  High-dimensional quantum entanglement can enrich the functionality of quantum
information processing. For example, it can enhance the channel capacity for
linear optic superdense coding and decrease the error rate threshold of quantum
key distribution. Long-distance distribution of a high-dimensional entanglement
is essential for such advanced quantum communications over a communications
network. Here, we show a long-distance distribution of a four-dimensional
entanglement. We employ time-bin entanglement, which is suitable for a fibre
transmission, and implement scalable measurements for the high-dimensional
entanglement using cascaded Mach-Zehnder interferometers. We observe that a
pair of time-bin entangled photons has more than 1 bit of secure information
capacity over 100 km. Our work constitutes an important step towards secure and
dense quantum communications in a large Hilbert space.
