  Relativistic spin-polarized positron beams are indispensable for future
electron-positron colliders to test modern high-energy physics theory with high
precision. However, present techniques require very large scale facilities for
those experiments.
  We put forward a novel efficient way for generating ultrarelativistic
polarized positron beams employing currently available laser fields. For this
purpose the generation of polarized positrons via multiphoton Breit-Wheeler
pair production and the associated spin dynamics in single-shot interaction of
an ultraintense laser pulse with an ultrarelativistic electron beam is
investigated in the quantum radiation-dominated regime. A specifically tailored
small ellipticity of the laser field is shown to promote splitting of the
polarized particles along the minor axis of laser polarization into two
oppositely polarized beams. In spite of radiative de-polarization, a dense
positron beam with up to about 90\% polarization can be generated in tens of
femtoseconds. The method may eventually usher high-energy physics studies into
smaller-scale laser laboratories.
