  Photon entanglement is an essential ingredient for linear optics quantum
computing schemes, quantum cryptographic protocols and fundamental tests of
quantum mechanics. Here we describe a setup that allows for the generation of
polarisation-entangled N-photon states on demand. The photons are obtained by
mapping the entangled state of N atoms, each of them trapped inside an optical
cavity, onto the free radiation field. The required initial state can be
prepared by performing postselective measurements on the collective emission
from the cavities through a multiport beamsplitter.

Quantum Physics