A bell-state measurement is the holy grail of QKD. It can only be done 25% of the time and with great difficulty 50% of the time. So your channel only works 25%

A bell-state measurement is the holy grail of QKD. It can only be done 25% of the time and with great difficulty 50% of the time. So your channel only works 25% of the time! For entanglement in a single qubit variable, only three distinct classes out of four Bell states are distinguishable using such linear optical techniques. This means two Bell states cannot be distinguished from each other, limiting the efficiency of quantum communication protocols such as teleportation. If a Bell state is measured from this ambiguous class, the teleportation event fails. >>http://en.wikipedia.org/wiki/Bell_state#Bell_state_measurement http://en.wikipedia.org/wiki/Bell_state#Bell_state_measureme...