“…While already actively being searched for in experiments using macroscopic magnetic fields in combination with continuous-wave lasers and high-finesse cavities [23][24][25], various recent theoretical studies have emphasized the possibility of its verification in an all-optical experiment, colliding an X-ray [26][27][28][29][30][31] or gamma-ray probe [32][33][34][35][36] with a high-intensity laser pulse. Other theoretical proposals have focused, e.g., on vacuum nonlinearity induced photon scattering phenomena in laser pulse collisions [37][38][39][40][41], interference effects [42][43][44], laser mode self-mixing [45], quantum reflection [46], higher-harmonic generation in an electromagnetized vacuum [47][48][49][50][51][52][53][54], photon splitting [21,22,[55][56][57][58][59][60][61] and photon merging [62][63][64][65].…”