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Measurement Sensors

Niedersachsen quantum communications testbed

Jaffar Emad Kadum, Ann‐Kathrin Kniggendorf, Alexander Kühl, Ali Hreibi, Shambo Mukherjee, Jochen Kronjäger, S. Kück

Published Jan 21, 2025• 1 citation
Publisher

Abstract

Quantum key distribution (QKD) is a prominent technology for exchanging secure keys between two authenticated remote parties. The security in a QKD system is unconditionally ensured by the laws of quantum mechanics which guarantee that any eavesdropping attempts will be identified. In this paper, we report the concept and structure of the Niedersachsen Quantum Link as a QKD testbed. The testbed consists of a 78 km long pair of dark fibers between the cities of Hannover and Braunschweig and is operated by Physikalisch-Technische Bundesanstalt and Leibniz Universität Hannover. One of the dark fibers is reserved for “quantum” applications, while the second fiber is dedicated to the dissemination of time and frequency reference signals as well as classical communication between two sites. This combination of dark “quantum” fiber and integrated time-frequency infrastructure is a unique feature of the Niedersachsen Quantum Link testbed.

DOI
10.1016/j.measen.2024.101774

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Quantum key distribution (QKD) is a prominent technology for exchanging secure keys between two authenticated remote parties. The security in a QKD system is unconditionally ensured by the laws of quantum mechanics which guarantee that any eavesdropping attempts will be identified. In this paper, we report the concept and structure of the Niedersachsen Quantum Link as a QKD testbed. The testbed consists of a 78 km long pair of dark fibers between the cities of Hannover and Braunschweig and is operated by Physikalisch-Technische Bundesanstalt and Leibniz Universität Hannover. One of the dark fibers is reserved for “quantum” applications, while the second fiber is dedicated to the dissemination of time and frequency reference signals as well as classical communication between two sites. This combination of dark “quantum” fiber and integrated time-frequency infrastructure is a unique feature of the Niedersachsen Quantum Link testbed.
We reconfigured an existing time and frequency dissemination (TFD) testbed on a deployed optical fibre pair for TFD and quantum key distribution (QKD) experiments using a separable fibre configuration to allow for one fibre to go dark while controlled TFD and optical communication for instrument control remains available via the second fibre. A lab-based model was used to establish and test the setup before deployment. Fractional frequency instability of the TFD available in separated fibre configuration was shown to be below 2 × 10 −13 at 1 s and at 1 × 10 −17 for 10 5 s averaging, while the looped-back configuration allows for an interferometric fibre link with a fractional frequency instability of 1 × 10 −15 (1 s averaging) and 1 × 10 −21 (10 5 s), respectively. A verified computer-based model of the link allows to test whether the photonic background from a given operation on the fibre is compatible with QKD operation in the same fibre.
Quantum key distribution (QKD), in which security is ensured by physical quantum laws, is shown as the most secure communication between two distant users [1]–[3]. Recent efforts in global research focus on providing the infrastructure that can support QKD and extending applications to large scale of users and distances. Three types of links are explored up to date: terrestrial free-space links (limited by visible distance), fiber-based links (repeater-free range limited to a few 100 km [1]) and finally satellite links, which allow to bridge intercontinental distances. For each of these scenarios, different types of hardware are involved and state encoding (polarization-, time- [1]–[2], spatial- and frequency-based [3]) is tailored to mitigate link impairments. Additionally, the effort towards higher QKD rates is given [1]–[3] that future global quantum communication networks support a wider range of use cases.
In a society based on the continuous exchange of sensitive data and information, the importance of secure and trustful communications is essential. Quantum key distribution (QKD) makes it possible to share data in an unconditionally secure way exploiting the basic principles of Quantum Mechanics [1]. During the last 30 years, many QKD protocols have been developed and tested, achieving long distance transmission [2] and secret key rates up to hundreds of Mbits per second [3]. However, this technology is still far from a large-scale deployment in existing fiber networks and telecom infrastructures, due to multiple factors: low secret-key rate, limited distance between users, lack of applications, high costs and high requirements in terms of low noise fiber links.
In this paper we present our work demonstrating the transfer of quantum cryptographic information over a 78 km optical fiber, as a base for future long distance public quantum links, using a commercial quantum key distribution (QKD) system based on the BBM92 protocol. The system communicates with pairs of polarization-entangled photons generated by the nonlinear optical process of spontaneous parametric down-conversion. The system was put to the test in the laboratory using optical fiber of different lengths up to 112 km and with different link losses up to 29 dB. The QKD system was implemented on single mode standard optical fiber link of 78 km length deployed between the two cities of Braunschweig and Hannover. Characterizations and optimizations of the QKD system and the fiber link have been carried out in order to improve the synchronization, to reduce the quantum bit error rate, and to successfully establish the intercity quantum communication.
In this study, we demonstrate the possibility to protect, with Quantum Key Distribution (QKD), a critical infrastructure as the fiber-based one used for time and frequency (TF) dissemination service. The proposed technique allows to disseminate secure and precise TF signals between two fiber-optic-connected locations, on a critical infrastructure, using both QKD and White Rabbit technique. This secure exchange enables the secret sharing of time information between two parties, allowing the synchronization of distant clocks with a stability of [Formula: see text] at 1 s, traceable to the Italian time scale. When encrypted, the time signals provide no useful information to a third party regarding the synchronization status, resulting in a time stability degraded by two orders of magnitude.
Quantum key distribution (QKD) promises unconditionally secure communication provided by fundamental laws of quantum mechanics. Current QKD systems suffer from range limitations either due to the earth’s curvature or attenuation in optical fibers. The Chinese and Austrian Academies of Science decided to jointly establish the required technologies and infrastuctures for a satellite based global quantum key distribution network. Here we present the proposed experiment as well as the proof-of-concept demonstrations conducted on a 143 km horizontal free-space link between the Canary Islands (Spain) and a 6 km horizontal link in Graz (Austria).