  Modulating the polarization of a beam of quantum particles is a powerful
method to tailor the macroscopic properties of the ensuing energy flux as it
directly influences the way in which its quantum constituents interact with
other particles, waves or continuum media. Practical polarizers, being well
developed for electric and electromagnetic energy, have not been proposed to
date for heat fluxes carried by phonons. Here we report on atomistic phonon
transport calculations demonstrating that ferroelectric domain walls can
operate as phonon polarizers when a heat flux pierces them. Our simulations for
representative ferroelectric perovskite PbTiO$_3$ show that the structural
inhomogeneity associated to the domain walls strongly suppresses transverse
phonons, while longitudinally polarized modes can travel through multiple walls
in series largely ignoring their presence.
