Publications
Involved institute: Theoretical Physics (from 2018)
Public type of publication: Articles ref. in Journals
"Online First" included
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
DOI: 10.1103/PhysRevD.104.L011902
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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
DOI: 10.1103/PhysRevLett.126.180605
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 Original PDF 417 kB Secondary publication
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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
DOI: 10.1007/JHEP08(2020)018
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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
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
DOI: 10.1103/PhysRevD.101.116019
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 Original PDF 285 kB Secondary publication
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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
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.

Contribution to WWW
arXiv:2001.00885: https://arxiv.org/abs/2001.00885 
EPL  Europhysics Letters 130(2020), 41001
DOI: 10.1209/02955075/130/41001
Downloads:
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 Fulltext from iopscience.iop.org
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.

Contribution to WWW
arXiv:1912.09924: https://arxiv.org/abs/1912.09924 
Physical Review D 101(2020)9, 096009
DOI: 10.1103/PhysRevD.101.096009
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
WKB approach to pair creation in spacetimedependent fields: The case of a spacetimedependent mass
Besides tunneling in static potential landscapes, for example, the WentzelKramersBrillouin (WKB) approach is a powerful nonperturbative approximation tool to study particle creation due to timedependent background fields, such as cosmological particle production or the SauterSchwinger effect, i.e., electronpositron pair creation in a strong electric field. However, our understanding of particle creation processes in background fields depending on both space and time is rather incomplete. In order to venture into this direction, we propose a generalization of the WKB method to truly spacetimedependent fields and apply it to the case of a spacetimedependent mass.

Physical Review D 99(2019)12, 125014
DOI: 10.1103/PhysRevD.99.125014 
Contribution to WWW
https://arxiv.org/abs/1603.00274
Boltzmann relaxation dynamics in the strongly interacting FermiHubbard model
Via the hierarchy of correlations, we study the Mott insulator phase of the FermiHubbard model in the limit of strong interactions and derive a quantum Boltzmann equation describing its relaxation dynamics. In stark contrast to the weakly interacting case, we find that the scattering cross sections strongly depend on the momenta of the colliding quasiparticles and holes. Therefore, the relaxation towards equilibrium crucially depends on the spectrum of excitations. For example, for particlehole excitations directly at the minimum of the (direct) Mott gap, the scattering cross sections vanish such that these excitations can have a very long lifetime.

Physical Review A 100(2019)5, 053617
DOI: 10.1103/PhysRevA.100.053617 
Contribution to WWW
https://arxiv.org/abs/1812.08581
DOI: 10.1103/PhysRevA.100.053617
Phonon Pair Creation by Inflating Quantum Fluctuations in an Ion Trap
Schützhold, R.; Wittemer, M.; Hakelberg, F.; Kiefer, P.; Schröder, J.P.; Warring, U.; Schaetz, T.; Fey, C.
Quantum theory predicts intriguing dynamics during drastic changes of external conditions. We switch the trapping field of two ions sufficiently fast to tear apart quantum fluctuations, i.e., create pairs of phonons and, thereby, squeeze the ions’ motional state. This process can be interpreted as an experimental analog to cosmological particle creation and is accompanied by the formation of spatial entanglement. Hence, our platform allows one to study the causal connections of squeezing, pair creation, and entanglement and might permit one to crossfertilize between concepts in cosmology and applications of quantum information processing.
Keywords: Inflation; Quantum Information with trapped Ions; Quantum simulation

Contribution to WWW
arXiv:1903.05523: https://arxiv.org/abs/1903.05523 
Physical Review Letters 123(2019)18, 180502
DOI: 10.1103/PhysRevLett.123.180502
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.

Contribution to WWW
arXiv:1910.04130: https://arxiv.org/abs/1910.04130 
Physical Review B 101(2020), 125131
DOI: 10.1103/PhysRevB.101.125131
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.

Contribution to WWW
arXiv:1909.12802: https://arxiv.org/abs/1909.12802 
Physical Review B 100(2020)24, 245110
DOI: 10.1103/PhysRevB.100.245110
Environment induced prethermalization in the MottHubbard model
Queißer, F.; Schützhold, R.
Via the hierarchy of correlations, we study the strongly interacting FermiHubbard model in the Mott insulator state and couple it to a Markovian environment which constantly monitors the particle numbers \hat n_\mu^\uparrow and \hat n_\mu^\downarrow for each lattice site \mu. As expected, the environment induces an imaginary part \gamma (i.e., decay rate) of the quasiparticle frequencies \omega_{\mathbf{k}}\to\omega_{\mathbf{k}}i\gamma and tends to diminish the correlations between lattice sites. Surprisingly, the environment does also steer the state of the system on intermediate time scales \mathcal{O}(1/\gamma) to a prethermalized state very similar to a quantum quench (i.e., suddenly switching on the hopping rate J). Full thermalization occurs via local onsite heating and takes much longer.

Physical Review B 99(2019)15, 155110
DOI: 10.1103/PhysRevB.99.155110 
Contribution to WWW
arXiv:1808.09906: https://arxiv.org/abs/1808.09906
Dynamically assisted nuclear fusion
Queißer, F.; Schützhold, R.
We consider deuteriumtritium fusion as a generic example for general fusion reactions. For initial kinetic energies in the keV regime, the reaction rate is exponentially suppressed due to the Coulomb barrier between the nuclei, which is overcome by tunneling. Here, we study whether the tunneling probability could be enhanced by an additional electromagnetic field, such as an xray free electron laser (XFEL). We find that the XFEL frequencies and field strengths required for this dynamical assistance mechanism should come within reach of presentday or nearfuture technology.

Contribution to WWW
arXiv:1902.04905: https://arxiv.org/abs/1902.04905 
Physical Review C 100(2019)4, 041601
DOI: 10.1103/PhysRevC.100.041601
Entangling continuous variables with a qubit array
Navez, P.; Sowa, A.; Zagoskin, A.
We show that an array of qubits embedded in a waveguide can emit entangled pairs of microwave photon beams. The quadratures obtained from the homodyne detection of these outputs beams form a pair of correlated continuous variables similar to the EinsteinPodolskyRosen experiment. The photon pairs are produced by the decay of plasmonlike collective excitations in the qubit array. The maximum intensity of the resulting beams is bounded by only the number of emitters. We calculate the excitation decay rate both into a continuum of the photon state and into a onemode cavity. We also determine the frequency of Rabilike oscillations resulting from a detuning.

Physical Review B 100(2019)14, 144506
DOI: 10.1103/PhysRevB.100.144506 
Contribution to WWW
arXiv:1903.06285 [quantph]: https://arxiv.org/abs/1903.06285
DOI: 10.1103/PhysRevB.100.144506
Reducible contributions to quantum electrodynamics in external fields
Ahmadiniaz, N.; Edwards, J. P.; Ilderton, A.
We consider oneparticle reducible (1PR) contributions to QED and scalar QED processes in external fields, at oneloop and twoloop order. We investigate three cases in detail: constant crossed fields, constant magnetic fields, and plane waves. We find that 1PR tadpole contributions in plane waves and constant crossed fields are nonzero, but contribute only divergences to be renormalised away. In constant magnetic fields, on the other hand, tadpole contributions give physical corrections to processes at oneloop and beyond. Our calculations are exact in the external fields and we give strong and weak field expansions in the magnetic case.

Journal of High Energy Physics 05(2019), 038
DOI: 10.1007/JHEP05(2019)038 
Contribution to WWW
arXiv:1901.09416 [hepth]: https://arxiv.org/abs/1901.09416
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
Analog of cosmological particle creation in electromagnetic waveguides
We consider an electromagnetic waveguide with a timedependent propagation speed v(t) as an analog for cosmological particle creation. In contrast to most previous studies which focus on the number of particles produced, we calculate the corresponding twopoint correlation function. For a small steplike variation delta v(t), this correlator displays characteristic signatures of particle pair creation. As another potential advantage, this observable is of first order in the perturbation delta v(t), whereas the particle number is second order in delta v(t) and thus more strongly suppressed for small delta v(t).

Physical Review D 100(2019)6, 065003
DOI: 10.1103/PhysRevD.100.065003 
Contribution to WWW
arXiv:1808.07453 [quantph]: https://arxiv.org/abs/1808.07453
Iontrap analog of particle creation in cosmology
Fey, C.; Schätz, T.; Schützhold, R.
We consider the transversal modes of ions in a linear radiofrequency trap where we control the timedependent axial confinement to show that we can excite quanta of motion via a twomode squeezing process. This effect is analogous to phenomena predicted to occur in the early universe, in general out of reach for experimental investigation. As a substantial advantage of this proposal in comparison to previous ones we propose to exploit the radial and axial modes simultaneously to permit experimental access of these effects based on stateoftheart technology. In addition, we propose to create and explore entanglement between the two ions.

Physical Review A (2018), 033407
DOI: 10.1103/PhysRevA.98.033407
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 Original PDF 520 kB Secondary publication
Discrete worldline instantons
Schneider, C.; Torgrimsson, G.; Schützhold, R.
The semiclassical approximation of the worldline path integral is a powerful tool to study nonperturbative electronpositron pair creation in spacetimedependent background fields. Finding solutions of the classical equations of motion, i.e., worldline instantons, is possible analytically only in special cases, and a numerical treatment is nontrivial as well. We introduce a completely general numerical approach based on an approximate evaluation of the discretized path integral that easily and robustly gives the full semiclassical pair production rate in nontrivial multidimensional fields, and apply it to some example cases.

Physical Review D (2018), 085009
DOI: 10.1103/PhysRevD.98.085009
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 Original PDF 1,5 MB Secondary publication
Interaction of a BoseEinstein condensate with a gravitational wave
Schützhold, R.
Partly motivated by recent proposals for the detection of gravitational waves, we study their interaction with BoseEinstein condensates. For homogeneous condensates at rest, the gravitational wave does not directly create phonons (to lowest order) but merely affects existing phonons or indirectly creates phonon pairs via quantum squeezingan effect which has already been considered in the literature. For inhomogeneous condensate flows such as a vortex lattice, however, the impact of the gravitational wave can directly create phonons. This more direct interaction can be more efficient and could perhaps help bring such a detection mechanism for gravitational waves a step closer towards experimental realizabilityeven though there is still a long way to go. Finally, we argue that superfluid helium might offer some advantages in this respect.

Physical Review D (2018), 105019
DOI: 10.1103/PhysRevD.98.105019
Downloads:
 Original PDF 176 kB Secondary publication