An Entanglement Filter

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.