  The ability to filter quantum states is a key capability in quantum
information science and technology, in which one-qubit filters, or polarizers,
have found wide application. Filtering on the basis of entanglement requires
extension to multi-qubit filters with qubit-qubit interactions. We demonstrated
an optical entanglement filter that passes a pair of photons if they have the
desired correlations of their polarization. Such devices have many important
applications to quantum technologies.
