Combine a beam splitter with two polarizing filters.
EDIT: I read the relevant paragraph, and what I described above probably doesn’t work. However, not all is lost: there is a device that more precisely does what Eliezer says: Beam-splitting polarizers.
Combine a beam splitter with two polarizing filters.
EDIT: I read the relevant paragraph, and what I described above probably doesn’t work. However, not all is lost: there is a device that more precisely does what Eliezer says: Beam-splitting polarizers.