD.101.116019
ISSN: 15366065
Cited 11 times in Scopus
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Quantum radiation in dielectric media with dispersion and dissipation
Lang, S.; Schützhold, R.; Unruh, W.
By a generalization of the Hopfield model, we construct a microscopic Lagrangian describing a dielectric medium with dispersion and dissipation. This facilitates a welldefined and unambiguous ab initio treatment of quantum electrodynamics in such media, even in timedependent backgrounds. As an example, we calculate the number of photons created by switching on and off dissipation in dependence on the temporal switching function. This effect may be stronger than quantum radiation produced by variations of the refractive index Δn(t) since the latter are typically very small and yield photon numbers of order (Δn)². As another difference, we find that the partner particles of the created medium photons are not other medium photons but excitations of the environment field causing the dissipation (which is switched on and off).

Contribution to WWW
arXiv:1912.09869: https://arxiv.org/abs/1912.09869 
Physical Review D 102(2020)12, 125020
DOI: 10.1103/PhysRevD.102.125020
Cited 3 times in Scopus
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Offshell Ward identities for Ngluon amplitudes
Offshell Ward identities in nonabelian gauge theory continue to be a subject of active research, since they are, in general, inhomogeneous and their form depends on the chosen gaugefixing procedure. For the threegluon and fourgluon vertices, it is known that a relatively simple form of the Ward identity can be achieved using the pinch technique or, equivalently, the backgroundfield method with quantum Feynman gauge. The latter is also the gaugefixing underlying the stringinspired formalism, and here we use this formalism to derive the corresponding form of the Ward identity for the oneloop N  gluon amplitudes.

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arXiv:2001.00885: https://arxiv.org/abs/2001.00885 
EPL  Europhysics Letters 130(2020), 41001
Online First (2020) DOI: 10.1209/02955075/130/41001
ISSN: 12864854
Cited 4 times in Scopus
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Pair production in temporally and spatially oscillating fields
Aleksandrov, I. A.; Kohlfürst, C.
Electronpositron pair production for inhomogeneous electric and magnetic fields oscillating in space and time is investigated. By employing accurate numerical methods (Furrypicture quantization and quantum kinetic theory), final particle momentum spectra are calculated and analyzed in terms of effective models. Furthermore, criteria for the applicability of approximate methods are derived and discussed. In this context, special focus is placed on the local density approximation, where fields are assumed to be locally homogeneous in space. Eventually, we apply our findings to the multiphoton regime. Special emphasis is on the importance of linear momentum conservation and the effect of its absence in momentum spectra within approximations based on local homogeneity of the fields.

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arXiv:1912.09924: https://arxiv.org/abs/1912.09924 
Physical Review D 101(2020)9, 096009
DOI: 10.1103/PhysRevD.101.096009
Cited 21 times in Scopus
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On the effect of timedependent inhomogeneous magnetic fields on the particle momentum spectrum in electronpositron pair production
Electronpositron pair production in spatially and temporally inhomogeneous electric and magnetic fields is studied within the DiracHeisenbergWigner formalism (quantum kinetic theory) through computing the corresponding Wigner functions. The focus is on discussing the particle momentum spectrum regarding signatures of Schwinger and multiphoton pair production. Special emphasis is put on studying the impact of a strong dynamical magnetic field on the particle distribution functions. As the equaltime Wigner approach is formulated in terms of partial integrodifferential equations an entire section of the manuscript is dedicated to present numerical solution techniques applicable to Wigner function approaches in general.

Contribution to WWW
arXiv:1912.09359: https://arxiv.org/abs/1912.09359 
Physical Review D 101(2020)9, 096003
DOI: 10.1103/PhysRevD.101.096003
Cited 26 times in Scopus
Relaxation dynamics in a Hubbard dimer coupled to fermionic baths: phenomenological description and its microscopic foundation
Schützhold, R.; Kleinherbers, E.; Szpak, N.; König, J.
We study relaxation dynamics in a stronglyinteracting twosite FermiHubbard model that is induced by fermionic baths. To derive the proper form of the Lindblad operators that enter an effective description of the systembath coupling in different temperature regimes, we employ a diagrammatic realtime technique for the reduced density matrix. An improvement on the commonlyused secular approximation, referred to as coherent approximation, is presented. We analyze the spectrum of relaxation rates and identify different time scales that are involved in the equilibration of the Hubbard dimer after a quantum quench.

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arXiv:1910.04130: https://arxiv.org/abs/1910.04130 
Physical Review B 101(2020), 125131
DOI: 10.1103/PhysRevB.101.125131
Cited 18 times in Scopus
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Hierarchy of doubletime correlations
Queißer, F.; Schützhold, R.
The hierarchy of correlations is an analytical approximation method which allows us to study nonequilibrium phenomena in strongly interacting quantum manybody systems on lattices in higher dimensions. So far, this method was restricted to equaltime correlators ⟨A ^ μ (t)B ^ ν (t)⟩ . In this work, we generalize this method to doubletime correlators ⟨A ^ μ (t)B ^ ν (t ′ )⟩ , which allows us to study effective light cones and Green functions and to incorporate finite initial temperatures.

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arXiv:1909.10938: https://arxiv.org/abs/1909.10938 
Journal of Statistical Mechanics: Theory and Experiment 5(2023), 053101
DOI: 10.1088/17425468/acccde
Cited 1 times in Scopus
Boltzmann relaxation dynamics of strongly interacting spinless fermions on a lattice
Queißer, F.; Schützhold, R.; Schreiber, S.; Kratzer, P.
Motivated by the recent interest in nonequilibrium phenomena in quantum manybody systems, we study strongly interacting fermions on a lattice by deriving and numerically solving quantum Boltzmann equations that describe their relaxation to thermodynamic equilibrium.The derivation is carried out by inspecting the hierarchy of correlations within the framework of the 1/Zexpansion. Applying the Markov approximation, we obtain the dynamic equations for the distribution functions. Interestingly, we find that in the strongcoupling limit, collisions between particles and holes dominate over particleparticle and holehole collisions  in stark contrast to weakly interacting systems. As a consequence, our numerical simulations show that the relaxation time scales strongly depend on the type of excitations (particles or holes or both) that are initially present.

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arXiv:1909.12802: https://arxiv.org/abs/1909.12802 
Physical Review B 100(2020)24, 245110
DOI: 10.1103/PhysRevB.100.245110
Cited 3 times in Scopus
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Comptonlike scattering of a scalar particle with N photons and one graviton
Ahmadiniaz, N.; Balli, F. M.; Corradini, O.; Dávila, J. M.; Schubert, C.
Treelevel scattering amplitudes for a scalar particle coupled to an arbitrary number N of photons and a single graviton are computed. We employ the worldline formalism as the main tool to compute the irreducible part of the amplitude, where all the photons and the graviton are directly attached to the scalar line, then derive a tree replacement rule to construct the reducible parts of the amplitude which involve irreducible pure Nphoton twoscalar amplitudes where one photon line emits the graviton. We test our construction by verifying the onshell gauge and diffeomorphism Ward identities, at arbitrary N.
Keywords: Scattering amplitudes; gravitons; Ward identities

Nuclear Physics B 950(2020), 114877
DOI: 10.1016/j.nuclphysb.2019.114877
Cited 9 times in Scopus