Probing QED Vacuum with Heavy Ions
  We recall how nearly half a century ago the proposal was made to explore the structure of the quantum vacuum using slow heavy-ion collisions. Pursuing this topic we review the foundational concept of spontaneous vacuum decay accompanied by observable positron emission in heavy-ion collisions and describe the related theoretical developments in strong fields QED. 
Johann Rafelski, Johannes Kirsch, Berndt M\"uller, Joachim Reinhardt,   and Walter Greiner
nucl-th hep-ph physics.atom-ph