  In this paper, a novel quantum dialogue (QD) protocol is proposed based on
single photons in both polarization and spatial-mode degrees of freedom. In the
proposed QD protocol, the initial states of single photons in both polarization
and spatial-mode degrees of freedom used for encoding are privately shared
between two communicants through the direct transmissions of their auxiliary
counterparts from one communicant to another. As a result, the information
leakage problem is avoided. Moreover, the detailed security analysis also shows
that the proposed QD protocol can resist Eve's several famous active attacks,
such as the Trojan horse attack, the intercept-resend attack, the
measure-resend attack and the entangle-measure attack. The proposed QD protocol
only needs single photons in both polarization and spatial-mode degrees of
freedom as quantum resource and adopts single-photon measurements. As a result,
it is feasible in practice as the preparation and the measurement of a single
photon in both polarization and spatial-mode degrees of freedom can be
accomplished with current experimental techniques.
