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IEEE International Conference on High Performance Computing and Communications; IEEE International Conference on Smart City; IEEE International Conference on Data Science and Systems (HPCC/SmartCity/DSS)

Optimal deployment design of repeaters and memories in quantum networks

Jiangyuan Yao, Kaiwen Zou, Deshun Li, Zheng Jiang

Published Dec 1, 2021• 2 citations
Publisher

Abstract

In quantum communication networks, the transmission success rate of entangled photons decreases exponentially with the transmission distance in fiber channel. With quantum repeaters, quantum networks can realize arbitrary long-distance links based on quantum entanglement swapping, quantum storage, and quantum teleportation. Thanks to the development of quantum memory technology, the throughput of quantum relay networks can be improved by increasing the number of quantum memories. In this paper, we study the routing metric, the goal of which is to achieve a higher throughput in a long-distance quantum network by reasonably allocating repeater positions with a given memory count between source and destination. A routing metric$E$was proposed, where the number and distance range of repeater deployments can be obtained by numerical analysis to support an optimal network throughput. According to the above principle, various repeaters were deployed on a linear quantum network to test the effectiveness of the proposed allocation mechanism with the given transmission loss, Bell-State Measurement (BSM) success rate, memory quantity, and distance. The results of evaluation show that increasing the number of repeaters blindly can lead to a waste of resources and a throughput decreasing, which implies that repeaters need to be placed at an appropriate deployment distance to achieve the optimal performance.

DOI
10.1109/hpcc-dss-smartcity-dependsys53884.2021.00072

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In quantum communication networks, the transmission success rate of entangled photons decreases exponentially with the transmission distance in fiber channel. With quantum repeaters, quantum networks can realize arbitrary long-distance links based on quantum entanglement swapping, quantum storage, and quantum teleportation. Thanks to the development of quantum memory technology, the throughput of quantum relay networks can be improved by increasing the number of quantum memories. In this paper, we study the routing metric, the goal of which is to achieve a higher throughput in a long-distance quantum network by reasonably allocating repeater positions with a given memory count between source and destination. A routing metricEEEwas proposed, where the number and distance range of repeater deployments can be obtained by numerical analysis to support an optimal network throughput. According to the above principle, various repeaters were deployed on a linear quantum network to test the effectiveness of the proposed allocation mechanism with the given transmission loss, Bell-State Measurement (BSM) success rate, memory quantity, and distance. The results of evaluation show that increasing the number of repeaters blindly can lead to a waste of resources and a throughput decreasing, which implies that repeaters need to be placed at an appropriate deployment distance to achieve the optimal performance.
First-generation quantum repeaters are essential in distributing entanglement over long distances since they could mitigate losses in optical fiber. Repeaters split the total distance into shorter, easier to generate links. Then, entanglement swapping establishes long range entanglement by performing local measurements at the repeaters and heralding their results. In this work, we develop an analytical model that estimates the impact of decoherence during heralding for a given protocol. Furthermore, we identify a competition between two phenomena: as repeater count increases, so does entanglement generation, but the additive impact of memory decoherence on final entanglement rate increases too. Therefore, we find that an optimal number of repeaters exists. These findings could lead the design of future quantum networks, both in the inter-repeater distance and swapping timings.
This paper investigates the scalability and transmission efficiency of entanglement swapping protocols within multi-node quantum networks. As the realization of a global quantum internet relies heavily on long-distance entanglement distribution, quantum repeaters utilizing entanglement swapping serve as a critical backbone. However, scalability remains a bottleneck due to photon loss, operational infidelities, and quantum memory coherence times. In this study, we analytically model a multi-node quantum chain and evaluate how the end-to-end entanglement fidelity degrades as the number of intermediate nodes scales up. Our analysis highlights the critical thresholds for Bell state measurement (BSM) success rates required to maintain quantum state integrity across large distances. The findings provide optimization constraints for designing scalable and fault-tolerant routing protocols in future quantum information networks.
Quantum repeater networks that allow long-distance entanglement distribution will be the backbone of distributed quantum information processing. In this paper we explore entanglement distribution using quantum repeaters with optimized buffer time, equipped with noisy quantum memories and performing imperfect entanglement purification and swapping. We observe that increasing the number of memories on end nodes leads to a higher entanglement distribution rate per memory and higher probability of high-fidelity entanglement distribution, at least for the case with perfect operations. When imperfect operations are considered, however, we make the surprising observation that the permemory entanglement rate decreases with increasing number of memories. Our results suggest that building quantum repeaters that perform well under realistic conditions requires careful modeling and design that takes into consideration the operations and resources that are finite and imperfect.
Quantum communications capacity using direct transmission over length-LLL optical fiber scales as $R\ensuremath{\sim}{e}^{\ensuremath{-}\ensuremath{\alpha}L}$, where $\ensuremath{\alpha}$ is the fiber's loss coefficient. The rate achieved using a linear chain of quantum repeaters equipped with quantum memories, probabilistic Bell state measurements (BSMs), and switches used for spatial multiplexing, but no quantum error correction, was shown to surpass the direct-transmission capacity. However, this rate still decays exponentially with the end-to-end distance, viz., $R\ensuremath{\sim}{e}^{\ensuremath{-}s\ensuremath{\alpha}L}$, with s<1s<1s<1. We show that the introduction of temporal multiplexing---i.e., the ability to perform BSMs among qubits at a repeater node that were successfully entangled with qubits at distinct neighboring nodes at different time steps---leads to a subexponential rate-vs-distance scaling, i.e., $R\ensuremath{\sim}{e}^{\ensuremath{-}t\sqrt{\ensuremath{\alpha}L}}$, which is not attainable with just spatial or spectral multiplexing. We evaluate analytical upper and lower bounds to this rate and obtain the exact rate by numerically optimizing the time-multiplexing block length and the number of repeater nodes. We further demonstrate that incorporating losses in the optical switches used to implement time multiplexing degrades the rate-vs-distance performance, eventually falling back to exponential scaling for very lossy switches. We also examine models for quantum memory decoherence and describe optimal regimes of operation to preserve the desired boost from temporal multiplexing. Quantum memory decoherence is seen to be more detrimental to the repeater's performance over switching losses.
We propose a repeat-until-success protocol to improve the performance of probabilistic quantum repeaters. Conventionally, these rely on passive static linear-optics elements and photodetectors to perform Bell-state measurements (BSMs) with a maximum success rate of 50%. This is a strong impediment for entanglement swapping between distant quantum memories. Every time a BSM fails, entanglement needs to be redistributed between the corresponding memories in the repeater link. The key ingredients of our scheme are repeatable BSMs. Under ideal conditions, these turn probabilistic quantum repeaters into deterministic ones. Under realistic conditions, our protocol too might fail. However, using additional threshold detectors now allows us to improve the entanglement generation rate by almost orders of magnitude, at a nominal distance of 1000 km, compared to schemes that rely on conventional BSMs. This improvement is sufficient to make the performance of our scheme comparable to the expected performance of some deterministic quantum repeaters.
Abstract Efficient long-distance entanglement distribution is a key requirement for a global quantum internet. Quantum repeaters overcome photon loss in optical fibers, offering a route to long-distance entanglement distribution. Here, we study heralded quantum repeater architectures that are based on two-photon interference and integrate local or remote swapping. We derive an analytical expression for the entanglement distribution time as a function of transmission distance for both heralded local-swapping and remote-swapping schemes. This expression provides physical insight into how repeaters overcome channel loss in long-haul fibers. Considering a realistic quantum memory, we present theoretical curves showing the effect of finite memory lifetime on the entanglement distribution time. By comparing optimal distribution times over distances up to thousands of kilometers, we determine the optimal nesting architectures for the single-repeater channel in each scheme and compare the optimal distribution rates and entanglement fidelities of these schemes at various distances. Finally, by examining the limiting cases of zero channel loss, zero device loss, and lossless repeater systems, we find fundamental performance bounds for quantum repeaters. These results provide practical design principles for scalable quantum repeater networks and pave the way toward continent-scale quantum communication.