  The zero-error capacity of a classical channel is expressed in terms of the
independence number of some graph and its tensor powers. This quantity is hard
to compute even for small graphs such as the cycle of length seven, so upper
bounds such as the Lovasz theta function play an important role in zero-error
communication. In this paper, we show that the Lovasz theta function is an
upper bound on the zero-error capacity even in the presence of entanglement
between the sender and receiver.
