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arXiv:2507.14836 · Optica

Improving the rate-loss scaling in polarization entanglement distribution using single-click entanglement swapping

Hikaru Shimizu, Joe Yoshimoto, Daiki Ichii, Junko Ishi‐Hayase, Rikizo Ikuta, Masahiro Takeoka

Published Aug 11, 2026• 0 citations
arXivHTMLPDF

Abstract

Polarization entanglement is widely used in optical quantum information processing as well as quantum communication due to its compatibility with standard optical components. However, distributing polarization entanglement is severely limited by photon loss in the channels. In conventional methods, the distribution rate of polarization entanglement scales linearly with the end-to-end channel transmittance, which decreases exponentially with distance. Here, we propose and experimentally demonstrate a protocol for distributing polarization entanglement with improved rate-loss scaling. The protocol combines single-click entanglement swapping with polarization–photon-number hybrid entanglement, in which the photon-number-basis modes are sent to a central swapping node while the polarization qubits are retained by the end users. In our experiment, we observed a square-root improvement in the rate-loss scaling compared with conventional methods and achieved a fidelity of 0.843 for distributed polarization-entangled photon pairs. These results provide a proof-of-principle demonstration that the favorable rate-loss scaling of single-click entanglement swapping can be transferred to polarization entanglement distribution.

DOI
10.1364/optica.590840

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Polarization entanglement is widely used in optical quantum information processing as well as quantum communication due to its compatibility with standard optical components. However, distributing polarization entanglement is severely limited by photon loss in the channels. In conventional methods, the distribution rate of polarization entanglement scales linearly with the end-to-end channel transmittance, which decreases exponentially with distance. Here, we propose and experimentally demonstrate a protocol for distributing polarization entanglement with improved rate-loss scaling. The protocol combines single-click entanglement swapping with polarization–photon-number hybrid entanglement, in which the photon-number-basis modes are sent to a central swapping node while the polarization qubits are retained by the end users. In our experiment, we observed a square-root improvement in the rate-loss scaling compared with conventional methods and achieved a fidelity of 0.843 for distributed polarization-entangled photon pairs. These results provide a proof-of-principle demonstration that the favorable rate-loss scaling of single-click entanglement swapping can be transferred to polarization entanglement distribution.
Entanglement generation between distant nodes is a fundamental process in distributed quantum information processing. Qudits, high-dimensional quantum states, are promising candidates for enhancing entanglement distribution capabilities. However, the success probability of qudit entanglement distribution using Bell measurements is typically lower than that of conventional qubit-based protocols. In this paper, we propose an entanglement swapping protocol specifically designed for qutrits (three-dimensional quantum states). Our protocol employs photon-number encoding combined with an additional mode basis, such as polarization, effectively increasing the success probability. We show that the proposed qutrit protocol can achieve higher entanglement generation rates compared to conventional two-photon detection-based qubit protocols, especially under conditions of limited photon generation probability. Furthermore, we evaluate our protocol under realistic experimental imperfections, including photon loss and threshold detection, and show that it achieves high fidelity with probabilistic photon sources.
Entanglement between distant parties is a key resource in quantum networks. However, photon losses in quantum channels significantly reduce the success probability of entanglement sharing, which scales quadratically with the channel transmittivity. Quantum repeaters using entanglement swapping can mitigate this effect, but usually require high-performance photonic quantum memories to synchronize photonic qubits. In this work, we theoretically and experimentally investigate an entanglement swapping protocol using photon-number-encoded states that can effectively alleviate quantum channel losses without requiring photonic quantum memories. We demonstrate that the protocol exhibits a success probability scaling linearly with the channel transmittivity. Furthermore, we show that while unbalanced channel losses can degrade the shared entanglement, this effect can be compensated by optimally adjusting the initial entangled states. Our results highlight the potential of photon-number encoding for realizing robust entanglement distribution in lossy quantum networks.
Quantum networks scale the advantages of quantum communication protocols to more than just two distant users. Here we present a fully connected quantum network architecture in which a single entangled photon source distributes quantum states to a multitude of users. Our network architecture thus minimizes the resources required of each user without sacrificing security or functionality. As no adaptations of the source are required to add users, the network can readily be scaled to a large number of clients, whereby no trust in the provider of the quantum source is required. Unlike previous attempts at multi-user networks, which have been based on active components, and thus limited to some duty cycle, our implementation is fully passive and thus provides the potential for unprecedented quantum communication speeds. We experimentally demonstrate the feasibility of our approach using a single source of bi-partite polarization entanglement which is multiplexed into 12 wavelength channels to distribute 6 states between 4 users in a fully connected graph using only 1 fiber and polarization analysis module per user. We then discuss practical usage scenarios to demonstrate the advantage of some of the salient features of our network topology. These include adaptations/hardware implementations that are more favorable for local area networks and those for long distance intercity links. Our implementation consisted of a single Type 0 MgO:PPLN crystal pumped bidirectionally in a Sagnac interferometer to produce polarization entangled photon pairs with an ≈ 60 nm bandwidth. These pair wise entangled photons were split into 12 different wavelength channels corresponding to 6 correlated wavelength pairs. Each of the 4 users in our demonstration received 3 wavelength channels multiplexed together via a solitary single mode fiber. Each user had just one detector unit that they used to simultaneously measure all input channels. We exploited small time delays between different channels to isolate the relevant signal from noise. We measured the quality of entanglement between each channel pair simultaneously and calculated the effective secure key rate using the E91 protocol. Our experiment showed the effectiveness of our network topology and its scalability against noise, number of users and losses. Further experiments are underway to increase the number of clients and improve the scalability of the topology. Also, we plan to demonstrate distributed computation tasks like secure auctions, Byzantine fault tolerance, and asynchronous reference frame agreement are feasible using our network architecture.
Quantum coherence and entanglement, which are essential resources for quantum information, are often degraded and lost due to decoherence. Here, we report a proof-of-principle experimental demonstration of high fidelity entanglement distribution over decoherence channels via qubit transduction. By unitarily switching the initial qubit encoding to another, which is insensitive to particular forms of decoherence, we have demonstrated that it is possible to avoid the effect of decoherence completely. In particular, we demonstrate high-fidelity distribution of photonic polarization entanglement over quantum channels with two types of decoherence, amplitude damping and polarization-mode dispersion, via qubit transduction between polarization qubits and dual-rail qubits. These results represent a significant breakthrough in quantum communication over decoherence channels as the protocol is input-state independent, requires no ancillary photons and symmetries, and has near-unity success probability.
Long-distance, entanglement distribution over thousands of kilometers is necessary to build a global-scale quantum network. Directly transferring entangled, telecom photons through fiber is relatively straight forward in local networks where the separation between nodes is below 100 km, corresponding to a 1% transmission. However, the exponentially greater loss at larger distances (e.g. only 10−6 transmission at 300 km) leads to unacceptably low transmission rates, which drop below the noise rates of typical photon detectors. Alternatively, the Earth’s rarefied atmosphere at high altitudes reduces the absorption and scattering of infrared photons. Therefore, free-space links between satellite and ground stations have been shown to enable entangled photon transmission over 2000 km. While this demonstration showed the viability of satellite-enhanced entanglement distribution, the source used in this demonstration produced distinguishable photons, which cannot be used for many fundamental protocols like teleportation. We are building a testbed to demonstrate the feasibility of distributing entangled, indistinguishable photons over a space-to-ground link using entanglement swapping.