Entanglement generation via diffraction

Quantum entanglement is an important resource for next-generation technologies. We show that diffracting systems can supplant beam splitters, and more generally interferometric networks, for entanglement generation -- systems as simple as screens with pinholes can create entanglement. We then discuss the necessary and sufficient conditions for entanglement to be generated by states input to any passive linear interferometric network. Entanglement generated in free space can now be harnessed in quantum-optical applications ranging from quantum computation and communication to quantum metrology.