Abstracts and Media
Antonino Di Piazza (Heidelberg, Max Planck Institute for Nuclear Physics)
QED in intense background fields: Foundation and modern applications
Quantum electrodynamics (QED) is a well established physical theory and its predictions have been confirmed experimentally in various regimes and with extremely high accuracy. However, there are still areas of QED that deserve theoretical and experimental investigation, especially when physical processes occur in the presence of intense background electromagnetic fields, i.e., of the order of the so-called "critical" field of QED. In the presence of electromagnetic fields of such a high strength the vacuum behaves as a birefringent and unstable medium, whose properties depend on the background field.
In the first of these two lectures I will review the general features of quantization of the Dirac field in a background electromagnetic field based on the Furry picture. Subjects like the stability of the vacuum in a given field will be discussed. In the second lecture I will report on modern applications of QED in intense background fields focusing to the case of a strong laser background field and, as examples, problems like radiation reaction and effects like vacuum polarization/pair production will be mentioned.
16:30-17:45 (JST), 8:30-9:45 (CET), Slides
18:00-19:15 (JST), 10:00-11:15 (CET), Slides
François Gelis (IPhT, Saclay)
Theoretical review of the Schwinger mechanism
In this presentation, I will review some theoretical aspects of the Schwinger mechanism, from the underlying quantum field theoretical foundations to some more practical issues. The presentation will cover the following topics:
- - Quantum field theories with external fields
- - Correlations in the Schwinger mechanism
- - Alternate formulations (Bogoliubov transformations, quantum kinetic equations, Wigner formalism)
- - Numerical lattice methods
- - Worldline formalism
- - Dynamically assisted Schwinger formalism
16:30-17:45 (JST), 8:30-9:45 (CET), Slides
18:00-19:15 (JST), 10:00-11:15 (CET), Slides
Takashi Oka (ISSP, U-Tokyo)
The Schwinger effect in solid-state physics
The Schwinger effect, i.e., tunnel creation of particle pairs in electric fields, is one of the oldest theoretical predictions in non-perturbative quantum field theory. I will explain how this effect provides the key to understand several laser-field induced nonequilibrium phenomena in solid-state systems.
(1) Application in strongly correlated electron systems:
In Mott insulators, charge excitations are frozen due to Coulomb repulsion. Strong dc and ac fields can create charge excitations, i.e., doublon-hole pairs, and melt the insulating state. I will explain this process using the Bethe ansatz approach with comparison to numerical results .
(2) Application in Dirac semimetals and geometric effects:
In weakly interacting Dirac semimetals, the non-adiabatic tunneling can be understood by a Landau-Zener like models. One interesting aspect is how geometric effects (Berry phase etc) appear in the dynamics leading to processes that break chiral symmetry .
(3) Floquet approach to Dirac semimetals in laser fields:
Floquet theory is a simple and powerful approach to study quantum systems driven by time-periodic perturbations . I will explain its application to Dirac semimetals in laser fields.
(4') Application in holographic QCD:
In holographic QCD, it is possible to study non-perturbative quantum dynamics using a classical field theory in a curved geometry. (Without explaining the detail,) I will present some results [4,5] compared with the above topics (1)-(3) to provide further research motivations.
 TO, PRB86, 075148 (2012).
 S. Takayoshi, J. Wu, TO, arXiv:2005.01755.
 TO, S. Kitamura, Ann. Rev. of CMP10, 387 (2019).
 K. Hashimoto, TO, JHEP 10 (2013) 116.
 K. Hashimoto, S. Kinoshita, K. Murata, TO, JHEP (2017) 127, S. Kinoshita, K. Murata, TO, JHEP (2018) 96.
16:30-17:45 (JST), 8:30-9:45 (CET), Slides
18:00-19:15 (JST), 10:00-11:15 (CET), Slides
James Daniel Brandenburg (Brookhaven National Laboratory & Center For Frontiers in Nuclear Science)
Observation of the Breit-Wheeler Process in Heavy-Ion Collisions
Ultra-relativistic heavy ion collisions are expected to produce some of the strongest magnetic fields (1013-1016 Tesla) in the Universe . Recently, there has been increased interest in the magnetic fields produced by heavy ion collisions and their possible observational impacts through emergent magnetohydrodynamical phenomena in Quantum Chromodynamics, like the Chiral Magnetic Effect . The initial strong electromagnetic fields produced in heavy ion collisions have been proposed as a source of linearly-polarized, quasi-real photons  that can interact via the Breit-Wheeler process to produce e+ e- pairs .
In this talk I present STAR measurements of e+ e- pair production in ultra-peripheral and peripheral Au+Au collisions at √sNN = 200 GeV. A comprehensive study of the pair kinematics is presented to distinguish the γγ → e+ e- process from other possible production mechanisms. Furthermore, the measured distribution of e+ e- pairs reveals a striking fourth-order angular modulation which demonstrates that the colliding photons are linearly polarized. The observed azimuthal asymmetry has implications for vacuum birefringence , a phenomenon predicted in 1936 in which empty space can split light according to its polarization components when subjected to a strong magnetic field. Furthermore, these measurements provide the first direct experimental evidence that ultra-relativistic heavy ion collisions are capable of producing the strongest magnetic fields in the known Universe over an extended spatial distribution.
 V. Skokov, A. Illarionov, and V. Toneev. International Journal of Modern Physics A 24 (2009): 5925–32.
 Kharzeev, D. E., et al. Prog. Part. Nucl. Phys., 88 (2016) 1–28
 C. Weizsäcker, Zeitschrift für Physik 88 (1934): 612–25.
 G. Breit and J. A. Wheeler. Physical Review 46 (1934): 1087
 Heisenberg, W., and H. Euler. Zeitschrift für Physik, (1936) arXiv: physics/0605038
Maxim Chernodub (Institut Denis Poisson, Tours, CNRS, France)
The Schwinger effect on the boundary and the scale invariance
The electrostatic field inside typical conductors drops down exponentially with a screening length. The length is determined by an intrinsic length scale of the system given by the density of mobile charge carriers. We show that in a classically conformal conducting medium, where the internal length scale is absent, the screening of a static electric field is governed by the quantum conformal (scale) anomaly associated with the renormalization of the electric charge. The electric field decays algebraically with a fractional power determined by the beta function of the system.
This 'anomalous conformal screening effect' is an indirect manifestation of the Schwinger pair production in relativistic field theory. Interestingly, this electrostatic screening is covariantly related to the generation of the anomalous currents localized at the boundary and produced by the conformal anomaly in the magnetic field background.
We discuss the experimental signatures of the proposed phenomena in Dirac semimetals.
Alexander Fedotov (National Research Nuclear University MEPhI, Moscow, Russia)
Enhancement and the final states in Strong Field QED processes
After a flash review of the essence of the SFQED approach, we discuss three examples of enhancement of QED processes in a strong external field:
- fake pair density blowup in the quantum kinetic description of Schwinger pair production  (not to be confused with the genuine dynamically assisted Schwinger mechanism!);
- onset of selfsustained QED cascades in a general type strong slowly varying background [2,3] (e.g., a single focused laser pulse - see the short talk by Arseny Mironov for derivations and further details);
- enhancement and an all-order resummation of the bubble-type radiative corrections (the Ritus-Narozhny conjecture) in a strong constant crossed field .
A special emphasis will be made on the importance of the hierarchies of scales involved and/or on a sticking point with properly setting the final states. Subtleties with the term 'non-perturbativity' in the context of strong-field QED will be further commented on.
 A.M. Fedotov, E.G. Gelfer, K.Yu. Korolev, S.A. Smolyansky, Physical Review D 83, 025011 (2011).
 A.M. Fedotov, N.B. Narozhny, G. Mourou, G. Korn, Physical Review Letters 105, 080402 (2010).
 A.A. Mironov, E.G. Gelfer, A.M. Fedotov, "Onset of electron-seeded cascades in generic electromagnetic fields", to be submitted.
 A.A. Mironov, S. Meuren, A.M. Fedotov, Physical Review D 102, 053005 (2020).
Kenji Fukushima (The University of Tokyo)
Particle Pair Production and Quantum Anomaly
I would like to discuss the fermion particle production problem under electromagnetic fields, which can be formulated by the spectral flow from the negative energy state to the positive energy state. However, there is a subtlety in theory about how to incorporate anti-particles. I would like to revisit this subtlety from the point of view of the chiral and the gauge anomaly.
Yoshimasa Hidaka (KEK, Tsukuba)
Thermalization of Yang-Mills theory in a (3+1) dimensional small lattice system
We study the real-time evolution of SU(2) Yang-Mills theory in a (3+1) dimensional small lattice system after interaction quench. We numerically solve the Schrodinger equation with the Kogut-Susskind Hamiltonian in the physical Hilbert space obtained by solving Gauss law constraints. We observe the thermalization of a Wilson loop to the canonical state; the relaxation time is insensitive to the coupling strength and estimated as 2π/T with temperatures T at steady states. We also compute the vacuum persistence probability (the Loschmidt echo) to understand the relaxation from the dynamics of the wave function.
Tatsuhiko Ikeda (Tokyo U., ISSP)
High-harmonic generation from solids: Floquet-theoretical approach and Dirac semimetals
High-harmonic generation (HHG), or frequency conversion to integer multiples of the input laser frequency, in solids has attracted renewed attention for the last decade (see Ref.  for review). This is a traditional nonlinear optical effect, but there have been, at least, two remarkable developments. First, very strong mid-infrared lasers have become available in experiments, and the HHG in deep nonperturbative regimes have been observed in semiconductors. Second, intense lasers have been available at as low as the terahertz (THz) frequency regime, uncovering the nonlinear optical responses of materials with small or no band gaps such as Dirac or Weyl semimetals.
In this talk, we review recent experimental and theoretical studies of the HHG in solids and then present two studies [2,3]. First, to analyze the HHG in solids, we discuss a Floquet-theoretical approach that enables us to describe electrons' dynamics under intense laser fields . Second, we discuss a THz HHG experiment in Dirac semimetals and its theoretical interpretation .
 S. Ghimire and D. A. Reiss, Nature Phys. 15, 10 (2019).
 T. N. Ikeda, K. Chinzei, and H. Tsunetsugu, Phys. Rev. A 98, 063426 (2018).
 B. Cheng*, N. Kanda*, T. N. Ikeda, R. Matsunaga et al. PRL 124, 117402 (2020).
Anton Ilderton (Plymouth University)
Quantum interference effects in pair production
Interference effects in pair production are well-known to be analogous to multiple-slit diffraction or Ramsey interferometry. We show that this analogy only holds semiclassically, and that exactly solvable cases reveal a richer interference structure which allows for quantum control of produced pair spectra.
Felix Karbstein (Helmholtz Inst., Jena and U. Jena)
Heisenberg-Euler effective action beyond one loop
The quantum vacuum is characterized by the omnipresence of quantum fluctuations of the underlying theory's particle degrees of freedom in the form of virtual processes. As electromagnetic fields couple to charges, the fluctuations of these virtual particles can give rise to effective nonlinear interactions among electromagnetic fields, thereby invalidating the classical Maxwell theory in vacuo. In this talk, we study the effective interactions of external electromagnetic fields induced by fluctuations of virtual particles in the vacuum of quantum electrodynamics (QED). These corrections to classical Maxwell theory are encoded in the Heisenberg-Euler effective action. Our main focus is on effective interactions beyond one-loop order. After sketching the emergence of the Heisenberg-Euler effective action from the underlying microscopic theory of QED, special emphasize is put on one-particle reducible contributions discovered only recently. Finally, we discuss the strong field behavior of the Heisenberg-Euler effective action and show that it is dominated by one-particle reducible contributions. The latter can efficiently be constructed from lower-order diagrams. This allows us to provide an explicit expression for the strong field limit of the Heisenberg-Euler effective action accounting for contributions from all loops. Remarkably, the leading strong field behavior of the all-loop result only requires input from the renowned one-loop expression derived by Heisenberg and Euler.
Sebastian Meuren (SLAC)
Probing Strong-field QED at FACET-II (SLAC E-320)
The Experiment 320 at FACET-II/SLAC aims at colliding the 13 GeV FACET-II electron beam with the highest possible laser intensities. With the existing laser intensities up to 10^20 W/cm^2 will be achievable with an f/2 off-axis parabola. This allows us to exceed the QED critical ("Schwinger") field in the electron rest frame. Even more importantly, the classical intensity parameter a0 = eE/(mc\omega) will be much larger than one, implying that pair production in the tunneling regime can be observed. Initial milestones are: measuring the transition from perturbative to non-perturbative nonlinear Compton scattering, studying quantum radiation reaction, investigating the validity of the local constant field approximation in addition to observing pair production in the tunneling regime.
The first major upgrade of the experiment will introduce a 2nd interaction point for producing polarized high-energy gamma photons via Compton backscattering. This will allow us to study photon-induced pair production in detail, including polarization effects and to scrutinize the validity of the two-step approximation for the trident process. A laser upgrade to the 100 TW-scale would allow us to explore a wider parameter range and provides access to novel phenomena, e.g., vacuum birefringence beyond the leading-order Euler-Heisenberg effective action.
On the time scale of 5-10 years we envision a major facility upgrade, which combines a 5-10 PW laser with a 30 GeV electron beam. The latter is obtained by linking the FACET-II with the LCLS-Cu LINAC. The combination of high-energy/intensity x-rays, multi-PW optical light, and a dense high-energy electon beam enables seminal, world-wide unique research opportunities in advance accelerator research (beamstrahlung mitigation at future linear collider, future gamma-gamma collider based on beamstrahlung), laboratory astrophysics (undestanding magnetars and other extreme astrophysical environments where the QED critical field is exceeded, highly relevant for the emerging field of multi-messenger astronomy), high-energy density and plasma physics (understanding relativistic electron-positron pair plasma in the QED regime and thus the interplay between strong-field quantum and collective plasma effects), as well as addressing fundamental questions in quantum field theory (first insights into the Ritus-Narozhny conjecture).
Shi Pu (USTC)
Strong electromagnetic fields in relativistic heavy ion collisions
We give a brief overview of recent theoretical results on the chiral magnetic effect, chiral kinetic theory and other effects related to the electromagnetic fields in heavy-ion collisions.
Sören Schlichting (Bielefeld U.)
Classical-statistical simulations of Schwinger pair production and Chiral turbulence in QED- and QCD-like theories
We will discuss the use of classical-statistical real-time lattice gauge theory simulations, to describe the non-perturbative quantum dynamics of fermions in the presence of strong (classical) gauge fields. Starting with benchmarks for the classical problem of Schwinger pair production in QED, with and without back-reaction of the produced electron-positron pairs, we will subsequently address the real-time dynamics of the chiral anomaly in chirality imbalanced QED- and QCD-like plasmas.
Ralf Schützhold (HZDR, Dresden and Dresden, Tech. U., ITP)
Sauter-Schwinger Effect and Dynamical Assistance
Kazuaki Takasan (UC, Berkeley)
Dielectric breakdown of strongly correlated insulators in one dimension
Application of sufficiently strong electric fields to insulators induces finite currents and then the insulators become metallic. This phenomenon is called dielectric breakdown and known as a fundamental nonequilibrium and nonlinear transport phenomenon in solids. For band insulators (i.e., free fermion systems), it is called Zener breakdown and well-understood . In contrast, the dielectric breakdown of strongly correlated insulators is less understood and has been actively studied even in recent years both experimentally  and theoretically . However, most of the previous studies are limited to specific examples, mainly fermionic Mott insulators. While dielectric breakdowns of other strongly correlated insulators, such as charge density-wave insulators, bosonic Mott insulators, and Kondo insulators, have been also experimentally relevant, universal properties common in the breakdown phenomena have not been well-understood.
In this talk, I would like to present our recent results about the universal aspects in the dielectric breakdown of one-dimensional strongly correlated insulators . Combining bosonization techniques with a theory of quantum tunneling (Dykhne-Davis-Pechukas formula), we develop an effective field-theoretical description of dielectric breakdown with a non-Hermitian sine-Gordon theory. Then, we derive an analytic formula of the threshold field which is a many-body generalization of the Landau-Zener formula. Importantly, we point out that the threshold field contains a previously overlooked factor originating from charges of elementary excitations, which should be significant when a system has fractionalized excitations. We apply our results to integrable lattice models and confirm that our formula is valid in a broad range including the weak coupling regime, indicating its wide and potential applicability.
The talk will start from the brief review of the studies of dielectric breakdown in solids and then move to our recent results. If time allows, I will also address some future issues and open problems in this field.
 C. Zener, Proc. R. Soc. London Ser. A 145, 523–529 (1934).
 e,g., Y. Kalcheim, et al., Nat. Comm. 11, 2985 (2020).
 e,g., V Tripathi and V. M. Vinokur, Sci. Rep. 10, 7304 (2020).
 K. Takasan, M. Nakagawa, and N. Kawakami, arXiv:1908.06107.
Koichiro Tanaka (Kyoto U.)
Extreme non-linear optics in solids - Zener-Tunneling and High-harmonic generation-
Recent development of femtosecond mid-infrared laser has been promoting nonlinear optics in solids to enter an extreme regime, where light-matter interaction energy (or Rabi frequency) becomes comparable with the scale of electronic transition energy. Higher-order harmonic generation (HHG) is a typical extreme nonlinear optics in solids and its initial step has been considered to be Zener-tunneling, which is nothing but a Schwinger-mechanism in solids. In this talk, we will review the state of the art of HHG in solids. Especially, HHG in several low dimensional materials is high-lighted from the several points of view: polarization selection rules, dynamical symmetries, and anisotropic response with the crystal axis.
Contributed Short Talks
Ivan Aleksandrov (St. Petersburg State University)
Total yield of electron-positron pairs produced in strong electromagnetic fields and the locally constant field approximation
The widely-used locally constant field approximation (LCFA) can be utilized in order to derive a closed-form expression for the total number of particles produced in a strong electromagnetic field of a general spatio-temporal configuration. This approximate approach is usually assumed to be justified if the external field varies slowly in space and time. We explore the validity of the LCFA by comparing its predictions to the results obtained by means of exact nonperturbative numerical techniques. To benchmark the LCFA in the regime of small field amplitudes and low frequencies, we employ a semiclassical approach. We identify the domain of the field parameters where the approximation is well justified. In particular, it is demonstrated that the Keldysh parameter is not a relevant quantity governing the accuracy of the LCFA.
Oliver Gould (University of Nottingham)
The Dual Schwinger Effect
Electromagnetic duality implies that strong magnetic fields will produce magnetic monopoles by the dual Schwinger effect, if indeed magnetic monopoles exist. As a consequence, strong magnetic fields provide perhaps the best avenue to search for magnetic monopoles: the existence of any given strong magnetic field provides a lower bound on their mass. This search strategy is currently being utilised at the LHC, where the strongest known magnetic fields are produced fleetingly by heavy-ion collisions.
Stanislav Iablokov (P.G. Demidov Yaroslavl State University)
Modified Fock-Schwinger method simplifies calculation of charged particle propagators in a constant magnetic field
We present a recently published modified Fock-Schwinger (MFS) method to find the exact solutions of the propagator equation for charged particles in the presence of a constant magnetic field directly in the momentum space as a sum over Landau levels. In contrast to the standard approaches for finding propagators, MFS method demonstrated several improvements in terms of computational complexity reduction and revealed simple internal structures in intermediate and final expressions, thus allowing us to obtain new useful representations of the propagators. We plan to apply this technique to a broader spectrum of electromagnetic field configurations (including constant electric field which is relevant for the Schwinger effect) to study different representations of propagators.
Ben King (University of Plymouth)
The locally monochromatic approximation and the LUXE experiment
The LUXE (Light Und X-fEl) experiment, planned to run at DESY, Germany, will combine a 40TW laser with a 16.5 GeV electron beam to study strong-field QED effects. A key experimental aim is to observe the transition from a perturbative to a non-perturbative dependency on the field intensity. Because a range of intensities will be considered at LUXE, the standard approach of including strong-field QED phenomena in numerical codes based upon the locally constant field approximation, is not sufficiently accurate to model the experiment. By combining a slowly-varying envelope approximation with a local phase expansion, we have developed the locally monochromatic approximation to strong-field QED, which is being used to calculate particle spectra and yields for the LUXE experiment.
Hiroyuki Kitamoto (FRIS, Tohoku University)
No-go theorem of anisotropic inflation via Schwinger mechanism
In the presence of a dilatonic coupling between an inflaton and a U(1) gauge field, a persistent electric field (i.e., an anisotropic inflation) is obtained as a solution of the classical field equations. We introduce charged, massive, and conformally coupled fields into this model, and study the pair production of charged particles. The semiclassical approach allows us to evaluate the induced current due to the pair production on general dilatonic factor and electric field. Solving the field equations with the induce current, we find that the electric field shows a damped oscillation, whose amplitude decays to zero regardless of the values of the masses of charged fields. In other words, we derive a no-go theorem of anisotropic inflation by taking into account the Schwinger mechanism.
Makoto Ochiai (Waseda University)
Field-theoretical analysis for Klein paradox in the temporal gauge
Canonical quantum field theory for relativistic fermions scattering off the one-dimensional step potential is discussed, using the complete sets of scattering wave functions. We treat the background field in the temporal gauge; the gauge transformation which turns the original scalar potential to zero is applied to the scattering wave functions and we use them as the mode functions of the Dirac field. In- and out-states are defined assuming the asymptotic freedom of the field, which leads to the Bogoliubov transformation between the creation-annihilation operators at the distant past and future. The reflection and transmission probabilities in the relativistic quantum mechanics are obtained through the S-matrix analysis. The counterintuitive behavior of the scattering which is known as the Klein paradox is appeared, and its mechanism is discussed based on the Schwinger effect.
Ilia Maltsev (St. Petersburg University)
How to observe the vacuum decay at a supercritical Coulomb field
In low-energy collision of heavy ions with the sufficiently large total charge, the initially neutral vacuum can decay spontaneously with for formation of the charged vacuum and emission of the two positrons. The detection of the emitted positrons would the direct evidence of the vacuum decay. However, the spontaneously produced particles cannot be distinguished from the dynamical background in the positron spectra. We show that the vacuum decay can nevertheless be detected using impact-sensitive measurements of pair-production probabilities.
Arseny Mironov (Prokhorov General Physics Institute of the Russian Academy of Sciences)
Onset of electron-seeded cascades in electromagnetic fields
When an electron or a positron is placed in a superstrong slowly varying electromagnetic field, a massive selfsustained QED cascade can arise. Under certain conditions this may happen even if the seed particle is initially at rest. On the basis of the locally-constant field approximation and the semi-classical approach to the description of the particle short-time dynamics, we obtain simple general Lorentz-invariant criteria for a cascade onset in terms of the external field. It can be applied, for example, to configure a (e.g. laser) field either to suppress or enhance the selfsustained cascade. As an illustration, we consider the onset of a cascade in a single focused laser pulse. We estimate the required field parameters and validate these considerations by the Monte Carlo simulations.
Gabriel Soares Rocha (Universidade Federal Fluminense, Brazil)
A novel Relaxation Time Approximation to the relativistic Boltzmann equation
In 1974 J. L. Anderson and H. R. Witting proposed in the seminal paper  the Relaxation Time Approximation (RTA) to the relativistic Boltzmann equation. This contribution is very relevant, and the obtention of the transport coefficients with it is much simplified. Recently, it has been employed to study the hydrodynamization of the matter produced in ultrarelativistic heavy ion collisions . However, the traditional RTA is inconsistent with the hydrodynamic equations unless one considers an energy-independent relaxation time and Landau matching conditions. The origin of such flaw is that the approximation violates basic properties of the collision integral. In this work, a generalization of RTA will be presented. This new version is also consistent with the second law of thermodynamics and we will see what are its consequences to the solution for the non-equilibrium Particle Distribution Function and to the transport properties, both obtained analytically.
 Anderson, J. L., Witting, H. R.. A relativistic relaxation-time model for the Boltzmann equation. Physica. 74 466–488 (1974).
 S. Kamata, M. Martinez, P. Plaschke, S. Ochsenfeld, S. Schlichting. Physical Review D 102 5, 056003 (2020)
Rajeev Singh (Institute of Nuclear Physics Polish Academy of Sciences)
Dynamics of spin polarization using relativistic spin hydrodynamics
Measurements made recently by the STAR collaboration show that the Lambda hyperons produced in relativistic heavy-ion collisions are subject to global spin polarization with respect to an axis coincident with the axis of rotation of the produced matter. Recently formulated formalism of relativistic hydrodynamics with spin, which is a generalization of the standard hydrodynamics, is a natural tool for describing the evolution of such systems. This approach is based on the conservation laws and the form of the energy-momentum tensor and spin tensor postulated by de Groot, van Leeuwen, and van Weert (GLW).
Maksim Valialshchikov (Skolkovo Institute of Science and Technology)
Narrow bandwidth gamma comb from nonlinear Compton scattering using the polarization gating technique
Nonlinear Compton scattering is a promising source of bright gamma-rays. Using readily available intense laser pulses to scatter off the energetic electrons on the one hand allows to significantly increase the total photon yield, but on the other hand leads to a dramatic spectral broadening of fundamental emission line as well as its harmonics due to the laser pulse shape induced ponderomotive effects. In this paper we propose to avoid ponderomotive broadening in harmonics by using the polarization gating technique - a well-known method to construct a laser pulse with temporally varying polarization. We show that by restricting harmonic emission only to the region near the peak of the pulse, where the polarization is linear, it is possible to generate a bright narrow bandwidth comb in the gamma region.