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Quantum key distribution beyond the repeaterless secret key capacity

Mariella Minder

Published Aug 24, 2020• 0 citations
Publisher

Abstract

Quantum communications promise to provide information theoretic security in the exchange of information. However, unlike their classical counterpart, they utilise dim optical pulses whose amplification is prohibited. Consequently, their transmission rate and range is confined below a theoretical limit known as repeaterless secret key capacity. Overcoming this limit with today’s technology was believed to be impossible until the recent proposal of Twin-field (TF) quantum key distribution (QKD), a scheme that uses phase-coherent optical signals and an auxiliary measuring station to distribute quantum information. Here, TF-QKD and its main variations are initially explored and compared in simulations, to assess their performance in different attributes. Such schemes are also practically implemented for the first time in two experiments. The first is a proof-of-principle implementation over significant channel losses, in excess of 90 dB. In the second, the setup is developed further and the protocol is implemented over real fibre channels exceeding 600 km in length, representing the first fibre-based secure quantum communication beyond the barriers of 600 km and 100 dB. In both cases, in the high loss/distance regime, the resulting secure key rates exceed the repeaterless secret key capacity, a result never achieved before.

DOI
10.17863/cam.62170

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Quantum communications promise to provide information theoretic security in the exchange of information. However, unlike their classical counterpart, they utilise dim optical pulses whose amplification is prohibited. Consequently, their transmission rate and range is confined below a theoretical limit known as repeaterless secret key capacity. Overcoming this limit with today’s technology was believed to be impossible until the recent proposal of Twin-field (TF) quantum key distribution (QKD), a scheme that uses phase-coherent optical signals and an auxiliary measuring station to distribute quantum information. Here, TF-QKD and its main variations are initially explored and compared in simulations, to assess their performance in different attributes. Such schemes are also practically implemented for the first time in two experiments. The first is a proof-of-principle implementation over significant channel losses, in excess of 90 dB. In the second, the setup is developed further and the protocol is implemented over real fibre channels exceeding 600 km in length, representing the first fibre-based secure quantum communication beyond the barriers of 600 km and 100 dB. In both cases, in the high loss/distance regime, the resulting secure key rates exceed the repeaterless secret key capacity, a result never achieved before.
Since they were first introduced, quantum communications have evolved into a mature technology that is making its way to market. Quantum key distribution (QKD), in particular, allows the exchange of information with security that stems from the very laws of quantum mechanics and hence is not impaired by future technological advances. These same laws, however, restrict the QKD transmission range, as they forbid a straightforward amplification of quantum signals. This leads to a fundamental rate-distance bound known as the “repeaterless secret key capacity” (SKC0), which was thought to be impossible to overcome without a full-fledged quantum repeater. The recent proposal of “Twin-Field QKD” challenged this belief and showed how to overcome the SKC0 with an effective quantum repeater, realisable with present-day technology. This allows to perform QKD over long-distance and high-loss channels. In my talk, I will review the main concepts and most significant results behind this novel idea.
Twin-field (TF) quantum key distribution (QKD) promises high key rates over long distances to beat the rate-distance limit. Here, applying the sending-or-not-sending TF QKD protocol, we experimentally demonstrate a secure key distribution that breaks the absolute key-rate limit of repeaterless QKD over a 509-km-long ultralow loss optical fiber. Two independent lasers are used as sources with remote-frequency-locking technique over the 500-km fiber distance. Practical optical fibers are used as the optical path with appropriate noise filtering; and finite-key effects are considered in the key-rate analysis. The secure key rate obtained at 509 km is more than seven times higher than the relative bound of repeaterless QKD for the same detection loss. The achieved secure key rate is also higher than that of a traditional QKD protocol running with a perfect repeaterless QKD device, even for an infinite number of sent pulses. Our result shows that the protocol and technologies applied in this experiment enable TF QKD to achieve a high secure key rate over a long distribution distance, and is therefore practically useful for field implementation of intercity QKD.
Quantum key distribution (QKD) allows users to generate shared encryption keys that are guaranteed to be theoretically secure by the laws of quantum mechanics. Most implementations use light pulses that are attenuated by the propagation medium. This leads to a fundamental rate-distance limit in QKD that was thought to be impossible to overcome with current technology. The recent proposal of "Twin-Field QKD", however, changed this belief and showed how to overcome this limit and perform long-distance QKD with present-day technology. In my talk I will review the most significant theoretical and experimental results in this emerging and rapidly growing research area.
Quantum Key Distribution (QKD) offers information-theoretic security, making it a cornerstone of future cryptographic infrastructures in the era of quantum computing. However, long-distance deployment of conventional QKD protocols is limited by channel losses, detector noise, and the fundamental repeaterless Pirandola-Laurenza-Ottaviani-Banchi (PLOB) bound. To address these challenges, this paper investigates the Twin-Field QKD (TF-QKD) protocol, which enables secure key exchange over long-haul optical fibers by exploiting single-photon interference at an untrusted intermediate node. An elaborate system model is provided with optical channel impairments, imperfections of detector and phase stabilization conditions. Finite-key analysis, error correction as well as privacy amplification are placed and made together in order to increase robustness of protocol against practical attacks. The simulation data show that TFQKD can reach secure key rates of over 500 km and is out of reach of decoy-state QKD but nearly close to fundamental limits. These results demonstrate that TF-QKD has the potential to practicalize quantumsecure communication at large scale and in a costeffective way, which can constitute a sub-unit of the future quantum internet.
Twin-field (TF) quantum key distribution (QKD) has rapidly risen as the most viable solution to long-distance secure fibre communication thanks to its fundamentally repeater-like rate-loss scaling. However, its implementation complexity, if not successfully addressed, could impede or even prevent its advance into real-world. To satisfy its requirement for twin-field coherence, all present setups adopted essentially a gigantic, resource-inefficient interferometer structure that lacks scalability that mature QKD systems provide with simplex quantum links. Here we introduce a technique that can stabilise an open channel without using a closed interferometer and has general applicability to phase-sensitive quantum communications. Using locally generated frequency combs to establish mutual coherence, we develop a simple and versatile TF-QKD setup that does not need service fibre and can operate over links of 100 km asymmetry. We confirm the setup's repeater-like behaviour and obtain a finite-size rate of 0.32 bit/s at a distance of 615.6 km.
With the help of quantum key distribution (QKD), two distant peers are able to share informationtheoretical secure key bits.Increasing the key rate is ultimately significant for the applications of QKD in the lossy channel.However, it has been proven that there is a fundamental rate-distance limit, called the linear bound, which restricts the performance of all existing repeaterless protocols and realizations.Surprisingly, a recently proposed protocol, called twin-field (TF) QKD, can beat the linear bound with no need for quantum repeaters.Here, we present one of the first implementations of the TF-QKD protocol and demonstrate its advantage of beating the linear bound at a channel distance of 300 km.In our experiment, a modified TF-QKD protocol that does not assume phase postselection is considered, and thus a higher key rate than the original one is expected.After controlling the phase evolution of the twin fields traveling through hundreds of kilometers of optical fibers, the implemented system achieves high-visibility singlephoton interference and allows stable and high-rate measurement-device-independent QKD.Our experimental demonstration and results confirm the feasibility of the TF-QKD protocol and its prominent superiority in long-distance key distribution services.