  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.
