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arXiv:1407.3362 · Physical Review A

Repeat-until-success quantum repeaters

David Edward Bruschi, Thomas M. Barlow, Mohsen Razavi, Almut Beige

Published Sep 5, 2014• 26 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physreva.90.032306

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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.
Quantum repeaters are necessary building blocks to realize not only long-distance quantum communication but also the quantum internet. Although quantum repeater protocols are composed of entanglement generation (between adjacent nodes) and entanglement swapping, the performance strongly depends on the success probability of the entanglement swapping. In this talk, we introduce a linear-optical “pairwise” fusion gate that probabilistically projects a pair of qudits onto a Bell state in a two-qubit subspace. The success probability of this gate increases with the dimension of the qudits. We show that the scaling of quantum repeater protocols can be improved dramatically if we use our fusion gates for the entanglement swapping, instead of conventional ones.
Quantum repeaters are vital ingredients for realization of long-distance quantum networks [1]. They can be realized by preparing nodes that can generate an entangled light-matter state, namely, a single photon entangled to a quantum memory. This allows for entangling two distant quantum memories, by propagating the single photons from each node to an intermediate place for Bell state measurements. A common approach for generating such entangled light-matter states is generating an entangled photon pair in a nonlinear source, and coupling one of the photons in the pair to a quantum memory for storage [2]. However, such an approach faces two main challenges: Photon-pair generation in nonlinear sources is a probabilistic process. Moreover, due to modal and spectral mismatches, coupling single photons from a nonlinear source to a quantum memory is generally not an efficient process. These make the successful generation of an entangled light-matter state highly probabilistic.
Quantum repeaters hold the promise to prevent the photon losses in communication channels. Most recently, the serious efforts have been applied to achieve scalable distribution of entanglement over long distances. However, the probabilistic nature of entanglement generation and realistic quantum memory storage times make the implementation of quantum repeaters an outstanding experimental challenge. We propose a quantum repeater protocol based on the deterministic storage of a single photon in atomic ensembles confined in distant high-finesse cavities and show that this system is capable of distributing the entanglement over long distances with a much higher rate as compared to previous protocols, thereby alleviating the limitations on the quantum memory lifetime by several orders of magnitude. Our scheme is robust with respect to phase fluctuations in the quantum channel, while the fidelity imperfection is fixed and negligibly small at each step of entanglement swapping.
Long-distance entanglement distribution is hindered by photon loss in optical fibers and the nocloning theorem. Optical quantum repeater (QR) protocols rely on Bell state measurements (BSMs), they are intrinsically limited to probabilistic photon operations and fail 50% of the time. We propose a hybrid approach to building quantum repeaters that combines the high transmission speed of photonic qubits in optical fiber with the high-fidelity quantum processing capabilities enabled by superconducting circuits. The transduction-enabled superconducting QR (TESQR) architecture eliminates the need for probabilistic BSMs and allows deterministic processing operations. The TESQR framework always yields a final state at the remote nodes rather than aborting on photon loss, manifesting deterministic entanglement distribution within certain parameter regimes. We evaluate the performance by assessing output-state fidelities and success probabilities of entanglement distribution using realistic noise models. Additionally, we integrate an entanglement purification scheme and evaluate the performance through numerical simulations in QuTiP environment. Our results show that, for entanglement swapping, the proposed scheme improves the entanglement distribution rate by an average of 63% and by up to 159% compared with photonic-only architectures. Moreover, after purification, the end-to-end fidelities exceed 0.8 over distances up to 20 km.
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.
We propose a realistic protocol to generate entanglement between quantum memories at neighboring nodes in hybrid quantum repeaters. Generated entanglement includes only one type of error, which enables efficient entanglement distillation. In contrast to the known protocols with such a property, our protocol with ideal detectors achieves the theoretical limit of the success probability and the fidelity to a Bell state, promising higher efficiencies in the repeaters. We also show that the advantage of our protocol remains even with realistic threshold detectors.