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arXiv:2512.17148 · Physical Review Applied

Zero-added-loss entanglement multiplexing using time-bin spectral shearing

Joseph C. Chapman, Muneer Alshowkan, Jack Postlewaite, Saikat Guha, Nageswara Rao

quant-phSubmitted Dec 19, 2025• Announced Dec 22, 2025• 0 citations
v1 · Dec 19, 2025v2 · May 3, 2026
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Abstract

High-quality quantum communications that enable important capabilities, such as distributed quantum computing and sensing, will require quantum repeaters for providing high-quality entanglement. To realize high-rate heralded entanglement for quantum repeaters, Chen et al. [Phys. Rev. Appl. 19, 054209 (2023)] proposed a scheme for heralded-multiplexed generation of quasi-deterministic entangled photon pairs, called zero-added-loss multiplexing (ZALM). Here, we propose a design of ZALM source using time-bin entanglement and spectral shearing. Additionally, we provide an analysis of experimentally relevant spectral-shearing parameters to optimize the spectral multiplexing. Moreover, we experimentally verify the compatibility of time-bin pulses and spectral shearing, as supported by observation of no phase shift when the same shearing is applied to both time bins. These results expand the benefits of applying a ZALM source to time-bin entanglement use cases. Moreover, more fully demonstrating time-bin and spectral shearing compatibility clears a path towards a broader use of spectral shearing that provides a deterministic frequency shift of high utility.

Journal reference
Phys. Rev. Applied 26, 014018 (2026)
DOI
10.1103/c8ss-8vyh
Comments
14 pages, 14 figures

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High-quality quantum communications that enable important capabilities, such as distributed quantum computing and sensing, will require quantum repeaters for providing high-quality entanglement. To realize high-rate heralded entanglement for quantum repeaters, Chen et al. [Phys. Rev. Appl. 19, 054209 (2023)] proposed a scheme for heralded-multiplexed generation of quasi-deterministic entangled photon pairs, called zero-added-loss multiplexing (ZALM). Here, we propose a design of ZALM source using time-bin entanglement and spectral shearing. Additionally, we provide an analysis of experimentally relevant spectral-shearing parameters to optimize the spectral multiplexing. Moreover, we experimentally verify the compatibility of time-bin pulses and spectral shearing, as supported by observation of no phase shift when the same shearing is applied to both time bins. These results expand the benefits of applying a ZALM source to time-bin entanglement use cases. Moreover, more fully demonstrating time-bin and spectral shearing compatibility clears a path towards a broader use of spectral shearing that provides a deterministic frequency shift of high utility.
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.
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Zero-added-loss multiplexing (ZALM)~[Phys. Rev. Appl. {\bf 19}, 054029 (2023)] is a midpoint-source scheme for entanglement distribution. Its quantum transmitter (QTX) employs two Sagnac sources of polarization-entangled pairs plus partial Bell-state measurements (BSMs) on their idler outputs to herald entanglement in their signal outputs, thus avoiding the switching losses suffered by other multiplexing approaches. When its Sagnac sources' spontaneous parametric downconverters (SPDCs) utilize χ(2)χ^{(2)}χ(2) crystals with phase-matched spectral islands, ZALM avoids the tradeoff between heralding efficiency and output-state fidelity incurred when conventional-crystal SPDCs' outputs are channelized by dense wavelength-division multiplexing filters. This paper reports an end-to-end fidelity and rate analysis for islands-based ZALM that includes: SPDC multipair emissions of all orders and partial-BSM losses in the ZALM QTX; loss in propagation from the ZALM QTX to Alice and Bob's quantum receivers (QRXs); loss and mode mismatch in the mode converters employed at Alice and Bob's QRXs; and loss in the partial-BSM loading of Alice and Bob's emissive quantum memories.
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
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 >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.