nonlinear, effective couplings between electromagnetic fields. While this has been realized theoretically already in the early days of QED [@Heisenberg:1935qt; @Weisskopf], the pure electromagnetic nonlinearity of the quantum vacuum still awaits its direct experimental verification on macroscopic scales.

The advent and planning of high-intensity laser facilities of the petawatt class has triggered a huge interest in ideas and proposals to probe quantum vacuum nonlinearities in realistic all-optical experimental set-ups; for recent reviews, see [@Dittrich:2000zu; @Marklund:2008gj; @Dunne:2008kc; @Heinzl:2008an; @DiPiazza:2011tq]. Typical examples are proposals intended to verify vacuum birefringence [@Toll:1952; @Baier; @BialynickaBirula:1970vy; @Adler:1971wn] that can be searched for using macroscopic magnetic fields [@Cantatore:2008zz; @Berceau:2011zz] or with the aid of high-intensity lasers [@Heinzl:2006xc], see also [@Dinu:2013gaa]. Alternative concepts suggest the use of time-varying fields and high-precision interferometry [@Zavattini:2008cr; @Dobrich:2009kd; @Grote:2014hja]. Other commonly studied nonlinear vacuum effects are direct light-by-light scattering [@Euler:1935zz; @Karplus:1950zz], photon splitting [@Adler:1971wn], and spontaneous vacuum decay in terms of Schwinger pair-production in electric fields [@Sauter:1931zz; @Heisenberg:1935qt; @Schwinger:1951nm]. Further optical signatures of quantum vacuum nonlinearities are those based on interference effects [@King:2013am; @Tommasini:2010fb; @Hatsagortsyan:2011], photon-photon scattering in the form of laser-pulse collisions [@King:2012aw; @Lundin:2006wu], quantum reflection [@Gies:2013yxa], photon merging [@Gies:2014jia], and harmonic generation from laser-driven vacuum [@DiPiazza:2005jc; @Fedotov:2006ii]. Related effects have 