It is argued that the title of this paper represents a misconception. Contrary to widespread beliefs it is electromagnetic field modes that are ``systems'' and can be entangled, not photons. The amount of entanglement in a given state is shown to depend on redefinitions of the modes; we calculate the minimum and maximum over all such redefinitions for several examples.
@article{arxiv.quant-ph/0206135,
title = {Entanglement of photons},
author = {S. J. van Enk},
journal= {arXiv preprint arXiv:quant-ph/0206135},
year = {2009}
}