  A process of preparation, transmission and subsequent projective measurement
of a qubit can be simulated by a classical model with only two bits of
communication and some amount of shared randomness. However no model for n
qubits with a finite amount of classical communication is known at present. A
lower bound for the communication cost can provide useful hints for a
generalization. It is known for example that the amount of communication must
be greater than c 2^n, where c~0.01. The proof uses a quite elaborate theorem
of communication complexity. Using a mathematical conjecture known as the
"double cap conjecture", we strengthen this result by presenting a geometrical
and extremely simple derivation of the lower bound 2^n-1. Only rank-1
projective measurements are involved in the derivation.
