  Quantum superdense coding protocols enhance channel capacity by using shared
quantum entanglement between two users. The channel capacity can be as high as
2 when one uses entangled qubits. However, this limit can be surpassed by using
high-dimensional entanglement. We report an experiment that exceeds the limit
using high-quality entangled ququarts with fidelities up to 0.98, demonstrating
a channel capacity of $2.09\pm0.01$. The measured channel capacity is also
higher than that obtained when transmitting only one ququart. We use the setup
to transmit a five-color image with a fidelity of 0.952. Our experiment shows
the great advantage of high-dimensional entanglement and will stimulate
research on high-dimensional quantum information processes.
