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arXiv:2403.13623 · Nature Communications

Fast delivery of heralded atom-photon quantum correlation over 12 km fiber through multiplexing enhancement

S.F. Zhang, Jixuan Shi, Yibo Liang, Yuedong Sun, Yukai Wu, L.-M. Duan, Yunfei Pu

Published Nov 27, 2024• 17 citations
arXivHTMLPDF

Abstract

Distributing quantum entanglement between distant parties is a significant but difficult task in quantum information science, as it can enable numerous applications but suffers from exponential decay in the quantum channel. Quantum repeaters are one of the most promising approaches towards this goal. In a quantum repeater protocol, it is essential that the entanglement generation speed within each elementary link is faster than the memory decoherence rate, and this stringent requirement has not been implemented over a fiber of metropolitan scale so far. As a step towards this challenging goal, in this work we experimentally realize multiplexing-enhanced generation of heralded atom-photon quantum correlation over a 12 km fiber. We successively generate 280 pairs of atom-photon quantum correlations with a train of photonic time-bin pulses filling the long fiber, and read out the excited memory modes on demand with either fixed or variable storage time after successful heralding. With the multiplexing enhancement, the heralding rate of atom-photon correlation can reach 1.95 kHz, and the ratio between the quantum correlation generation rate to memory decoherence rate can be improved to 0.46 for a fiber length of 12 km. This work therefore constitutes an important step towards the realization of a large-scale quantum repeater network.

DOI
10.1038/s41467-024-54691-3

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Distributing quantum entanglement between distant parties is a significant but difficult task in quantum information science, as it can enable numerous applications but suffers from exponential decay in the quantum channel. Quantum repeaters are one of the most promising approaches towards this goal. In a quantum repeater protocol, it is essential that the entanglement generation speed within each elementary link is faster than the memory decoherence rate, and this stringent requirement has not been implemented over a fiber of metropolitan scale so far. As a step towards this challenging goal, in this work we experimentally realize multiplexing-enhanced generation of heralded atom-photon quantum correlation over a 12 km fiber. We successively generate 280 pairs of atom-photon quantum correlations with a train of photonic time-bin pulses filling the long fiber, and read out the excited memory modes on demand with either fixed or variable storage time after successful heralding. With the multiplexing enhancement, the heralding rate of atom-photon correlation can reach 1.95 kHz, and the ratio between the quantum correlation generation rate to memory decoherence rate can be improved to 0.46 for a fiber length of 12 km. This work therefore constitutes an important step towards the realization of a large-scale quantum repeater network.
Quantum network and quantum repeater are promising ways to scale up a quantum information system. In a functional quantum network, it is required that the distribution rate of heralded remote entanglement should be higher than the decoherence rate of each local node. A promising scheme to accelerate the remote entanglement distribution is through multiplexing enhancement. In this work, we experimentally realize a multiplexed quantum network node based on a chain of 40Ca+ ions. We employ a hybrid multiplexing scheme in which maximally 44 time-bin modes are generated and sent through a long fiber to boost the entangling rate. Via this scheme, we can generate heralded ion-photon entanglement with a success rate of 4.28 s−1 over a 12 km fiber. In addition, a dual-type framework is utilized to protect quantum information from the destructive ion-photon entangling attempts and a memory coherence time of 366 ms is achieved, which has exceeded the entanglement generation time. Despite recent advances with trappedion-based platforms, achieving quantum networks with link efficiency greater than unity on metropolitan scales is still a challenge. Here, the authors demonstrate a multiplexed quantum network generating heralded entanglement at a rate faster than local decoherence.
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Realization of long-distance quantum communication through the quantum repeater network requires a combination of some key elements, including a multiplexed quantum memory for storage of entanglement and a telecom photon for propagation of information through the fiber.Although impressive experimental advances have demonstrated the individual elements, combining these key capabilities together and realizing them in a single experimental system remains a significant challenge.Here, we report an experimental realization of long-distance entanglement between a multiplexed quantum memory with 49 individually accessible memory cells and a telecom photon after transmission in a 10-km optical fiber.Excitation of an atomic ensemble generates narrow-band polarization entanglement between a telecom photon of 1530-nm wavelength and another photon of 780-nm wavelength, which is then stored into a memory cell of a multiplexed atomic quantum memory and read out after a controllable delay.The entanglement is verified through quantum-state tomography after quantum storage in the atomic memory and fiber transmission of the telecom photon.This experiment demonstrates an important step towards realization of long-distance quantum communication networks.
Duan-Lukin-Cirac-Zoller quantum repeater protocol provides a feasible scheme to implement long-distance quantum communication and large-scale quantum networks. The elementary link, namely the entanglement between two atomic ensembles, is a fundamental component of quantum repeater. For practical quantum repeater, it is required that the elementary link can be prepared with high yield and the spin waves stored in atoms can be efficiently converted into photons on demand. However, so far, such quantum repeater link has not been demonstrated in experiments. Here, we demonstrate a proof-of-principle multiplexed quantum repeater link by entangling two temporally multiplexed quantum memory. Compared with a single-mode link, the successful preparation rate of the multiplexed link is increased by one order of magnitude. By using the cavity-enhanced scheme, the on-demand retrieval efficiency of atomic spin waves is improved to 70%, which is beneficial for the subsequent entanglement swapping between adjacent links. The realization of temporally multiplexed quantum repeater link with high retrieval efficiency lays a foundation for the development of practical quantum networks.
We present a quantum repeater protocol for distributing entanglement over long distances, where a dedicated communication stage enables trial rates not limited by the travel time between repeater nodes. To accomplish this, each node contains several qubits that can couple to one single-photon emitter. Photons from the emitters in neighboring nodes generate heralded entanglement between qubits in these nodes. The protocol leaves the emitters disentangled from the rest of the system immediately after emitting the photons, thus allowing them to be reused to entangle other qubits without waiting for the repeater link round-trip time. This time multiplexing increases the protocol trial rate by up to an order of magnitude. The protocol is then combined with conventional deterministic entanglement swapping and heralded entanglement purification to extend the entanglement distance and reduce the entanglement error, respectively. We perform a complete protocol analysis by considering all relevant error sources, such as initialization, two-qubit gate, and qubit measurement errors, as well as the exponential decoherence of the qubits with time. The latter is particularly important since we analyze the protocol performance for a broad range of experimental parameters and obtain secret key rates ranging from 1 to 1000 Hz at a distance of 1000 km. Our results suggest that it is important to reach a qubit memory coherence time of around 1 s, and two-qubit gate and measurement errors on the order of 10 − 3 to obtain reasonable secret key rates over distances longer than achievable with direct transmission. While this work focuses on optimizing secret key rates, the protocol can also be used for Einstein-Podolsky-Rosen pair generation and is thus also relevant for, e.g., distributed quantum computing.
Abstract Quantum repeaters based on heralded entanglement require quantum nodes that are able to generate multimode quantum correlations between memories and telecommunication photons. The communication rate scales linearly with the number of modes, yet highly multimode quantum storage remains challenging. In this work, we demonstrate an atomic frequency comb quantum memory with a time-domain mode capacity of 1250 modes and a bandwidth of 100 MHz. The memory is based on a Y2SiO5 crystal doped with 171Yb3+ ions, with a memory wavelength of 979 nm. The memory is interfaced with a source of non-degenerate photon pairs at 979 and 1550 nm, bandwidth-matched to the quantum memory. We obtain strong non-classical second-order cross correlations over all modes, for storage times of up to 25 μs. The telecommunication photons propagated through 5 km of fiber before the release of the memory photons, a key capability for quantum repeaters based on heralded entanglement and feed-forward operations. Building on this experiment should allow distribution of entanglement between remote quantum nodes, with enhanced rates owing to the high multimode capacity.