  Sources of entanglement are an enabling resource in quantum technology, and
pushing the limits of generation rate and quality of entanglement is a
necessary pre-requisite towards practical applications. Here, we present an
ultra-bright source of polarization-entangled photon pairs based on
time-reversed Hong-Ou-Mandel interference. By superimposing four pair-creation
possibilities on a polarization beam splitter, pairs of identical photons are
separated into two spatial modes without the usual requirement for wavelength
distinguishability or non-collinear emission angles. Our source yields
high-fidelity polarization entanglement and high pair-generation rates without
any requirement for active interferometric stabilization, which makes it an
ideal candidate for a variety of applications, in particular those requiring
indistinguishable photons.

Quantum Physics