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arXiv:2007.00362 · Quantum Science and Technology

Experimentally optimizing QKD rates via nonlocal dispersion compensation

Sebastian Philipp Neumann, Domenico Ribezzo, Martin Bohmann, Rupert Ursin

Published Feb 12, 2021• 30 citations
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Abstract

Abstract Quantum key distribution (QKD) enables unconditionally secure communication guaranteed by the laws of physics. The last decades have seen tremendous efforts in making this technology feasible under real-life conditions, with implementations bridging ever longer distances and creating ever higher secure key rates. Readily deployed glass fiber connections are a natural choice for distributing the single photons necessary for QKD both in intra- and intercity links. Any fiber-based implementation however experiences chromatic dispersion which deteriorates temporal detection precision. This ultimately limits maximum distance and achievable key rate of such QKD systems. In this work, we address this limitation to both maximum distance and key rate and present an effective and easy-to-implement method to overcome chromatic dispersion effects. By exploiting entangled photons’ frequency correlations, we make use of nonlocal dispersion compensation to improve the photons’ temporal correlations. Our experiment is the first implementation utilizing the inherently quantum-mechanical effect of nonlocal dispersion compensation for QKD in this way. We experimentally show an increase in key rate from 6.1 to 228.3 bits/s over 6.46 km of telecom fiber. Our approach is extendable to arbitrary fiber lengths and dispersion values, resulting in substantially increased key rates and even enabling QKD in the first place where strong dispersion would otherwise frustrate key extraction at all.

DOI
10.1088/2058-9565/abe5ee

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Abstract Quantum key distribution (QKD) enables unconditionally secure communication guaranteed by the laws of physics. The last decades have seen tremendous efforts in making this technology feasible under real-life conditions, with implementations bridging ever longer distances and creating ever higher secure key rates. Readily deployed glass fiber connections are a natural choice for distributing the single photons necessary for QKD both in intra- and intercity links. Any fiber-based implementation however experiences chromatic dispersion which deteriorates temporal detection precision. This ultimately limits maximum distance and achievable key rate of such QKD systems. In this work, we address this limitation to both maximum distance and key rate and present an effective and easy-to-implement method to overcome chromatic dispersion effects. By exploiting entangled photons’ frequency correlations, we make use of nonlocal dispersion compensation to improve the photons’ temporal correlations. Our experiment is the first implementation utilizing the inherently quantum-mechanical effect of nonlocal dispersion compensation for QKD in this way. We experimentally show an increase in key rate from 6.1 to 228.3 bits/s over 6.46 km of telecom fiber. Our approach is extendable to arbitrary fiber lengths and dispersion values, resulting in substantially increased key rates and even enabling QKD in the first place where strong dispersion would otherwise frustrate key extraction at all.
Quantum Key distribution (QKD) enables two parties, Alice and Bob, to share a sequence of random bit whose secrecy is guaranteed by the fondations of quantum physics. The natural counterpart of this genuine inviolability lies in moderate key generation rates and achievable distances, due to severe exper- imental constraints, such as photon generation probabilities, detector's inefficiencies, detector's dark counts, and losses in fiber distribution channels. As well as improving these exper- imental resources since the last couple of years, several strate- gies are developed for increasing this bit rate. One promising way consists in exploiting the spectral correlations of time- energy entangled photons, enabling higher information con- tent per paired photon. Our idea is to combine the maturity of QKD schemes with high performance standard telecom com- ponents to propose a practical scheme allowing the distribu- tion of secret bits over 200 km at unprecedented rates.
Dispersive optics quantum key distribution (DO-QKD) based on energy-time entangled photon pairs is an important QKD scheme. In DO-QKD, the arrival time of photons is used in key generation and security analysis, which would be greatly affected by fiber dispersion. In this work, we established a theoretical model of the entanglement-based DO-QKD system, considering the protocol, physical processes (such as fiber transmission and single-photon detection), and the analysis of security tests. Based on this theoretical model, we investigate the influence of chromatic dispersion introduced by transmission fibers on the performance of DO-QKD. By analyzing the benefits and costs of dispersion compensation, the system performance under G.652 and G.655 optical fibers are shown, respectively. The results show that dispersion compensation is unnecessary for DO-QKD systems in campus networks and even metro networks. Whereas, it is still required in DO-QKD systems with longer fiber transmission distances.
Encoding quantum information via time-bin entangled states has had a profound impact on the development of quantum communications. However, dispersive propagation limits their achievable transmission distances. Here we describe a regime for nonlocal dispersion cancellation where the sum of arrival times of photons undergoing identical dispersion remains highly correlated. We exploit this effect to mitigate dispersive effects in a quantum key distribution fiber link, allowing an increase in the secret key rate by over a factor of 5 after 80 km of optical fiber dispersion.
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.
To increase dramatically the distance and the secure key generation rate of quantum key distribution (QKD), the idea of quantum decoys--signals of different intensities--has recently been proposed. Here, we present the first experimental implementation of decoy state QKD. By making simple modifications to a commercial quantum key distribution system, we show that a secure key generation rate of 165 bit/s, which is 1/4 of the theoretical limit, can be obtained over 15 km of a telecommunication fiber. We also show that with the same experimental parameters, not even a single bit of secure key can be extracted with a non-decoy-state protocol. Compared to building single photon sources, decoy state QKD is a much simpler method for increasing the distance and key generation rate of unconditionally secure QKD.