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arXiv:2501.04131 · Physical Review X

Heralded Entanglement of On-Demand Spin-Wave Solid-State Quantum Memories for Multiplexed Quantum Network Links

Jonathan Hänni, Alberto E. Rodríguez-Moldes, Félicien Appas, Sören Wengerowsky, Dario Lago-Rivera, Markus Teller, Samuele Grandi, Hugues de Riedmatten

Published Aug 15, 2025• 13 citations
arXivHTMLPDF

Abstract

The ability to distribute heralded entanglement between distant matter nodes is a primitive for the implementation of large-scale quantum networks. Some of the most crucial requirements for future applications include high heralding rates at telecom wavelengths, multiplexed operation, and on-demand retrieval of stored excitations for synchronization of separate quantum links. Despite tremendous progress in various physical systems, the demonstration of telecom-heralded entanglement between quantum nodes featuring both multiplexed operation and on-demand retrieval remains elusive. In this work, we combine narrow band parametric photon-pair sources and solid-state quantum memories based on rare-earth doped crystals to demonstrate telecom-heralded entanglement between spatially separated spin-wave quantum memories with fully adjustable recall time and temporal multiplexing of 15 modes. In a first experiment, the storage in the spin state is conditioned on the entanglement heralding. We take advantage of the control over readout pulse phase to achieve feedforward conditional phase shifts on the stored photons depending on which heralding detector clicked. We exploit this effect to double the entanglement heralding rate for a given quantum state up to 510 counts / s , with an associated detection rate of 0.32 counts / s and measured positive concurrence by up to 6 standard deviations. In a second experiment, we simulate the communication time of a long-distance link by implementing an unconditional storage scheme with a dead time of 100 μ s . We take advantage of temporal multiplexing to increase the entanglement rates by a factor of 15 with respect to single mode storage, reaching a value of 22 counts / s per heralding detector. These results establish our architecture as a prime candidate for the implementation of scalable high-rate quantum network links.

DOI
10.1103/wvv1-6lg8

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The ability to distribute heralded entanglement between distant matter nodes is a primitive for the implementation of large-scale quantum networks. Some of the most crucial requirements for future applications include high heralding rates at telecom wavelengths, multiplexed operation, and on-demand retrieval of stored excitations for synchronization of separate quantum links. Despite tremendous progress in various physical systems, the demonstration of telecom-heralded entanglement between quantum nodes featuring both multiplexed operation and on-demand retrieval remains elusive. In this work, we combine narrow band parametric photon-pair sources and solid-state quantum memories based on rare-earth doped crystals to demonstrate telecom-heralded entanglement between spatially separated spin-wave quantum memories with fully adjustable recall time and temporal multiplexing of 15 modes. In a first experiment, the storage in the spin state is conditioned on the entanglement heralding. We take advantage of the control over readout pulse phase to achieve feedforward conditional phase shifts on the stored photons depending on which heralding detector clicked. We exploit this effect to double the entanglement heralding rate for a given quantum state up to 510 counts / s , with an associated detection rate of 0.32 counts / s and measured positive concurrence by up to 6 standard deviations. In a second experiment, we simulate the communication time of a long-distance link by implementing an unconditional storage scheme with a dead time of 100 μ s . We take advantage of temporal multiplexing to increase the entanglement rates by a factor of 15 with respect to single mode storage, reaching a value of 22 counts / s per heralding detector. These results establish our architecture as a prime candidate for the implementation of scalable high-rate quantum network links.
Entanglement between photons at telecommunication wavelengths and long-lived quantum memories is one of the fundamental requirements of long-distance quantum communication. Quantum memories featuring on-demand readout and multimode operation are additional precious assets that will benefit the communication rate. In this Letter, we report the first demonstration of entanglement between a telecom photon and a collective spin excitation in a multimode solid-state quantum memory. Photon pairs are generated through widely nondegenerate parametric down-conversion, featuring energy-time entanglement between the telecom-wavelength idler and a visible signal photon. The latter is stored in a Pr^{3+}:Y_{2}SiO_{5} crystal as a spin wave using the full atomic frequency comb scheme. We then recall the stored signal photon and analyze the entanglement using the Franson scheme. We measure conditional fidelities of 92(2)% for excited-state storage, enough to violate a Clauser-Horne-Shimony-Holt inequality, and 77(2)% for spin-wave storage. Taking advantage of the on-demand readout from the spin state, we extend the entanglement storage in the quantum memory for up to 47.7 μs, which could allow for the distribution of entanglement between quantum nodes separated by distances of up to 10 km.
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Multiplexed quantum memories increase the entanglement distribution rate in long-distance quantum repeater architectures by harnessing storage in several degrees of freedom. Here, we report on the distribution of light-matter quantum correlations using an array of time-multiplexed solid-state quantum memories. We store telecom-heralded single photons sequentially in up to ten memory cells using the full atomic frequency comb protocol with on-demand read-out in a Pr3+^{3+}3+:Y2_22​SiO5_55​ crystal. Leveraging both spatial and temporal multiplexing, we demonstrate quantum correlations between the telecom photon and up to 60 spatio-temporal modes of the quantum memory array. We then transmit the heralding telecom photon over 39.1 km of deployed optical fiber in the Metropolitan Area of Barcelona. In a realistic scenario where the generation rate is limited by the two-way communication time, we show that up to 15 % of the 393μs393 μs393μs round-trip communication time is filled with communication trials, leading to a 60-fold enhancement in the rate of detected telecom photons correlated with the quantum memory array, compared to a single-mode memory. With increased storage times and efficiencies, our multiplexed quantum memory array will constitute the backbone of a long-distance quantum network, establishing remote entanglement at high rates.
As the reach of quantum technologies extends ever further in communication and information science, a reliable way of transferring quantum information between distant locations becomes ever more crucial. While photons are widely accepted as excellent carriers due to their speed and low decoherence, losses of transmission (in free space or fibre) and the impossibility of cloning quantum information still pose a great challenge. The quantum repeater architecture was suggested as a solution to both problems [1]. In a quantum repeater the information encoded in an input state is transferred to a new one through entanglement swapping, that is then sent on along the channel. In this work we present our advances towards the realisation of a quantum repeater. Our system of choice combines a solid-state quantum memory with a source of photon pairs. The memory is based on a Rare-Earth Doped crystal, where quantum information can be stored in Pr3+ ions as a collective excitation using the Atomic Frequency Comb technique. On demand retrieval of the information is realised by transferring the excitation to a long-lived spin state. Record values of storage times and retrieval efficiencies have been demonstrated in this system [2]. Entangled pairs of single photons are generated by parametric down conversion in a periodically poled crystal placed inside a bow-tie cavity. This allows us to generate narrow band photons pairs, where the signal is spectrally matched to the memory (606nm), while the idler is in the telecom band [3]. Such a configuration allows us to benefit from the high performance of the memory, that also allows for temporal [2] and frequency [4] multimodality, while at the same time overcoming the high optical losses of 606nm photons by pair generation of a telecom photon. The first stepping stone, progress towards which is presented in this work, is the successful demonstration of energy-time entanglement between the telecom idler photon and the signal photon, stored as spin-wave excitation. The entanglement of the original pair is maintained by the memory temporal multimodality. The entanglement analysis will be made through time-bin qubit analysers made of a fibre-based Mach-Zehnder interferometer, for the former, and a solid-state equivalent based on two AFC with different storage times, for the latter [5]. In this direction we have already doubled the efficiency of the AFC storage protocols, that will be beneficial to count rates and signal-to-noise ratio. With respect to [2], we also increased the spectral-matching between the source and the memory [4]. Our experiment will provide an increase in storage time of 3 orders of magnitude with respect to previous demonstrations, as well as introducing for the first time on-demand read-out in a highly multi-mode memory. Demonstration of the successful transfer of quantum information between the signal photon and the long-lived solid-state excitation will open the way to the demonstration of long-distance entanglement between individual nodes in a quantum network.
Abstract The exploitation of multimodality in different degrees of freedom is one of the most promising ways to increase the rate of heralded entanglement between distant quantum nodes. In this paper, we realize a spatially-multiplexed solid-state quantum memory array with ten individually controllable spin-wave memory cells featuring on-demand read-out and temporal multiplexing. By combining spatial and temporal multiplexing, we store weak coherent pulses at the single-photon level in up to 250 spatio-temporal modes, with an average signal-to-noise ratio of 10(2). We perform a thorough characterization of the whole system, including its multiplexing and demultiplexing stage. We verify that the memory array exhibits low cross-talk even at the single-photon level. The measured performance indicates readiness for storing non-classical states and promises a speed-up in entanglement distribution rates.