  An unsymmetrical quantum key distribution scheme is proposed, its security is
guaranteed by the correlation of the Greenberger-Horne-Zeilinger triplet state.
In the proposed protocol, the distribution of quantum states are unsymmetrical.
This unsymmetrical characteristic makes the transmission qubits (except the
loss qubits) be useful. Sequentially the proposed protocol has excellent
efficiency and security which are very useful in the practical application.
