  Quantum key distribution (QKD) guarantees the secure communication between
legitimate parties with quantum mechanics. High-dimensional QKD (HDQKD) not
only increases the secret key rate but also tolerates higher quantum bit error
rate (QBER). Many HDQKD experiments have been realized by utilizing
orbital-angular-momentum (OAM) photons as the degree of freedom (DOF) of OAM of
the photon is a prospective resource for HD quantum information. In this work
we proposed and characterized that a high-quality HDQKD based on
polarization-OAM hybrid states can be realized by utilizing state mapping
between different DOFs. Both the preparation and measurement procedures of the
proof-of-principle verification experiment are simple and stable. Our
experiment verified that $(0.60\pm 0.06)\%$ QBER and $1.849\pm 0.008$ bits
secret key rate per sifted signal can be achieved for a four-dimensional QKD
with the weak coherent light source and decoy state method.
