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Russian Microelectronics

High-Fidelity Distribution of Macroscopic Entanglement over Network of BEC Qubits

Alexey N. Pyrkov, I. D. Lazarev

Published Dec 1, 2025• 0 citations
Publisher

Abstract

Abstract High-fidelity entanglement distribution is a crucial objective for various applications in quantum technologies, including quantum communication, distributed quantum computing, and quantum metrology. Current methods for that, mainly centered around quantum repeaters, focus on distributing only microscopic amounts of entanglement at a time. In this paper, we discuss a novel approach that was proposed for efficient distributing a macroscopic amount of entanglement across a chain of macroscopic Bose–Einstein condensates (BECs) or cold atomic ensembles. The framework employs BECs at each node, which are connected via a shared optical mode and enables the high-fidelity long-distance distribution of macroscopic entanglement while requiring only local measurements on the intermediate ensembles, the ensembles at the ends of the chain remain entangled, under ideal conditions without decoherence. We discuss that at specific “magic” interaction times the macroscopic entanglement can be distributed with perfect fidelity, regardless of the chain length. It paves a way for development of quantum repeaters for macroscopic entanglement distribution.

DOI
10.1134/s1063739725602061

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Abstract High-fidelity entanglement distribution is a crucial objective for various applications in quantum technologies, including quantum communication, distributed quantum computing, and quantum metrology. Current methods for that, mainly centered around quantum repeaters, focus on distributing only microscopic amounts of entanglement at a time. In this paper, we discuss a novel approach that was proposed for efficient distributing a macroscopic amount of entanglement across a chain of macroscopic Bose–Einstein condensates (BECs) or cold atomic ensembles. The framework employs BECs at each node, which are connected via a shared optical mode and enables the high-fidelity long-distance distribution of macroscopic entanglement while requiring only local measurements on the intermediate ensembles, the ensembles at the ends of the chain remain entangled, under ideal conditions without decoherence. We discuss that at specific “magic” interaction times the macroscopic entanglement can be distributed with perfect fidelity, regardless of the chain length. It paves a way for development of quantum repeaters for macroscopic entanglement distribution.
Abstract Distributing long‐distance entanglement is a fundamental goal that is necessary for a variety of tasks such as quantum communication, distributed quantum computing, and quantum metrology. Currently quantum repeater schemes typically aim to distribute one ebit at a time, the equivalent of one Bell pair's worth of entanglement. Here, a scheme is presented to distribute a macroscopic amount of entanglement across long‐distances using a number of operations that scales only linearly with the chain length in the ideal case (without decoherence). The scheme involves ensembles of qubits and entangling them with an interaction, which can be realized using atomic gas ensembles coupled by a shared optical mode. Using only local measurements on the intermediate ensembles, this leaves the ensembles at the ends of the chain entangled. It is showed that there are particular “magic” interaction times that allow for distribution of entanglement with perfect fidelity, with no degradation with chain length. The scheme is deterministic, such that with suitable local conditional unitary corrections, the same entangled state can always be prepared with good approximation.
The distribution of entanglement is a crucial task for quantum communication towards realizing a globe-spanning quantum internet. Recently a protocol for deterministic long-distance distribution of macroscopic entanglement over a network of ensembles of qubits was introduced [Adv. Quantum Technol. 2025, 8, 2400524]. It was shown that this protocol allows for the propagation of macroscopic amounts of entanglement with a protocol complexity that is independent on the ensemble size. However, questions remained on whether the scheme is viable, particularly for a large particle number, which is the case for realistic atomic ensembles. Here we develop improved numerical techniques that allow calculation of realistic ensemble sizes up to 10^6 with a negligible loss of accuracy. We find that moderate dephasing leaves the entanglement largely intact at the magic times, whereas stronger noise monotonically suppresses the entanglement. Our results demonstrate that the protocol retains its functionality towards the macroscopic regime and provides quantitative benchmarks for its robustness under a realistic level of decoherence.
Quantum networks create a completely new way for communication, and the most important function of a quantum network is to generate long-distance quantum entanglement to serve a number of quantum applications. As the scale of the network expands, in order to establish end-to-end entanglement between two remote nodes, entangled pairs need to be generated and distributed among multiple repeaters along the path from the source to the destination, which requires a specific protocol to negotiate resource allocations and quantum operations among the repeaters. Thus, to realize such remote entanglement distribution in a quantum network, designing a stable and reliable protocol becomes urgent and necessary. In this article, we focus on how to guarantee the generation rate of entangled pairs between any two quantum nodes to meet the requirements of various quantum applications in a large-scale quantum network. We present a connection-oriented entanglement distribution protocol inspired by the connection-oriented communication model in classical networks. Our protocol is located in the network layer of the quantum network to enable end-to-end quantum communication. Three main features are provided by the proposed protocol: 1) it is reliable and can guarantee the successful entanglement generation; 2) it can reduce the influence of quantum decoherence via reducing the latency caused by resource competition; and 3) it can guarantee the rate of generating entanglement between two quantum nodes according to the requirement of quantum applications.
We present a hybrid quantum repeater protocol for the long-distance distribution of atomic entangled states beyond qubits. In our scheme, imperfect noisy entangled pairs of two qudits, i.e., two discrete-variable ddd-level systems, each of, in principle, arbitrary dimension ddd, are initially shared between the intermediate stations of the channel. This is achieved via local, sufficiently strong light-matter interactions, involving optical coherent states and their transmission after these interactions, and optical measurements on the transmitted field modes, especially efficient continuous-variable homodyne detections (``hybrid'' here refers to the simultaneous exploitation of discrete- and continuous-variable degrees of freedom for the local processing and storage of entangled states as well as their nonlocal distribution, respectively) and unambiguous state discrimination. For qutrits we quantify the light-matter entanglement that can be effectively shared through an elementary lossy channel, and for a repeater spacing of up to 10 km we show that the realistic (lossy) qutrit entanglement is even larger than any ideal (loss-free) qubit entanglement. After including qudit entanglement purification and swapping procedures, we calculate the long-distance entangled-pair distribution rates and the final entangled-state fidelities for total communication distances of up to 1280 km. For example, employing unambiguous state discrimination, with three rounds of purification, entangled qudit pairs of near-unit fidelity can be distributed over 1280 km at an ideal maximal rate (assuming perfect gate operations) on the order of 100 Hz.
Quantum communication technology provides an absolute security guarantee for information transmitted between quantum applications. To establish practical quantum networks, providing entangled pairs of stable quality, suitable for varying distances, is crucial. However, due to the inherent losses and imperfections of the operations during the entanglement distribution process, the fidelity of end-to-end entangled pairs will be greatly reduced. Consequently, establishing fidelity-guaranteed entangled pairs will be challenging. In this work, we introduce an entanglement distribution method that ensures the fidelity requirements of applications across different distances, while maintaining high efficiency. The method first decomposes the end-to-end fidelity requirements to target fidelity for purification at each link, based on the worst-case consumption. Then it performs multiple-round symmetrical entanglement purification based on the backup scheme to purify each entangled pair to the target value. The experimental results indicate that the scheme has acceptable performance in resource consumption and great feasibility while ensuring high efficiency.
Constructing a quantum memory node with the ability of long-distance atom-photon distribution is the essential task for future quantum networks, enabling distributed quantum computing, quantum cryptography, and remote sensing. Here we report the demonstration of a quantum-network node with a simple cavity-free cold atomic ensemble. This node gives an initial retrieval efficiency of approximately 55% and memory lifetime of 160 μs for atomic qubits. With the aid of a high-efficiency and polarization-independent quantum frequency conversion (QFC) module, the generated entangled photon in the node at 780-nm wavelength is converted to telecom S band at 1522 nm, enabling atom-photon distribution over long distance. We observe an entanglement fidelity between the atoms and telecom photon exceeding 80% after photon transmission over 20-km fiber with an end excitation probability of 0.2% and repetition of 1.7 kHz, the remaining infidelity being dominated by atomic decoherence. The low-noise QFC with an external efficiency up to 48.5% gives a signal-to-noise ratio of 6.9 for transmitted photons with fiber length up to 100 km, laying the cornerstone for entanglement distribution at a hundred-km level. This result provides a new platform toward the realization of a long-distance quantum network.