Abstract Quantum repeaters are enabling technologies for long-distance quantum communications. Despite the significant progress in the field, we still not only face implementation challenges but also need theoretical solutions that better meet all the desired design criteria. Preliminary solutions for quantum repeaters often do not scale well, while the most advanced solutions are so demanding that their implementation may take a long time and require substantial changes to current telecom infrastructure. In this paper, we propose a compromise solution that is not only scalable in the mid-to-long term but also adapts well to the realities of the backbone networks in the current Internet infrastructure. The key ideas behind our solution are twofold. First, we use a hop-by-hop approach to entanglement swapping, allowing our solution to benefit from the same features as packet-switched networks. Second, we employ simple error detection, rather than more complicated error correction, techniques to make our solution sufficiently scalable in the face of errors. This is achieved without requiring overly demanding specifications for the physical devices needed in the network. We test this idea in a quantum key distribution (QKD) setting over a repeater chain and demonstrate how trust-free continental QKD can be achieved through several stages of development.
Abstract Quantum repeaters are enabling technologies for long-distance quantum communications. Despite the significant progress in the field, we still not only face implementation challenges but also need theoretical solutions that better meet all the desired design criteria. Preliminary solutions for quantum repeaters often do not scale well, while the most advanced solutions are so demanding that their implementation may take a long time and require substantial changes to current telecom infrastructure. In this paper, we propose a compromise solution that is not only scalable in the mid-to-long term but also adapts well to the realities of the backbone networks in the current Internet infrastructure. The key ideas behind our solution are twofold. First, we use a hop-by-hop approach to entanglement swapping, allowing our solution to benefit from the same features as packet-switched networks. Second, we employ simple error detection, rather than more complicated error correction, techniques to make our solution sufficiently scalable in the face of errors. This is achieved without requiring overly demanding specifications for the physical devices needed in the network. We test this idea in a quantum key distribution (QKD) setting over a repeater chain and demonstrate how trust-free continental QKD can be achieved through several stages of development.
Long-distance quantum communication faces significant challenges due to photon loss and operational errors in fiber optic channels. Quantum repeaters (QRs) offer a promising solution by extending the communication range and enabling secure exchange of quantum information over continental distances. This article provides an overview of the three generations of QRs, comparing their technological requirements and challenges. We introduce the basic techniques, such as entanglement swapping, distillation, and error correction, and discuss their relevance to each generation of QRs. In addition, we present a future direction that integrates superconducting qubits with optical fibers, enabled by recent advances in microwave-optical transduction. This new hybrid architecture offers promising prospects for scalable and efficient quantum networks. It is also applicable in quantum key distribution protocols, such as E91.
Abstract: This chapter explores the engineering of quantum repeaters, a critical technology for enabling secure and scalable long-distance quantum communication. Quantum repeaters extend the range of quantum communication networks by preserving entanglement and mitigating signal loss over large distances. The chapter covers the key principles of entanglement swapping, quantum memory, and quantum error correction, which are essential for maintaining the integrity of quantum states. It also examines advanced repeater architectures, including multi-mode repeaters and entanglement purification techniques, to improve scalability and robustness. Case studies highlight real-world implementations of quantum repeaters in quantum key distribution (QKD) networks, demonstrating their role in achieving secure global communication. Keywords: quantum repeaters, long-distance quantum communication, entanglement swapping, quantum memory, quantum error correction, entanglement purification, multi-mode repeaters, quantum key distribution, QKD, scalable quantum networks, secure communication.
When built, quantum repeater networks will require classical network protocols to control the quantum operations. However, existing work on repeaters has focused on the quantum operations themselves, with less attention paid to the contents, semantics, ordering and reliability of the classical control messages. In this work we define and describe our implementation of the classical control protocols. The state machines and packet sequences for the three protocol layers are presented, and operation confirmed by running the protocols over simulations of the physical network. We also show that proper management of the resources in a bottleneck link allows the aggregate throughput of two end-to-end flows to substantially exceed that of a single flow. Our layered architectural framework will support independent evolution of the separate protocol layers. Networks of quantum repeaters utilize three concepts to execute a distributed algorithm that creates entangled quantum states between nodes that are far apart: a basic entanglement mechanism which depends on the physical implementation, error management (in this work, we study a method known as purification), and finally a quantum state propagation layer (here we implement entanglement swapping, which builds multi-hop connections from single-hop connections). Some researchers are investigating approaches that are substantially different from entanglement swapping [30, 24, 19]. Here we focus on swapping, but the layered architecture approach is broadly applicable, allowing other implementations to replace only a single layer in the protocol stack. Previous work primarily focused on the physical and mathematical tools for building repeaters. Classical information is also needed to enable teleportation and swapping, as many quantum operations are not deterministic, and results of quantum measurements need to be reported to distant partners before further operations can proceed. Also, operations in the middle of the network must be coordinated to route and swap properly. This requires classical messages to make operations robust, but message propagation times penalize performance. Even though this delay is usually included in repeater simulations, prior work has not defined the protocols in detail, especially with respect to how all of the nodes make consistent decisions in a timely fashion. In this work, we introduce a protocol stack for networks of quantum repeaters that considers all the necessary classical messages and which can be easily adapted for different approaches at all three protocol layers. We run simulations of competing flows on a dumbbell topology in order to increase our confidence in the behavior of our network protocols. By adjusting the fidelity thresholds required for entanglement swapping, we show that some configurations boost the aggregate throughput for multiple flows significantly above the maximum for a single flow, taking advantage of resources that would otherwise sit idle. The operation of such complex networks and such delicate tuning of the system without formal protocol definitions would not be possible. Previous work has also concentrated almost exclusively on the dedicated use of a single line or chain of repeaters, delivering Bell pairs only to the two nodes at the ends of the chain. Networks, however, typically have more than two end points, and allow any pair of these end nodes to communicate. More topologically complex networks, with numerous end nodes, are obviously much more scalable than connecting every possible pair of nodes using a dedicated line of repeaters. we investigate how classical multiplexing schemes translate to the domain of quantum repeaters, in order to manage shared resources and active communication flows of data from different stations. We simulate four sharing protocols in a complex network with competing traffic. Circuit switching gives any individual flow the best performance, but makes poor use of the overall network and is inflexible. We use circuit switching as our baseline case to compare time division multiplexing, buffer space multiplexing, and statistical multiplexing, and show that all multiplexing schemes are better than circuit switching. For the particular network simulated, we find that statistical multiplexing outperforms time division multiplexing by 28% and buffer space multiplexing by 13%. We find that all three multiplexing schemes are fair; each flow is penalized a similar percentage of its throughput as the the total number of users in the network increases. Finally, statistical multiplexing requires no network-wide coordination of the use of quantum memory or channels, and is easier to implement than a robust, scalable scheme for the other protocols. Our current results suggest that the best strategy for quantum repeater networks is statistical multiplexing. (This is in fact the quantum analogue of the basis on which the Internet works.) However, our current simulations are done with no degradation of memory over time, using the assumption of quantum error-corrected memory at each repeater; inclusion of decoherence and a finite qubit lifetime remains as future work.
Purpose of the study: systematization and critical analysis of existing approaches to error suppression in quantum repeaters, as well as to assess their advantages and limitations for the implementation of scalable quantum networks and the quantum internet. Methods of research: the paper provides a detailed analysis of modern literature, including a comparison of various quantum repeater schemes, as well as an assessment of resource costs and performance. Result(s): quantum repeaters can be divided into three generations, each of which demonstrates optimal efficiency under certain conditions. The first generation, implementing probabilistic error suppression by generating heralded entanglement and two-way entanglement distillation, is easy to implement and provides the basic functionality of a quantum network, but requires significant time due to the need to exchange classical signals and long-term storage of quantum states. The second generation combines probabilistic entanglement generation with deterministic operation error suppression using quantum error correction, which reduces the requirements for long-term quantum memory, although the exchange of classical signals between neighboring nodes remains mandatory. The third generation relies entirely on deterministic error suppression using one-way classical communication, which allows for a significant reduction in time delays and high entanglement generation rates, despite the need for a denser arrangement of repeaters and high-quality local gates. In addition, the review covers new areas such as memoryless repeaters and all-photonic repeaters. The conducted comparative analysis of resource costs demonstrates that optimization of repeater operating parameters is a key factor for the implementation of scalable quantum networks, which is of direct importance for quantum key distribution, quantum metrology, and distributed quantum computing. Scientific novelty: the scientific novelty lies in the integration of disparate approaches to the implementation of quantum repeaters into a single holistic representation, which allows for an objective assessment of their efficiency by key parameters. The review highlights the potential for new classes of repeaters, such as memoryless repeaters and all-photonic circuits, to become important elements in the development of a quantum internet.
Quantum networking, heralded as the next frontier in communication networks, envisions a realm where quantum computers and devices collaborate to unlock capabilities beyond what is possible with the Internet. A critical component for realizing a long-distance quantum network, and ultimately, the Quantum Internet, is the quantum repeater. As with the race to build a scalable quantum computer with different technologies, various schemes exist for building quantum repeaters. This article offers a gentle introduction to the two-way “all-photonic quantum repeaters,” a recent addition to quantum repeater technologies. In contrast to conventional approaches, these repeaters eliminate the need for quantum memories, offering the dual benefits of higher repetition rates and intrinsic tolerance to both quantum operational errors and photon losses. Using visualization and simple rules for manipulating graph states, we describe how all-photonic quantum repeaters work. We discuss the problem of the increased volume of classical communication required by this scheme, which places a huge processing requirement on the end nodes. We address this problem by presenting a solution that decreases the amount of classical communication by three orders of magnitude. We conclude by highlighting other key open challenges in translating the theoretical all-photonic framework into real-world implementation, providing insights into the practical considerations and future research directions of all-photonic quantum repeater technology.
Quantum networks (QNs) are a promising platform for secure communications, enhanced sensing, and efficient distributed quantum computing. However, due to the fragile nature of quantum states, these networks face significant challenges in terms of scalability. In this paper, the scaling limits of quantum repeater networks (QRNs) are analyzed. The goal of this work is to maximize the overall length, or scalability of QRNs such that long-distance quantum communications is achieved while application-specific quality-of-service (QoS) requirements are satisfied. In particular, a novel joint optimization framework that aims at maximizing QRN scalability, while satisfyingQoS constraints on the end-to-end fidelity and rate is proposed. The proposed approach optimizes the number of QRN repeater nodes, their separation distance, and the number of distillation rounds to be performed at both link and end-to-end levels. Extensive simulations are conducted to analyze the tradeoffs between QRN scalability, rate, and fidelity under gate and measurement errors. The obtained results characterize the QRN scaling limits for a given QoS requirement. The proposed approach offers a promising solution and design guidelines for future QRN deployments.