  QED cascades play an important role in extreme astrophysical environments
like magnetars. They can also be produced by passing a relativistic electron
beam through an intense laser field. Signatures of collective pair plasma
effects in these QED cascades are shown to appear, in exquisite detail, through
plasma-induced frequency upshifts in the laser spectrum.
  Remarkably, these signatures can be detected even in small plasma volumes
moving at relativistic speeds. Strong-field quantum and collective pair plasma
effects can thus be explored with existing technology, provided that
ultra-dense electron beams are colocated with multi-PW lasers.
