QED's nonlinear light
ARTUR K. EKERT, Sarah Law · Physics World · 1990
It was in the 1930s that quantum electrodynamicists demonstrated that virtural particles should mediate photon-photon interactions in a vacuum. Hitherto, the resulting nonlinear phenomena had seemed undetectable given technological limitations. But now, Y J Ding and A E Kaplan from John Hopkins University, Baltimore, have demonstrated that new nonlinear magneto optical effects, giving rise to optical secondharmonic generation, could provide a signature of nonlinear behaviour that is detectable with state-of-the-art laser beams and photon detectors (1989 Phys. Rev. Lett. 63 2752)