Abstract
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.
Highlights
Long-distance distribution of quantum entanglement[1,2,3,4,5,6] is essential for quantum communications
The state after fibre transmission is evaluated by using the quantum state tomography (QST) scheme proposed by the authors, with which we can significantly reduce the complexity of the experimental procedure[36]
The pump pulses were launched into another periodically poled lithium niobate (PPLN) waveguide to create a four-dimensional time-bin maximally entangled state via spontaneous parametric down-conversion (SPDC)
Summary
Long-distance distribution of quantum entanglement[1,2,3,4,5,6] is essential for quantum communications. Entangled qudits can be used to overcome the channel capacity limit for linear photonic superdense coding[28] They enable us to perform high-dimensional quantum key distribution. Using high-dimensional entanglement, we can decrease the threshold of the symbol error rate and increase the information capacity of a secure channel[29,30], which has been demonstrated with a free-space optical setup in the laboratory[31] For such quantum communications over a communications network, it is necessary to distribute high-dimensional entanglement. Filtering to generate the maximally entangled state reduces the generation rate of the entanglement, leading to a longer measurement time Another problem is qudit degradation caused by various disturbances in a transmission channel. The results indicate that the measured photon pair has a secure information capacity of more than 1 bit even after the distribution over 100 km
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