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arXiv:2206.03670 · Physical Review Applied

Zero-Added-Loss Entangled-Photon Multiplexing for Ground- and Space-Based Quantum Networks

Kevin C. Chen, Prajit Dhara, Mikkel Heuck, Yuan Lee, Wenhan Dai, Saikat Guha, Dirk Englund

Published May 9, 2023• 42 citations
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Abstract

We propose a scheme for optical entanglement distribution in quantum networks based on a quasideterministic entangled photon-pair source. By combining heralded photonic Bell-pair generation with spectral mode conversion to interface with quantum memories, the scheme eliminates switching losses due to multiplexing in the source. We analyze this ``zero-added-loss multiplexing'' (ZALM) Bell-pair source for the particularly challenging problem of long-baseline entanglement distribution via satellites and ground-based memories, where it unlocks additional advantages: (i) the substantially higher channel efficiency $\ensuremath{\eta}$ of downlinks versus uplinks with realistic adaptive optics, and (ii) photon loss occurring before interaction with the quantum memory---i.e., Alice and Bob receiving rather than transmitting---improve entanglement generation rate scaling by $\mathcal{O}(\sqrt{\ensuremath{\eta}})$. Based on numerical analyses, we estimate our protocol to achieve $>10\phantom{\rule{0.2em}{0ex}}\text{ebit/s}$ at memory multiplexing of ${10}^{2}$ spin qubits for ground distance $>{10}^{2}\phantom{\rule{0.2em}{0ex}}\text{km}$, with the spin-spin Bell-state fidelity exceeding 99%. Our architecture presents a blueprint for realizing global-scale quantum networks in the near term.

DOI
10.1103/physrevapplied.19.054029

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We propose a scheme for optical entanglement distribution in quantum networks based on a quasideterministic entangled photon-pair source. By combining heralded photonic Bell-pair generation with spectral mode conversion to interface with quantum memories, the scheme eliminates switching losses due to multiplexing in the source. We analyze this ``zero-added-loss multiplexing'' (ZALM) Bell-pair source for the particularly challenging problem of long-baseline entanglement distribution via satellites and ground-based memories, where it unlocks additional advantages: (i) the substantially higher channel efficiency \ensuremathη\ensuremath{\eta}\ensuremathη of downlinks versus uplinks with realistic adaptive optics, and (ii) photon loss occurring before interaction with the quantum memory---i.e., Alice and Bob receiving rather than transmitting---improve entanglement generation rate scaling by O(\ensuremathη)\mathcal{O}(\sqrt{\ensuremath{\eta}})O(\ensuremathη​). Based on numerical analyses, we estimate our protocol to achieve >10ebit/s>10\phantom{\rule{0.2em}{0ex}}\text{ebit/s}>10ebit/s at memory multiplexing of 102{10}^{2}102 spin qubits for ground distance >102km>{10}^{2}\phantom{\rule{0.2em}{0ex}}\text{km}>102km, with the spin-spin Bell-state fidelity exceeding 99%. Our architecture presents a blueprint for realizing global-scale quantum networks in the near term.
Boosting the communication rate of quantum networks is a central challenge in quantum information science. Recently, an efficient entanglement distribution scheme employing quasi-deterministic Bell-pair sources based on time-frequency multiplexing, referred to as zero-added-loss multiplexing~(ZALM), has been proposed. Its implementation, however, requires high-fidelity entanglement swapping across densely multiplexed time-frequency modes, which has remained an experimental challenge. Here we demonstrate entanglement swapping across 16 parallel frequency modes with a high average fidelity of 93.9±\pm±\SI{1.4}{\%}. Notably, polarization-entangled photon pairs in each frequency mode are spectrally single-mode using only off-the-shelf 50-GHz dense wavelength-division multiplexing~(DWDM) filters, eliminating the need for additional narrowband filtering. Furthermore, in order to fully exploit the temporal degree of freedom, the pump pulse is operated with a repetition frequency of \SI{3.0}{GHz}. By combining the frequency and time multiplexing, the total swapping rate reaches 5.38$\pm$0.17\,\si{pairs\,s^{-1}}, which corresponds to the ZALM Bell-pair rate of \SI{8.2e2}{pairs\,s^{-1}}. Our results establish the key experimental capabilities required for ZALM and demonstrate a scalable route toward practical high-rate quantum repeaters and long-haul quantum networks.
The recently proposed zero-added-loss multiplexing (ZALM) source of entangled photons enables higher efficiency in entanglement distribution than spontaneous parametric down-conversion sources and can be carried out using both space-to-ground and ground-to-ground links. We demonstrate the flexibility of ZALM architectures to be adapted to alternative entanglement distribution protocols. Focusing on the counterintuitive result that entanglement can be generated between distant parties without using any entanglement as a resource, we analyze two protocols for entanglement distribution to memories via separable states. Modeling them in a ZALM setup, we consider the effects of noise both in the communication channels and in the memories. We thereby identify the optimal protocol to use with respect to the highest entanglement generated, given the noise conditions of the network. Published by the American Physical Society 2024
High-rate, high-fidelity entanglement distribution is essential to the creation of a quantum internet, but recent achievements in fiber (248 km at a 9- s − 1 rate) and satellite-based (1200 km at a 1.1- s − 1 rate) entanglement distribution fall far short of what is needed. Chen . [Phys. Rev. Appl. , 054209 (2023)] proposed a means for dramatically increasing entanglement-distribution rates via a scheme they called zero-added-loss multiplexing (ZALM). ZALM’s quantum transmitter employs a pair of Sagnac-configured spontaneous parametric down-converters (SPDCs), channelization via dense wavelength-division multiplexing (DWDM) filtering, and partial Bell-state measurements (BSMs) to realize a heralded source of frequency-multiplexed polarization-entangled biphotons. Each biphoton is transmitted to Alice and Bob along with a classical message identifying its frequency channel and whether a ψ − singlet or a ψ + triplet was heralded. Alice’s and Bob’s quantum receivers then use DWDM filtering and temporal-mode conversion to interface their received biphotons to intracavity color-center quantum memories. This paper delves deeply into ZALM’s SPDCs, partial BSMs, and Duan-Kimble loading of Alice’s and Bob’s quantum memories. Its principal results—the density operators for the SPDC sources and the quantum memories—allow heralding probability, heralding efficiency, and fidelity to be evaluated for both the polarization-entangled biphotons and the loaded quantum memories, thus enabling exploration of the parameter space for optimizing ZALM’s performance. Even without a comprehensive optimization analysis, the paper’s examples already demonstrate two critical features of the ZALM architecture: (1) the necessity of achieving a near-separable channelized biphoton wave function to ensure that the biphoton sent to Alice and Bob is of high purity; and (2) the premium placed on Alice’s and Bob’s temporal-mode converters enabling narrowband push-pull memory loading to ensure that the arriving biphoton’s state is faithfully transferred to the intracavity color centers. Published by the American Physical Society 2024
Spontaneous parametric down-converters (SPDCs) are the best available entanglement sources for distributing entanglement in a quantum internet. However, their intrinsically probabilistic nature, and their need to operate at low brightness to suppress multipair events, dictate that multiplexed SPDC arrays are required for high-rate distribution in that application. Early SPDC multiplexing proposals involved path switching, whose switching losses significantly degrade performance. The present paper proposes and analyzes a scheme for spectral multiplexing that provides entanglement-distribution rates well in excess of the state of the art. It builds on zero-added-loss multiplexing (ZALM)~[Phys. Rev. Appl. {\bf 19}, 054029 (2023)] for high-rate heralded entanglement generation, which does not require a switched array of SPDCs. Our ZALM's SPDCs rely on nonlinear crystals with NIN_INI​ phase-matched spectral islands, each generating two-mode squeezed-vacuum states. Also, our ZALM's multiplexing protocol uses both same-island and cross-island heralding, which allows the entanglement-delivery rate to approximately scale as NI2N_I^2NI2​ in the realistic weak-squeezing regime. As a result, our scheme uses an order of magnitude fewer spectral channels than the original ZALM proposal, which may enable near-term implementations of satellite-to-ground or fiber-optic based ZALM architectures.
Photonic entanglement distribution is key for the emerging quantum internet. Here, the authors propose a spectral-multiplexing scheme that delivers favorable entanglement rates by combining domain-engineered crystals with zero-added-loss multiplexing. Their scheme uses a down-conversion crystal that can produce photon pairs in distinct, spectrally factorable ``islands'', and by combining same- and cross-island heralding, they dramatically reduce the number of required spectral channels.
The realization of a global quantum network holds the potential to enable groundbreaking applications such as secure quantum communication and blind quantum computing. However, building such a network remains a formidable challenge, primarily due to photon loss in optical fibers. In this work, we propose a quantum repeater architecture for distributing entanglement over intercontinental distances by leveraging low-Earth-orbit satellites equipped with spontaneous parametric down-conversion (SPDC) photon-pair sources and ground stations utilizing single-atom memories in optical cavities and single-photon detectors to implement the cavity-assisted photon scattering (CAPS) gates for high-fidelity entanglement mapping. The efficient entanglement swapping is achieved by performing high-fidelity Rydberg gates and readouts. We evaluate the entanglement distribution rates and fidelities by analyzing several key imperfections, including time-dependent two-photon transmission and time-dependent pair fidelity, for various satellite heights and ground station distances. We also investigate the impact of pair source fidelity and spin decoherence rate on the repeater performance. Furthermore, we introduce a spatial-frequency multiplexing strategy within this architecture to enhance the design's performance. Finally, we discuss in detail the practical implementation of this architecture. Our results show that this architecture enables entanglement distribution over intercontinental distances. For example, it can distribute over 10000 pairs per flyby over 10000 km with a fidelity above 90%, surpassing the capabilities of terrestrial quantum repeaters