Quantum process tomography of a high-dimensional quantum communication channel

Frédéric Bouchard1, Felix Hufnagel1, Dominik Koutný2, Aazad Abbas1, Alicia Sit1, Khabat Heshami3, Robert Fickler1,4,5, and Ebrahim Karimi1,6

1Department of Physics, University of Ottawa, 25 Templeton Street, Ottawa, ON, K1N 6N5, Canada.
2Department of Optics, Palacký University, 17. listopadu 12, 771 46 Olomouc, Czech Republic.
3National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario K1A 0R6, Canada
4Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, A-1090 Vienna, Austria.
5Current address: Photonics Laboratory, Physics Unit, Tampere University, Tampere, FI-33720, Finland.
6Department of Physics, Institute for Advanced Studies in Basic Sciences, 45137-66731 Zanjan, Iran.

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Abstract

The characterization of quantum processes, e.g. communication channels, is an essential ingredient for establishing quantum information systems. For quantum key distribution protocols, the amount of overall noise in the channel determines the rate at which secret bits are distributed between authorized partners. In particular, tomographic protocols allow for the full reconstruction, and thus characterization, of the channel. Here, we perform quantum process tomography of high-dimensional quantum communication channels with dimensions ranging from 2 to 5. We can thus explicitly demonstrate the effect of an eavesdropper performing an optimal cloning attack or an intercept-resend attack during a quantum cryptographic protocol. Moreover, our study shows that quantum process tomography enables a more detailed understanding of the channel conditions compared to a coarse-grained measure, such as quantum bit error rates. This full characterization technique allows us to optimize the performance of quantum key distribution under asymmetric experimental conditions, which is particularly useful when considering high-dimensional encoding schemes.

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