  In a photonic realization of qubits the implementation of quantum logic is
rather difficult due the extremely weak interaction on the few photon level. On
the other hand, in these systems interference is available to process the
quantum states. We formalize the use of interference by the definition of a
simple class of operations which include linear optical elements, auxiliary
states and conditional operations.
  We investigate an important subclass of these tools, namely linear optical
elements and auxiliary modes in the vacuum state. For this tools, we are able
to extend a previous quantitative result, a no-go theorem for perfect Bell
state analyzer on two qubits in polarization entanglement, by a quantitative
statement. We show, that within this subclass it is not possible to
discriminate unambiguously four equiprobable Bell states with a probability
higher than 50 %.
