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The quantum vacuum constitutes a fascinating medium of study, in particular since near-future laser facilities will be able to probe the nonlinear nature of this vacuum. There has been a large number of proposed tests of the low-energy, high intensity regime of quantum electrodynamics (QED) where the nonlinear aspects of the electromagnetic vacuum...
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... that the vacuum is disturbed by a strong exter- nal electric field. A virtual electron-positron pair may then become real if the work exercised by the electric field on an electron over the distance of approximately a Compton wavelength is of the order of the electron rest mass (see Fig. 3). Pair production has been observed in experiment where high frequency photons interacted with an intense electromagnetic field, [11] (see Fig. 4). Pair creation by focused laser beams has also been discussed in e.g. Refs. ...
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Citations
... Such devices are the best way to investigate classical and quantum electrodynamics in the strong field regime. The recent LASERs may, in the near future, be used in a detailed inspection of the structure of the vacuum and, therefore, of the electromagnetic nonlinearities [19][20][21][22]. ...
In this work, we study a model in nonlinear electrodynamics in the presence of a CPT-even term that violates Lorentz symmetry. The Lorentz-breaking vector, in addition to the usual background magnetic field, produces interesting effects in the dispersion relations. The consequences on the vacuum refractive index and the group velocity are studied. Vacuum birefringence is discussed in the case the nonlinear electrodynamics is a Euler–Heisenberg model.
... with A being a measure of quadratic field strength normalized to the critical field, like (E/E crit ) 2 or (B/B crit ) 2 for static fields or I/I crit for a beam intensity I, see Sec. 3. Hence, the difference of the phase velocities yields a birefringence of vacuum [17][18][19][29][30][31][32][33][34][35][36] whereas the difference from n = 1 yields a refraction in general. Furthermore, the external field, polarizing the vacuum, can be realized by static fields or by electromagnetic waves. ...
... Currently, thanks to the development of ultra-intense optical lasers and XFELs, researchers [18][19][20]30,33,35,36,77,78] proposed to probe characteristics of the QED vacuum. Here, the highly purified linearly polarized XFEL interacts with an intense optical laser in vacuum. ...
Polarimetry is a highly sensitive method to quantify changes of the polarization state of light when passing through matter and is therefore widely applied in material science. The progress of synchrotron and X-ray free electron laser (XFEL) sources has led to significant developments of X-ray polarizers, opening perspectives for new applications of polarimetry to study source and beamline parameters as well as sample characteristics. X-ray polarimetry has shown to date a polarization purity of less than $1.4\times {10}^{-11}$ , enabling the detection of very small signals from ultrafast phenomena. A prominent application is the detection of vacuum birefringence. Vacuum birefringence is predicted in quantum electrodynamics and is expected to be probed by combining an XFEL with a petawatt-class optical laser. We review how source and optical elements affect X-ray polarimeters in general and which qualities are required for the detection of vacuum birefringence.
... However, the results of recent theoretical investigations suggest that vacuum e − e + pair production is possible through careful combination and shaping of the laser pulse field [5][6][7][8][9]. One may expect that some schemes will make experiments possible with advanced laser facilities in the near future [10][11][12][13]. ...
The effect of the carrier envelope phase on electron-positron pair production is studied in a spatially inhomogeneous electric field with symmetrical frequency chirp. In both high- and low-frequency fields, we find periodic variations in the momentum spectrum and in the reduced particle number induced by the phase. In particular, the changes are more sensitive in the case of a low-frequency field. Under certain conditions, the reduced particle number can increase by over one order of magnitude, induced by the phase. Some different types of symmetries for two correlated phases and an interesting individual self-symmetry are revealed in the momentum spectrum. The combined roles of phase and chirping in the periodic and symmetrical behaviour of the momentum spectrum and the reduced particle number provide a more flexible parameter choice to control and optimise the pair production.
... To probe the scattering of light with light in detail in an earth-based laboratory effectively putting these non-linear interaction terms at display poises strong-field experiments to become one of the most anticipated experiments at modern laser facilities [16][17][18][19][20][21]. Prominent examples in the context of pair production are the SLAC e-340 at FACET-II at the linear collider in Stanford (LCLS) [22] or the LUXE experiment at DESY in Hamburg [23]. ...
We discuss the Heisenberg-Wigner phase-space formalism in quantum electrodynamics as well as scalar quantum electrodynamics with respect to transverse fields. In regard to the special characteristics of such field types we derive modified transport equations such that particle momenta perpendicular to the propagation direction of the waves show up as external parameters only. In case of spatially oscillating fields we further demonstrate how to transform momentum derivative operators of infinite order into simple coupling terms.
... As it became evident decades ago, a self-consistent theory of electromagnetic interactions should properly incorporate vacuum fluctuations, which can give rise to essentially nonlinear fundamental phenomena, such as light-by-light scattering [1][2][3][4], vacuum birefringence [5][6][7], and electron-positron pair production in strong fields [8][9][10] (for review, see Refs. [11][12][13][14][15][16][17]). The latter process is especially intriguing, as in the case of quasistatic external backgrounds, it can be described only by means of nonperturbative methods. ...
We investigate the phenomenon of electron–positron pair production in intense external backgrounds within the strong-field regime. We perform nonperturbative calculations by solving the quantum kinetic equations, and obtain the momentum distributions of particles created and the total number of pairs. In particular, we analyze the validity of the locally constant field approximation (LCFA), which represents a powerful method for treating inhomogeneous external backgrounds. We consider a combination of two consecutive time-dependent Sauter pulses and thoroughly examine the effects of quantum interference and the role of the Pauli exclusion principle. It is shown that the latter can be approximately incorporated within the LCFA when computing the momentum distributions, while the closed-form LCFA expression for the total particle yield completely disregards Pauli blocking. It is demonstrated that in the presence of multiple turning points of classical electron trajectories, one observes interference patterns in the particle spectra, and the LCFA may significantly overestimate the number of pairs. To further elaborate this issue, we perform the analogous calculations in the case of scalar QED. It is shown that the quantum statistics effects enhance the number of bosons produced.
... Quantum field theory however reveals that all fundamental degrees of freedom come in the form of fields permeating all space and whose rest state can never be strictly assigned, so that an inert vacuum can in principle not exist nor a single field be in perfect isolation, each field constantly interacting with all the other ones through real or virtual states [2]. As laser technology is making steady progress toward unprecedented intensities [3], the interaction of light with the quantum vacuum newly stands as a promising probe for fundamental physics [4][5][6][7][8][9][10]. According to the standard model, first deviations from free propagation stem from the coupling to the electron field, controlled by the electron mass m and the coupling constant e. Together they define the characteristic field scale of quantum electrodynamics (QED), F S = m 2 /e in Lorentz-Heaviside units with c = = 1, that is F S 1.32 × 10 18 V m −1 4.4 × 10 9 T, which marks the onset of the spontaneous coherent field decay into real electron-positron pairs known as the nonperturbative Schwinger process [11][12][13]. ...
The advent of Petawatt-class laser systems allows generating electro-magnetic fields of unprecedented strength in a controlled environment, driving in-creasingly more efforts to probe yet unobserved processes through their interaction with the quantum vacuum. Still, the lowest intensity scale governing these effects lies orders of magnitude beyond foreseen capabilities, so that such endeavor is ex- pected to remain extremely challenging. In recent years, however, plasma mirrors have emerged as a promising bridge across this gap, by enabling the conversion of intense infrared laser pulses into coherently focused Doppler harmonic beams lying in the X-UV range. In this work, we present quantitative predictions on the quantum vacuum signatures produced when such beams are focused to intensities between 10 ²⁴ and 10 ²⁸ W.cm ⁻² . These signatures, which notably include photon-photon scattering and electron-positron pair creation, are obtained using state-of-the-art massively par- allel numerical tools. In view of identifying experimentally favorable configurations, we also consider the coupling of the focused harmonic beam with an auxiliary optical beam, and provide comparison with other established schemes. Our results show that a single coherently focused harmonic beam can produce as much scattered photons as two infrared pulses in head-on collision, and confirm that the coupling of the harmonic beam to an auxiliary beam gives rise to significant levels of inelastic scattering, and hence holds the potential to strongly improve the attainable signal to noise ratios in experiments.
... It was found that Maxwell's Lagrangian gains additional quantum corrections which lead to remarkable nonlinear phenomena such as light-by-light scattering [1][2][3][4] and Sauter-Schwinger electron-positron pair production [2,5,6] (for review, see, e.g., Refs. [7][8][9][10][11][12][13][14][15]). To galvanize the quantum fluctuations, one basically strives to make them interact with a strong background field. ...
According to quantum electrodynamics (QED), a strong electromagnetic field can make the vacuum state decay via the production of electron-positron pairs. Here we investigate the emission of soft photons which accompanies a nonperturbative process of pair production. Our analysis is carried out within the Furry picture to first order in the fine-structure constant. Also, it is shown that the presence of photons in the initial state gives rise to an additional (stimulated) channel of photon emission besides the pure vacuum one. On the other hand, the number of final (signal) photons includes also a negative contribution due to photon absorption within the pair production process. These contributions are evaluated and compared. To obtain quantitative predictions in the domain of realistic field parameters, we employ the Wentzel-Kramers-Brillouin approach. We propose using an optical-probe photon beam, whose intensity changes as it traverses a spatial region where a strong electric component of a background laser field is present. It is demonstrated that relative intensity changes on the level of 1% can be experimentally observed once the intensity of the strong background field exceeds 1027 W/cm2 within a large laser-wavelength interval. This finding is expected to significantly support possible experimental investigations of nonlinear QED phenomena in the nonperturbative regime.
... Focusing on slowly varying fields characterized by typical frequencies much smaller than m e , quantum vacuum nonlinearities can be reliably studied on the basis of the leading contribution to the Heisenberg-Euler effective Lagrangian L HE [1][2][3][4][5]. See also the pertinent reviews [6][7][8][9][10][11][12] and references therein. In the Heaviside-Lorentz system with units where ...
... respectively [12]. Equation (8) only amounts to a solution of the paraxial Helmholtz equation for z R ¼ ωw 2 0 =ð2M 2 Þ with M 2 ≡ 1. On the other hand, a beam quality factor M 2 ≠ 1 is widely employed to phenomenologically capture important characteristics of the field profiles of experimentally realistic laser beams deviating from ideal fundamental Gaussian beams [64,65]. ...
... Furthermore, we note that by construction the approximate beam profiles (8) and (9) are only expected to allow for a reliable description of the colliding laser pulses as long as the radial far-field divergence of the probe (pump) beam ...
We study the nonlinear QED signature of x-ray vacuum diffraction in the head-on collision of optical high-intensity and x-ray free-electron laser pulses at finite spatiotemporal offsets between the laser foci. The high-intensity laser driven scattering of signal photons outside the forward cone of the x-ray probe constitutes a prospective experimental signature of quantum vacuum nonlinearity. Resorting to a simplified phenomenological ad hoc model, it was recently argued that the angular distribution of the signal in the far-field is sensitive to the wavefront curvature of the probe beam in the interaction region with the high-intensity pump. In this work, we model both the pump and probe fields as pulsed paraxial Gaussian beams and reanalyze this effect from first principles. We focus on vacuum diffraction both as an individual signature of quantum vacuum nonlinearity and as a potential means to improve the signal-to-background separation in vacuum birefringence experiments.
... Fortunately, the development of ultraintense laser technology [8][9][10] might make the experimental observation of pair production possible in the near future. Especially the application of chirped pulse amplification (CPA) technique has greatly improved the laser intensity [11], for example, the current laser intensity has reached about 10 22 W=cm 2 [12], and the expected intensity to be 10 25 ∼ 10 26 W=cm 2 for many planned facilities [13,14]. ...
Enhanced dynamically assisted pair production in frequency chirped spatially inhomogeneous electric fields is studied using the Dirac-Heisenberg-Wigner formalism. The effects of the chirp parameter on the reduced momentum spectrum and the total particle yield are investigated in detail for the single field or the dynamically assisted combined fields at various spatial scales. For the single high frequency field and the combined fields, the interference effect of momentum spectrum becomes more and more remarkable with the increasing chirp, meanwhile, in the chirped dynamically assisted case, the reduced total yield is enhanced significantly when chirping of two fields are present. Specifically, in the assisted case, the reduced particle number is increased by more than one order of magnitude at the small spatial scale, while it is enhanced about two times at the large spatial scale. We also obtain some optimal chirp and spatial scale parameters for the total yield and the enhancement factor for various scenarios.
... The Schwinger effect [1], whereby nonperturbative quantum effects in a background electric field lead to electron-positron pair production, has received much attention (for example, see the reviews [2][3][4][5][6]). Heuristically, the electric field pulls apart the electron-positron pairs that are fluctuating in and out of the vacuum. ...
... We will evolve the quantum variables, Q a i , assuming that they are in their noninteracting ground state at t ¼ 0. We insert (4) in (2), then use the background (5), and expand to quadratic order in the Q a μ to obtain, ...
... We have considered the fate of a uniform color electric field in a pure gauge Yang-Mills theory with SU(2) gauge group. 5 Our approach assumes initial conditions with a uniform electric field and quantum excitations in their noninteracting ground state. We then truncate the excitations to quadratic order in the action and evolve the system using the CQC. ...
We study the Schwinger process in a uniform non-Abelian electric field using a dynamical approach in which we evolve an initial quantum state for gluonic excitations. We evaluate the spectral energy density and number density in the excitations as functions of time. The total energy density has an ultraviolet divergence which we argue gets tamed due to asymptotic freedom, leading to g4E4t4 growth, where g is the coupling and E the electric field strength. We also find an infrared divergence in the number density of excitations whose resolution requires an effect such as confinement.
