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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Practical repeaters for ultra-long distance quantum communication

Scott E. Vinay, Pieter Kok

Published Feb 20, 2017• 0 citations
Publisher

Abstract

The problem of distributing a secret key by quantum channels is one of the preeminent problems in quantum information. To construct such a quantum channel, it is useful to use quantum repeaters, which shorten the distance over which individual photons need to be sent, and thus overcoming the problem of attenuation. Here, we present a method by which a quantum repeater network may be constructed using only existing technology. By taking advantage of the robustness of the processes of double-heralding and brokering, a thorough analysis of the system shows that this gives secret key rates on the order of kilohertz, even over distances of thousands of kilometres.

DOI
10.1117/12.2250497

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The problem of distributing a secret key by quantum channels is one of the preeminent problems in quantum information. To construct such a quantum channel, it is useful to use quantum repeaters, which shorten the distance over which individual photons need to be sent, and thus overcoming the problem of attenuation. Here, we present a method by which a quantum repeater network may be constructed using only existing technology. By taking advantage of the robustness of the processes of double-heralding and brokering, a thorough analysis of the system shows that this gives secret key rates on the order of kilohertz, even over distances of thousands of kilometres.
Quantum repeaters enable long-range quantum communication in the presence of attenuation. Here we propose a method to construct a robust quantum repeater network using only existing technology. We combine the ideas of brokered graph-state construction with double-heralded entanglement generation to form a system that is able to perform all parts of the procedure in a way that is highly tolerant to photon loss and imperfections in detectors. We show that when used in quantum key distribution this leads to secure kilohertz bit rates over intercontinental distances.
Long-distance communication data cannot be safely sent over the Internet without strong key distribution mechanisms and long-term, secure communication routes. In recent days, distributing a secret key across faraway entities connected by an optical fibre or open space has been the goal of quantum key distribution. The previous quantum method challenge in guaranteeing the integrity of qubits makes long-distance communication a challenging endeavour. With a growing number of cyber dangers, it is more important than ever to guarantee safe interaction. Quantum computers can easily crack traditional key exchange techniques, which create shared secret keys for protection. A quantum key distributionbased secure communication model (QKD-SCM) has been suggested for large-scale networks to rapidly ascertain the bare minimum of quantum repeaters needed to provide end-to-end connections among all end hosts. The suggested QKD-SCM evaluates the safety and efficiency of existing point-to-point QKD systems. The QKD-SCM with Quantum repeaters setup breaks the channel into quantum memory (QM), which is climbable and allows for long-distance quantum communication. The end of the channel is used to produce entangled qubits. Experiments show that QKD-SCM can succeed in making long-distance networks very secure with less communication delay.
Quantum key distribution over probabilistic quantum repeaters is addressed. We compare, under practical assumptions, two such schemes in terms of their secret key generation rates per quantum memory. The two schemes under investigation are the one proposed by Duan et al. [Nature (London) 414, 413 (2001)] and that of Sangouard et al. [Phys. Rev. A 76, 050301 (2007)]. We consider various sources of imperfection in both protocols, such as nonzero double-photon probabilities at the sources, dark counts in detectors, and inefficiencies in the channel, photodetectors, and memories. We also consider memory decay and dephasing processes in our analysis. For the latter system, we determine the maximum value of the double-photon probability beyond which secret key distillation is not possible. We also find crossover distances for one nesting level to its subsequent one. We finally compare the two protocols in terms of their achievable secret key generation rates at their optimal settings. Our results specify regimes of operation where one system outperforms the other.
The feasibility of trust-free long-haul quantum key distribution (QKD) is addressed. We combine measurement-device-independent QKD, as an access technology, with a quantum repeater setup, at the core of future quantum communication networks. This will provide a quantum link none of whose intermediary nodes need to be trusted, or, in our terminology, a trust-free QKD link. As the main figure of merit, we calculate the secret key generation rate when a particular probabilistic quantum repeater protocol is in use. We assume the users are equipped with imperfect single photon sources, which can possibly emit two single photons, or laser sources to implement decoy-state techniques. We consider apparatus imperfection, such as quantum efficiency and dark count of photodetectors, path loss of the channel, and writing and reading efficiencies of quantum memories. By optimizing different system parameters, we estimate the maximum distance over which users can share secret keys when a finite number of memories are employed in the repeater setup.
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.
Quantum key distribution allows for the generation of a secret key between distant parties connected by a quantum channel such as optical fibre or free space. Unfortunately, the rate of generation of a secret key by direct transmission is fundamentally limited by the distance. This limit can be overcome by the implementation of so-called quantum repeaters. Here, we assess the performance of a specific but very natural setup called a single sequential repeater for quantum key distribution. We offer a fine-grained assessment of the repeater by introducing a series of benchmarks. The benchmarks, which should be surpassed to claim a working repeater, are based on finite-energy considerations, thermal noise and the losses in the setup. In order to boost the performance of the studied repeaters we introduce two methods. The first one corresponds to the concept of a cut-off, which reduces the effect of decoherence during the storage of a quantum state by introducing a maximum storage time. Secondly, we supplement the standard classical post-processing with an advantage distillation procedure. Using these methods, we find realistic parameters for which it is possible to achieve rates greater than each of the benchmarks, guiding the way towards implementing quantum repeaters.