  The quantum key distribution protocol BB84, published by C. H. Bennett and G.
Brassard in 1984, describes how two spatially separated parties can generate a
random bit string fully known only to them by transmission of single-qubit
quantum states. Any attempt to eavesdrop on the protocol introduces disturbance
which can be detected by the legitimate parties. In this Master's Thesis a
novel modification to the BB84 protocol is analyzed. Instead of sending single
particles one-by-one as in BB84, they are grouped and a non-local
transformation is applied to each group before transmission. Each particle is
sent to the intended receiver, always delaying the transmission until the
receiver has acknowledged the previous particle on an authenticated classical
channel, restricting eavesdropping to accessing the quantum transmission one
particle at a time. Hence, an eavesdropper cannot undo the non-local
transformation perfectly. Even if perfect cloning of quantum states was
possible the state of the group could not be cloned. We calculate the maximal
information on the established key provided by an intercept-resend attack and
the induced disturbance for different transformations. We observe that it is
possible to significantly reduce the eavesdropper's maximal information on the
key--to one eighth of that in BB84 for a fixed, reasonable amount of
disturbance. We also show that the individual access to the particles poses a
fundamental restriction to the eavesdropper, and discuss a novel attack type
against the proposed protocol.
