  We propose a high-efficiency three-party quantum key agreement protocol, by
utilizing two-photon polarization-entangled Bell states and a few single-photon
polarization states as the information carriers, and we use the quantum dense
coding method to improve its efficiency. In this protocol, each participant
performs one of four unitary operations to encode their sub-secret key on the
passing photons which contain two parts, the first quantum qubits of Bell
states and a small number of single-photon states. At the end of this protocol,
based on very little information announced by other, all participants involved
can deduce the same final shared key simultaneously. We analyze the security
and the efficiency of this protocol, showing that it has a high efficiency and
can resist both outside attacks and inside attacks. As a consequence, our
protocol is a secure and efficient three-party quantum key agreement protocol.
