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Proceedings Paper

Towards high-dimensional quantum key distribution over long-distance free-space links (Conference Presentation)
Author(s): Sebastian Ecker; Fabian Steinlechner; Matthias Fink; Bo Liu; Jessica Bavaresco; Marcus Huber; Thomas Scheidl; Rupert Ursin

Paper Abstract

The distribution of quantum entanglement between distant parties is a key challenge in the pursuit of a worldwide quantum network. Quantum repeaters and optical satellite links have both been proposed to overcome the distance limitations of fiber-based quantum networks. In order to test the viability of a world spanning quantum satellite network, several proof-of-concept studies have already demonstrated high-fidelity transmission of photonic entanglement via terrestrial long-distance free-space links. With the recent launch of the Micius quantum satellite by the Chinese Academy of Sciences, the technological challenge of implementing this scheme in situ was overcome. All of these free-space experiments utilized entanglement in a two-dimensional state space, which in most cases was encoded in the polarization of photons. However, high-dimensional entanglement can yield significant benefits in quantum key distribution by increasing the secure key rate and enhancing the resilience to eavesdropping attacks. Energy-time entanglement is an established photonic degree of freedom (DOF); it is routinely used for the distribution of entanglement in fiber-based quantum cryptography networks but has only recently been considered as a viable option for atmospheric free-space quantum communications. The dimensionality can be further increased by exploiting simultaneous entanglement in several DOF. This so-called hyperentanglement has previously been used in various quantum protocols, but not yet been demonstrated outside a protected laboratory environment. For our proof-of-concept experiment (doi:10.1038/ncomms15971), we use energy-time and polarization hyperentanglement to distribute, for the first time, 4-dimensional entanglement via a 1.2-km-long intra-city free-space link. A source of hyperentangled photons and a detection module (Alice) were located in our lab and a receiver station (Bob) was located at a different university building. The source produced polarization entangled photon pairs via type-0 down-conversion in a Sagnac loop configuration. The emission time of a photon pair is uncertain within the significantly longer coherence time of the pump laser, thus resulting in an energy-time and polarization hyperentangled state. One photon was guided to a local measurement module and the partner photon was guided to a transmitter telescope on the roof of the institute. After transmission over the 1.2-km-long free-space link, the photons were received by a telescope and detected in Bob’s measurement module. The measurement modules for Alice and Bob each featured a polarization and an energy-time analyzer. We observe high-visibility two-photon interference in both polarization and energy-time subspaces. The measured visibilities certify entanglement in both subspaces individually. Additionally, they establish a lower bound on the Bell-state fidelity of the hyperentangled state of 0.94, thus certifying genuine 4-dimensional entanglement and 1.47 ebits of entanglement of formation. We have thus successfully distributed hyperentangled photon pairs via an intra-city free-space link under conditions of strong atmospheric turbulence. The transmission of quantum information embedded in a genuine high-dimensional state space under real-world link conditions is a first important step towards real-world implementations of advanced quantum information processing protocols in the future. In particular, it enables the implementation of high-dimensional QKD protocols over long-distance free-space links, and, ultimately, over satellite links with only minor changes to existing mission proposals.

Paper Details

Date Published: 11 October 2018
Proc. SPIE 10799, Emerging Imaging and Sensing Technologies for Security and Defence III; and Unmanned Sensors, Systems, and Countermeasures, 107990E (11 October 2018); doi: 10.1117/12.2326600
Show Author Affiliations
Sebastian Ecker, Institut für Quantenoptik und Quanteninformation (Austria)
Fabian Steinlechner, Institut für Quantenoptik und Quanteninformation (Austria)
Matthias Fink, Institut für Quantenoptik und Quanteninformation (Austria)
Bo Liu, Institut für Quantenoptik und Quanteninformation (Austria)
Jessica Bavaresco, Institut für Quantenoptik und Quanteninformation (Austria)
Marcus Huber, Institut für Quantenoptik und Quanteninformation (Austria)
Thomas Scheidl, Institut für Quantenoptik und Quanteninformation (Austria)
Rupert Ursin, Institut für Quantenoptik und Quanteninformation (Austria)

Published in SPIE Proceedings Vol. 10799:
Emerging Imaging and Sensing Technologies for Security and Defence III; and Unmanned Sensors, Systems, and Countermeasures
Gerald S. Buller; Markus Mueller; Richard C. Hollins; Robert A. Lamb, Editor(s)

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