We show that the quantum interference between down-converted photon pairs and photons from coherent laser light can produce a maximally path entangled $N$-photon output component with a fidelity greater than 90% for arbitrarily high photon numbers. A simple beam splitter operation can thus transform the two-photon coherence of down-converted light into an almost optimal $N$-photon coherence.

• Received 23 April 2007

DOI:https://doi.org/10.1103/PhysRevA.76.031806