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arXiv:2212.02695 · Quantum Science and Technology

High-dimensional quantum key distribution using energy-time entanglement over 242 km partially deployed fiber

Jingyuan Liu, Zhihao Lin, Dongning Liu, Xue Feng, Fang Liu, Kaiyu Cui, Yidong Huang, Wei Zhang

Published Sep 28, 2023• 42 citations
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Abstract

Abstract Entanglement-based quantum key distribution (QKD) is an essential ingredient in quantum communication, owing to the property of source-independent security and the potential on constructing large-scale quantum communication networks. However, implementation of entanglement-based QKD over long-distance optical fiber links is still challenging, especially over deployed fibers. In this work, we report an experimental QKD using energy-time entangled photon pairs that transmit over optical fibers of 242 km (including a section of 19 km deployed fibers). The QKD is realized through the protocol of dispersive-optics QKD (DO-QKD) with high-dimensional encoding to utilize coincidence counts more efficiently. A reliable, high-accuracy time synchronization technology for long-distance entanglement-based QKD is developed based on the distribution of optical pulses in quantum channels. Our system operates continuously for more than 7 d without active polarization or phase calibration. We ultimately generate secure keys with secure key rates of 0.22 bps and 0.06 bps in the asymptotic and finite-size regimes, respectively. It shows that entanglement-based DO-QKD is reliable for long-distance realization in the field if its high requirement on time synchronization is satisfied.

DOI
10.1088/2058-9565/acfe37

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Abstract Entanglement-based quantum key distribution (QKD) is an essential ingredient in quantum communication, owing to the property of source-independent security and the potential on constructing large-scale quantum communication networks. However, implementation of entanglement-based QKD over long-distance optical fiber links is still challenging, especially over deployed fibers. In this work, we report an experimental QKD using energy-time entangled photon pairs that transmit over optical fibers of 242 km (including a section of 19 km deployed fibers). The QKD is realized through the protocol of dispersive-optics QKD (DO-QKD) with high-dimensional encoding to utilize coincidence counts more efficiently. A reliable, high-accuracy time synchronization technology for long-distance entanglement-based QKD is developed based on the distribution of optical pulses in quantum channels. Our system operates continuously for more than 7 d without active polarization or phase calibration. We ultimately generate secure keys with secure key rates of 0.22 bps and 0.06 bps in the asymptotic and finite-size regimes, respectively. It shows that entanglement-based DO-QKD is reliable for long-distance realization in the field if its high requirement on time synchronization is satisfied.
We present a field-deployed quantum key distribution (QKD) link based on energy-time entanglement. Three nodes are connected, bridging the cities of Nice and Sophia Antipolis, by means of commercial grade optical fibers, spanning over a total distance of 50 km. We have built a packaged and resilient source of energy-time entangled photon pairs and implemented remote interferometer stabilization. Moreover, the clocks of the two end users are synchronized exploiting photon-counting techniques only, therefore requiring no dedicated channel. The system runs over the ITU 100-GHz standard telecom grid, through which a secure key rate of 7.0 kbit/s is extracted for one pair of channels. A homemade software performs all the necessary postprocessing procedures enabling to establish secret keys in real time. We present the first fully operational entanglement-based metropolitan QKD system being totally self-sufficient, i.e., requiring no additional overheads to be operated on a conventional metropolitan fiber network.
An energy-time entanglement-based dispersive optics quantum key distribution (DO-QKD) is demonstrated experimentally over optical fibers of 20 km. In the experiment, the telecom band energy-time entangled photon pairs are generated through spontaneous four-wave mixing in a silicon waveguide. The arrival time of photons is registered for key generation and security test. High-dimensional encoding in the arrival time of photons is used to increase the information per coincidence of photon pairs. The bin sifting process is optimized by a three-level structure, which significantly reduces the raw quantum bit error rate (QBER) due to timing jitters of detectors and electronics. A raw key generation rate of 151 kbps with a QBER of 4.95% is achieved, in a time bin encoding format with 4 bits per coincidence. This experiment shows that the entanglement-based DO-QKD can be implemented in an efficient and convenient way, which has great potential for quantum secure communication networks in the future.
Abstract Quantum key distribution (QKD) has established itself as a groundbreaking technology, showcasing inherent security features that are fundamentally proven. Qubit-based QKD protocols that rely on binary encoding encounter an inherent constraint related to the secret key capacity. This limitation restricts the maximum secret key capacity to one bit per photon. On the other hand, qudit-based QKD protocols have their advantages in scenarios where photons are scarce and noise is present, as they enable the transmission of more than one secret bit per photon. While proof-of-principle entangled-based qudit QKD systems have been successfully demonstrated over the years, the current limitation lies in the maximum distribution distance, which remains at 20 km fiber distance. Moreover, in these entangled high-dimensional QKD systems, the witness and distribution of quantum steering have not been shown before. Here we present a high-dimensional time-bin QKD protocol based on energy-time entanglement that generates a secure finite-length key capacity of 2.39 bit/coincidences and secure cryptographic finite-length keys at 0.24 Mbits s −1 in a 50 km optical fiber link. Our system is built entirely using readily available commercial off-the-shelf components, and secured by nonlocal dispersion cancellation technique against collective Gaussian attacks. Furthermore, we set new records for witnessing both energy-time entanglement and quantum steering over different fiber distances. When operating with a quantum channel loss of 39 dB, our system retains its inherent characteristic of utilizing large-alphabet. This enables us to achieve a secure key rate of 0.30 kbits s −1 and a secure key capacity of 1.10 bit/coincidences, considering finite-key effects. Our experimental results closely match the theoretical upper bound limit of secure cryptographic keys in high-dimensional time-bin QKD protocols (Mower et al 2013 Phys. Rev. A 87 062322; Zhang et al 2014 Phys. Rev. Lett. 112 120506), and outperform recent state-of-the-art qubit-based QKD protocols in terms of secure key throughput using commercial single-photon detectors (Wengerowsky et al 2019 Proc. Natl Acad. Sci. 116 6684; Wengerowsky et al 2020 npj Quantum Inf. 6 5; Zhang et al 2014 Phys. Rev. Lett. 112 120506; Zhang et al 2019 Nat. Photon. 13 839; Liu et al 2019 Phys. Rev. Lett. 122 160501; Zhang et al 2020 Phys. Rev. Lett. 125 010502; Wei et al 2020 Phys. Rev. X 10 031030). The simple and robust entanglement-based high-dimensional time-bin protocol presented here provides potential for practical long-distance quantum steering and QKD with multiple secure bits-per-coincidence, and higher secure cryptographic keys compared to mature qubit-based QKD protocols.
Quantum entanglement is one of the most extraordinary effects in quantum physics, with many applications in the emerging field of quantum information science. In particular, it provides the foundation for quantum key distribution (QKD), which promises a conceptual leap in information security. Entanglement-based QKD holds great promise for future applications owing to the possibility of device-independent security and the potential of establishing global-scale quantum repeater networks. While other approaches to QKD have already reached the level of maturity required for operation in absence of typical laboratory infrastructure, comparable field demonstrations of entanglement-based QKD have not been performed so far. Here, we report on the successful distribution of polarization-entangled photon pairs between Malta and Sicily over 96 km of submarine optical telecommunications fiber. We observe around 257 photon pairs per second, with a polarization visibility above 90%. Our results show that QKD based on polarization entanglement is now indeed viable in long-distance fiber links. This field demonstration marks the longest-distance distribution of entanglement in a deployed telecommunications network and demonstrates an international submarine quantum communication channel. This opens up myriad possibilities for future experiments and technological applications using existing infrastructure.
Quantum key distribution (QKD) is a secure communication scheme for sharing symmetric cryptographic keys based on the laws of quantum physics, and is considered a key player in the realm of cyber-security. A critical challenge for QKD systems comes from the fact that the ever-increasing rates at which digital data are transmitted require more and more performing sources of quantum keys, primarily in terms of secret key generation rate. High-dimensional QKD based on path encoding has been proposed as a candidate approach to address this challenge. However, while proof-of-principle demonstrations based on lab experiments have been reported in the literature, demonstrations in realistic environments are still missing. Here we report the generation of secret keys in a 4-dimensional hybrid time-path-encoded QKD system over a 52-km deployed multicore fiber link forming by looping back two cores of a 26-km 4-core optical fiber. Our results indicate that robust high-dimensional QKD can be implemented in a realistic environment by combining standard telecom equipment with emerging multicore fiber technology.
We present the implementation of a time synchronization protocol as a part of an experimentally deployed entanglement-based quantum key distribution (QKD) link. The system is deployed over 48 km of optical fibers across the Métropole Côte d'Azur and enables secret cryptographic key exchange between two remote users, with an average rate of 7 kbps. We exploit the time correlation of paired photons generated by a high-quality source of energy-time entanglement implemented in the QKD link to synchronize two rubidium clocks located at the end stations. The level of stability achieved guarantees a time offset between the clocks under 12 ps at all time. We also show that this protocol requires less hardware than a typical synchronization protocol for QKD that would distribute a reference clock signal between the users.