  Although existing technology cannot yet directly produce fields at the
Schwinger level, experimental facilities can already explore strong-field QED
phenomena by taking advantage of the Lorentz boost of energetic electron beams.
Recent studies show that QED cascades can create electron-positron pairs at
sufficiently high density to exhibit collective plasma effects. Signatures of
the collective pair plasma effects can appear in exquisite detail through
plasma-induced frequency upshifts and chirps in the laser spectrum. Maximizing
the magnitude of the QED plasma signature demands high pair density and low
pair energy, which suits the configuration of colliding an over
$10^{18}{Jm^{-3}}$ energy-density electron beam with a
$10^{22}\mathrm{-}10^{23}{Wcm^{-2}}$ intensity laser pulse. The collision
creates pairs that have a large plasma frequency, made even larger as they slow
down or reverse direction due to both the radiation reaction and laser
pressure. This paper explains at a tutorial level the key properties of the QED
cascades and laser frequency upshift, and at the same time finds the minimum
parameters that can be used to produce observable QED plasma.
