  Two-qubit quantum codes have been suggested to obtain better efficiency and
higher loss tolerance in quantum key distribution. Here, we propose a two-qubit
quantum key distribution protocol based on a mixed basis consisting of two Bell
states and two states from the computational basis. All states can be generated
from a single entangled photon pair resource by using local operations on only
one auxiliary photon. Compared to other schemes it is also possible to
deterministically discriminate all states using linear optics. Additionally,
our protocol can be implemented with today's technology. When discussing the
security of our protocol we find a much improved resistance against certain
attacks as compared to the standard BB84 protocol.
