  A beam splitter is a simple, readily available device which can act to
entangle the output optical fields. We show that a necessary condition for the
fields at the output of the beam splitter to be entangled is that the pure
input states exhibit nonclassical behavior. We generalize this proof for
arbitrary (pure or impure) Gaussian input states. Specifically, nonclassicality
of the input Gaussian fields is a necessary condition for entanglement of the
field modes with the help of the beam splitter. We conjecture that this is a
general property of the beam splitter: Nonclassicality of the inputs is a
necessary condition for entangling fields in the beam splitter.
