Additional Information:
Funding details: Engineering and Physical Sciences Research Council, EPSRC, EP/M018156/1 Funding details: W911NF-16-1-0044 Funding details: Science and Technology Facilities Council, STFC, ST/G008248/1 Funding text 1: D.S. acknowledges valuable discussions with J. Gratus, T. Heinzl, A. Ilderton, B. King, and M. Korostelev, and support from the Science and Technology Facilities Council, Grant No. ST/G008248/1. C.P.R. and D.D.S. acknowledge support from Engineering and Physical Sciences Grant No. EP/M018156/1. A.G.R.T. acknowledges support from U. S. DOD under Grant No. W911NF-16-1-0044. APPENDIX A: References: Patrignani, C., The review of particle physics (2016) Chin. Phys. C, 40, p. 100001; David Jackson, J., (1983) Klassische Elektrodynamik, , 2nd ed. (Walter de Gruyter, Berlin); Anselmino, M., Efremov, A., Leader, E., The theory and phenomenology of polarized deep inelastic scattering (1995) Phys. Rep., 261, p. 1; Steven, D., Bass, The Proton Spin Puzzle: Where Are We Today? (2009) Mod. Phys. Lett. A, 24, p. 1087; Prescott, C.Y., Atwood, W.B., Cottrell, R.L.A., DeStaebler, H., Garwin, E.L., Gonidec, A., Miller, R.H., Jentschke, W., Parity non-conservation in inelastic electron scattering (1978) Phys. Lett. B, 77, p. 347; Labzowsky, L.N., Nefiodov, A.V., Plunien, G., Soff, G., Marrus, R., Liesen, D., Parity-violation effect in heliumlike gadolinium and europium (2001) Phys. Rev. A, 63, p. 054105; Vauth, A., List, J., Beam polarization at the ilc: Physics case and realization (2016) Int. J. Mod. Phys. Conf. Ser., 40, p. 1660003; Mane, S.R., Shatunov, Yu.M., Yokoya, K., Spin-polarized charged particle beams in high-energy accelerators (2005) Rep. Prog. Phys., 68, p. 1997; Subashiev, A.V., Mamaev, Yu.A., Yashin, Yu.P., Clendenin, J.E., Spin polarized electrons: Generation and applications (1999) Phys. Low-Dim. Struct., 1, p. 1; Mamaev, Yu.A., Gerchikov, L.G., Yashin, Yu.P., Vasiliev, D.A., Kuzmichev, V.V., Ustinov, V.M., Zhukov, A.E., Vasiliev, A.P., Optimized photocathode for spin-polarized electron sources (2008) Appl. Phys. Lett., 93, p. 081114; Sokolov, A.A., Ternov, I.M., (1968) Synchrotron Radiation, , (Akademie Verlag, Berlin); Ternov, I.M., Synchrotron radiation (1995) Phys. Usp., 38, p. 409; Esarey, E., Schroeder, C.B., Leemans, W.P., Physics of laser-driven plasma-based electron accelerators (2009) Rev. Mod. Phys., 81, p. 1229; Vieira, J., Huang, C.-K., Mori, W.B., Silva, L.O., Polarized beam conditioning in plasma based acceleration (2011) Phys. Rev. Spec. Top.-Accel. Beams, 14, p. 071303; Del Sorbo, D., Seipt, D., Blackburn, T.G., Thomas, A.G.R., Murphy, C.D., Kirk, J.G., Ridgers, C.P., Spin polarization of electrons by ultraintense lasers (2017) Phys. Rev. A, 96, p. 043407; Del Sorbo, D., Seipt, D., Thomas, R.A.G., Ridgers, C.P., Electron spin polarization in realistic trajectories around the magnetic node of two counter-propagating, circularly polarized, ultra-intense lasers (2018) Plasma Phys. Controlled Fusion, 60, p. 064003; Cole, J.M., Behm, K.T., Gerstmayr, E., Blackburn, T.G., Wood, J.C., Baird, C.D., Duff, M.J., Symes, D.R., Experimental Evidence of Radiation Reaction in the Collision of a High-Intensity Laser Pulse with a Laser-Wakefield Accelerated Electron Beam (2018) Phys. Rev. X, 8, p. 011020; Poder, K., Tamburini, M., Sarri, G., Di Piazza, A., Kuschel, S., Baird, C.D., Behm, K., Symes, D., Evidence of Strong Radiation Reaction in the Field of an Ultra-Intense Laser (2018) Phys. Rev. X, 8, p. 031004; Kirsebom, K., Mikkelsen, U., Uggerhøj, E., Elsener, K., Ballestrero, S., Sona, P., Vilakazi, Z.Z., First Measurements of the Unique Influence of Spin on the Energy Loss of Ultrarelativistic Electrons in Strong Electromagnetic Fields (2001) Phys. Rev. Lett., 87, p. 054801; Panek, P., Kamiński, J.Z., Ehlotzky, F., Laser-induced Compton scattering at relativistically high radiation powers (2002) Phys. Rev. A, 65, p. 022712; Dumlu, C.K., Dunne, G.V., Interference effects in Schwinger vacuum pair production for time-dependent laser pulses (2011) Phys. Rev. D, 83, p. 065028; Boca, M., Dinu, V., Florescu, V., Spin effects in nonlinear Compton scattering in a plane-wave laser pulse (2012) Nucl. Instruments Methods Phys. Res., Sect. B, 279, p. 12; Jansen, M.J.A., Kamiński, J.Z., Krajewska, K., Müller, C., Strong-field Breit-Wheeler pair production in short laser pulses: Relevance of spin effects (2016) Phys. Rev. D, 94, p. 013010; Bagrov, V.G., Fedosov, N.I., Kopytov, G.F., Oxsyzyan, S.S., Tlyachev, V.B., Radiational self-polarization of electrons moving in the electromagnetic plane-wave field (1989) Nuovo Cimento B Ser. 11, 103, p. 549; Bol'Shedvorsky, E., Polityko, S., Misaki, A., Spin of scattered electrons in the nonlinear Compton effect (2000) Prog. Theor. Phys., 104, p. 769; Bolshedvorsky, E.M., Polityko, S.I., Polarization of the final electron in the field of an intense electromagnetic wave (2000) Russ. Phys. J., 43, p. 913; Yu Ivanov, D., Kotkin, G.L., Serbo, V.G., Complete description of polarization effects in emission of a photon by an electron in the field of a strong laser wave (2004) Eur. Phys. J. C, 36, p. 127; Karlovets, D.V., Radiative polarization of electrons in a strong laser wave (2011) Phys. Rev. A, 84, p. 062116; Krajewska, K., Kamiński, J.Z., Spin effects in nonlinear Compton scattering in ultrashort linearly-polarized laser pulses (2013) Laser Part. Beams, 31, p. 503; Krajewska, K., Kamiński, J.Z., Frequency scaling law for nonlinear Compton and Thomson scattering: Relevance of spin and polarization effects (2014) Phys. Rev. A, 90, p. 052117; King, B., Elkina, N., Ruhl, H., Photon polarization in electron-seeded pair-creation cascades (2013) Phys. Rev. A, 87, p. 042117; King, B., Double Compton scattering in a constant crossed field (2015) Phys. Rev. A, 91, p. 033415; Meuren, S., Di Piazza, A., Quantum Electron Self-Interaction in a Strong Laser Field (2011) Phys. Rev. Lett., 107, p. 260401; Ahrens, S., Bauke, H., Keitel, C.H., Müller, C., Spin Dynamics in the Kapitza-Dirac Effect (2012) Phys. Rev. Lett., 109, p. 043601; Barth, I., Smirnova, O., Spin-polarized electrons produced by strong-field ionization (2013) Phys. Rev. A, 88, p. 013401; Klaiber, M., Hatsagortsyan, K.Z., Spin-asymmetric laser-driven relativistic tunneling from (Equation presented) states (2014) Phys. Rev. A, 90, p. 063416; Zille, D., Seipt, D., Möller, M., Fritzsche, S., Gräfe, S., Müller, C., Paulus, G.G., Spin-dependent rescattering in strong-field ionization of helium (2017) J. Phys. B: At., Mol. Opt. Phys., 50, p. 065001; Zille, D., Seipt, D., Möller, M., Fritzsche, S., Paulus, G.G., Milošević, D.B., Spin-dependent quantum theory of high-order above-threshold ionization (2017) Phys. Rev. A, 95, p. 063408; Dellweg, M.M., Müller, C., Controlling electron spin dynamics in bichromatic Kapitza-Dirac scattering by the laser field polarization (2017) Phys. Rev. A, 95, p. 042124; Nikishov, A.I., Ritus, V.I., Quantum processes in the field of a plane electromagnetic wave and in a constant field. i (1964) Zh. Eksp. Teor. Fiz., 46, p. 776; Nikishov, A.I., Ritus, V.I., (1964) Sov. Phys. JETP, 19, p. 529. , [, ()]; Nikishov, A.I., Ritus, V.I., Quantum processes in the field of a plane electromagnetic wave and in a constant field (1964) Zh. Eksp. Teor. Fiz., 46, p. 1768; Nikishov, A.I., Ritus, V.I., (1964) Sov. Phys. JETP, 19, p. 1191. , [, ()]; Goldman, I.I., Intensity effects in Compton scattering (1964) Phys. Lett., 8, p. 103; Brown, L.S., Kibble, T.W.B., Interaction of intense laser beams with electrons (1964) Phys. Rev., 133, p. A705; Nikishov, A.I., Ritus, V.I., Nonlinear effects in Compton scattering and pair production owing to absorption of several photons (1964) Zh. Eksp. Teor. Fiz., 47, p. 1130; Nikishov, A.I., Ritus, V.I., (1965) Sov. Phys. JETP, 20, p. 757. , [, ()]; Narozhnyi, N.B., Nikishov, A.I., Ritus, V.I., Quantum processes in the field of a circularly polarized electromagnetic wave (1964) Zh. Eksp. Teor. Fiz., 47, p. 930; Narozhnyi, N.B., Nikishov, A.I., Ritus, V.I., (1965) Sov. Phys. JETP, 20, p. 622. , [, ()]; Seipt, D., Kämpfer, B., Non-linear Compton scattering of ultrashort and intense laser pulses (2011) Phys. Rev. A, 83, p. 022101; Wolkow, D.M., Über eine Klasse von Lösungen der Diracschen Gleichung (1935) Z. Phys., 94, p. 250; Raicher, E., Eliezer, S., Analytical solutions of the Dirac and the Klein-Gordon equations in plasma induced by high-intensity laser (2013) Phys. Rev. A, 88, p. 022113; King, B., Hu, H., Classical and quantum dynamics of a charged scalar particle in a background of two counterpropagating plane waves (2016) Phys. Rev. D, 94, p. 125010; Heinzl, T., Ilderton, A., King, B., Classical and quantum particle dynamics in univariate background fields (2016) Phys. Rev. D, 94, p. 065039; Ilderton, A., Torgrimsson, G., Scattering in plane-wave backgrounds: Infrared effects and pole structure (2013) Phys. Rev. D, 87, p. 085040; Berestetskii, V., Lifshitz, E., Pitaevskii, L., (1982) Quantum Electrodynamics, , Course of Theoretical Physics, 4 (Butterworth, Washington, DC); Mitter, H., Quantum Electrodynamics in Laser Fields (1975) Acta Phys. Austriaca, Suppl. XIV, p. 397; Ritus, V.I., Quantum effects of the interaction of elementary particles with an intense electromagnetic field (1985) J. Sov. Laser Res., 6, p. 497; Harvey, C., Heinzl, T., Ilderton, A., Signatures of high-intensity Compton scattering (2009) Phys. Rev. A, 79, p. 063407; Seipt, D., Kharin, V., Rykovanov, S., Surzhykov, A., Fritzsche, S., Analytical results for nonlinear Compton scattering in short intense laser pulses (2016) J. Plasma Phys., 82, p. 655820203; Fradkin, D.M., Good, R.H., Electron polarization operators (1961) Rev. Mod. Phys., 33, p. 343; Bauke, H., Ahrens, S., Keitel, C.H., Grobe, R., Electron-spin dynamics induced by photon spins (2014) New J. Phys., 16, p. 103028; Bliokh, K.Y., Dennis, M.R., Nori, F., Position, spin, and orbital angular momentum of a relativistic electron (2017) Phys. Rev. A, 96, p. 023622; Itzykson, C., Zuber, J.-B., (1980) Quantum Field Theory, , (McGraw-Hill, New York); Lorcé, C., New explicit expressions for Dirac bilinears (2018) Phys. Rev. D, 97, p. 016005; Bargmann, V., Michel, L., Telegdi, V.L., Precession of the Polarization of Particles Moving in a Homogeneous Electromagnetic Field (1959) Phys. Rev. Lett., 2, p. 435; Spohn, H., Semiclassical limit of the Dirac equation and spin precession (2000) Ann. Phys. (NY), 282, p. 420; Bulanov, S.S., Schroeder, C.B., Esarey, E., Leemans, W.P., Electromagnetic cascade in high-energy electron, positron, and photon interactions with intense laser pulses (2013) Phys. Rev. A, 87, p. 062110; Blum, K., (2012) Density Matrix Theory and Applications, , 3rd ed., Springer Series on Atomic, Optical, and Plasma Physics, 64 (Springer, Berlin); Baier, V.N., Katkov, V.M., Strakhovenko, V.M., (1998) Electromagnetic Processes at High Energies in Oriented Single Crystals, , (World Scientific, Singapore); Balashov, V.V., Grum-Grzhimailo, A.N., Kabachnik, N.M., (2000) Polarization and Correlation Phenomena in Atomic Collisions, , (Springer, Berlin); Surzhykov, A., Fritzsche, S., Stöhlker, T., Effects of the target polarization on the diagnostics of spin-polarized heavy ions (2007) Radiat. Phys. Chem., 76, p. 392; Lötstedt, E., Jentschura, U.D., Nonperturbative Treatment of Double Compton Backscattering in Intense Laser Fields (2009) Phys. Rev. Lett., 103, p. 110404; Kämpfer, T., Uschmann, I., Wu, Z.W., Surzhykov, A., Fritzsche, S., Förster, E., Paulus, G.G., Linear polarization of the characteristic x-ray lines following inner-shell photoionization of tungsten (2016) Phys. Rev. A, 93, p. 033409; Peshkov, A.A., Seipt, D., Surzhykov, A., Fritzsche, S., Photoexcitation of atoms by Laguerre-Gaussian beams (2017) Phys. Rev. A, 96, p. 023407; Stock, S., Surzhykov, A., Fritzsche, S., Seipt, D., Compton scattering of twisted light: Angular distribution and polarization of scattered photons (2015) Phys. Rev. A, 92, p. 013401; Angioi, A., Mackenroth, F., Di Piazza, A., Nonlinear single Compton scattering of an electron wave packet (2016) Phys. Rev. A, 93, p. 052102; Thomas, A.G.R., Algorithm for calculating spectral intensity due to charged particles in arbitrary motion (2010) Phys. Rev. Spec. Top.-Accel. Beams, 13, p. 020702; Ahrens, S., Sun, C.-P., Spin in Compton scattering with pronounced polarization dynamics (2017) Phys. Rev. A, 96, p. 063407; Esarey, E., Ride, S.K., Sprangle, P., Nonlinear Thomson scattering of intense laser pulses from beams and plasmas (1993) Phys. Rev. e, 48, p. 3003; Kotkin, G.L., Serbo, V.G., Telnov, V.I., Electron (positron) beam polarization by Compton scattering on circularly polarized laser photons (2003) Phys. Rev. Spec. Top.-Accel. Beams, 6, p. 011001; Seipt, D., Kämpfer, B., Asymmetries of azimuthal photon distributions in nonlinear Compton scattering in ultrashort intense laser pulses (2013) Phys. Rev. A, 88, p. 012127; Titov, A.I., Kämpfer, B., Hosaka, A., Nousch, T., Seipt, D., Determination of the carrier envelope phase for short, circularly polarized laser pulses (2016) Phys. Rev. D, 93, p. 045010; Seipt, D., Kämpfer, B., Two-photon Compton process in pulsed intense laser fields (2012) Phys. Rev. D, 85, p. 101701; Ilderton, A., Torgrimsson, G., Radiation reaction from QED: Lightfront perturbation theory in a plane wave background (2013) Phys. Rev. D, 88, p. 025021; Dinu, V., Harvey, C., Ilderton, A., Marklund, M., Torgrimsson, G., Quantum Radiation Reaction: From Interference to Incoherence (2016) Phys. Rev. Lett., 116, p. 044801; Ridgers, C.P., Kirk, J.G., Duclous, R., Blackburn, T.G., Brady, C.S., Bennett, K., Arber, T.D., Bell, A.R., Modelling gamma-ray photon emission and pair production in high-intensity laser-matter interactions (2014) J. Comput. Phys., 260, p. 273; Gonoskov, A., Bastrakov, S., Efimenko, E., Ilderton, A., Marklund, M., Meyerov, I., Muraviev, A., Wallin, E., Extended particle-in-cell schemes for physics in ultrastrong laser fields: Review and developments (2015) Phys. Rev. e, 92, p. 023305; Green, D.G., Harvey, C.N., SIMLA: Simulating particle dynamics in intense laser and other electromagnetic fields via classical and quantum electrodynamics (2015) Comput. Phys. Commun., 192, p. 313; Harvey, C.N., Ilderton, A., King, B., Testing numerical implementations of strong-field electrodynamics (2015) Phys. Rev. A, 91, p. 013822; Blackburn, T.G., Seipt, D., Bulanov, S.S., Marklund, M., Benchmarking semiclassical approaches to strong-field QED: Nonlinear Compton scattering in intense laser pulses (2018) Phys. Plasmas, 25, p. 083108; Dinu, V., Exact final-state integrals for strong-field QED (2013) Phys. Rev. A, 87, p. 052101; Di Piazza, A., Tamburini, M., Meuren, S., Keitel, C.H., Implementing nonlinear Compton scattering beyond the local constant field approximation (2018) Phys. Rev. A, 98, p. 012134; Elkina, N.V., Fedotov, A.M., Yu Kostyukov, I., Legkov, M.V., Narozhny, N.B., Nerush, E.N., Ruhl, H., QED cascades induced by circularly polarized laser fields (2011) Phys. Rev. Spec. Top.-Accel. Beams, 14, p. 054401; Milošević, D.B., Paulus, G.G., Bauer, D., Becker, W., Above-threshold ionization by few-cycle pulses (2006) J. Phys. B: At., Mol. Opt. Phys., 39, p. R203; Baier, V.N., Katkov, V.M., Radiative Polarization of Electrons in a Magnetic Field (1967) Zh. Eksp. Teor. Fiz., 52, p. 1422; Baier, V.N., Katkov, V.M., (1967) Sov. Phys. JETP, 25, p. 944. , [, ()]; Ritus, V.I., Radiative corrections in quantum electrodynamics with intense field and their analytical properties (1972) Ann. Phys. (NY), 69, p. 555; Kibble, T.W.B., Salam, A., Strathdee, J., Intensity-dependent mass shift and symmetry breaking (1975) Nucl. Phys. B, 96, p. 255; Harvey, C., Heinzl, T., Ilderton, A., Marklund, M., The Intensity Dependent Mass Shift: Existence, Universality and Detection (2012) Phys. Rev. Lett., 109, p. 100402; Di Piazza, A., Completeness and orthonormality of the Volkov states and the Volkov propagator in configuration space (2018) Phys. Rev. D, 97, p. 056028; Hebenstreit, F., Ilderton, A., Marklund, M., Zamanian, J., Strong field effects in laser pulses: The Wigner formalism (2011) Phys. Rev. D, 83, p. 065007