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International Conference on Transparent Optical Networks

Quantum Repeater Links with Rare-Earth Solid-State Quantum Memories

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

Published Jul 6, 2025• 0 citations
Publisher

Abstract

In this talk, we report on recent progress towards the realization of quantum repeaters based on rare-earth doped crystal quantum memories (QM) and cavity-enhanced SPDC photon-pair sources. We first present our demonstration of on-demand telecom-heralded entanglement between multimode solid-state quantum memories with high rate and fully adjustable recall time. Then, we show how we successfully established long-distance non-classical correlations between a telecom photon and a QM in a field-deployed experiment in the city of Barcelona.

DOI
10.1109/icton67126.2025.11125216

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In this talk, we report on recent progress towards the realization of quantum repeaters based on rare-earth doped crystal quantum memories (QM) and cavity-enhanced SPDC photon-pair sources. We first present our demonstration of on-demand telecom-heralded entanglement between multimode solid-state quantum memories with high rate and fully adjustable recall time. Then, we show how we successfully established long-distance non-classical correlations between a telecom photon and a QM in a field-deployed experiment in the city of Barcelona.
Quantum correlations between long-lived quantum memories and telecom photons that can propagate with low loss in optical fibers are an essential resource for the realization of large-scale quantum information networks.Significant progress has been realized in this direction with atomic and solid-state systems.Here, we demonstrate quantum correlations between a telecom photon and a multimode ondemand solid state quantum memory.This is achieved by mapping a correlated single photon onto a spin collective excitation in a Pr 3þ ∶Y 2 SiO 5 crystal for a controllable time.The stored single photons are generated by cavity-enhanced spontaneous parametric down-conversion and heralded by their partner photons at telecom wavelength.These results represent the first demonstration of a multimode on-demand solid state quantum memory for external quantum states of light.They provide an important resource for quantum repeaters and pave the way for the implementation of quantum information networks with distant solid state quantum nodes.
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.
Quantum correlations between long-lived quantum memories and telecom photons that can propagate with low loss in optical fibers are an essential resource for the realization of large-scale quantum information networks. Significant progress has been realized in this direction with atomic and solid-state systems. Here, we demonstrate quantum correlations between a telecom photon and a multimode ondemand solid state quantum memory. This is achieved by mapping a correlated single photon onto a spin collective excitation in a Pr 3+ :Y 2 SiO 5 crystal for a controllable time. The stored single photons are generated by cavity-enhanced spontaneous parametric down-conversion and heralded by their partner photons at telecom wavelength. These results represent the first demonstration of a multimode on-demand solid state quantum memory for external quantum states of light. They provide an important resource for quantum repeaters and pave the way for the implementation of quantum information networks with distant solid state quantum nodes.
Quantum memories are very important in quantum communication and information as they provide a quantum interface between photons, used for long distance communication, and matter, in which stationary qubits can be stored. Moreover they are fundamental building blocks in quantum repeaters. Rare-earth doped crystals (REDC) are promising candidates for the implementation of quantum memories because of their excellent coherence properties at cryogenic temperatures and because solid state offers good prospect for scalability and integration.
The ability of storing and retrieving quantum states of light is an important experimental challenge in quantum information science. A powerful quantum memory for light is required in a quantum repeater, which would allow long distance (>500km) quantum communications. To be implemented in such application, the quantum memory must allow on-demand readout, with high fidelity and efficiency, and a long storage time. Additionally a multimode capacity (for temporal or spatial modes) would allow multiplexing. Our approaches focus on rare-earth doped crystals, i.e. solid state quantum memory. I present, in this work, our contributions for a solid-state quantum storage, with good performances in every criteria. In particular, I present the preservation of quantum entanglement during the storage, which paves the way for the implementation of quantum memories in quantum repeaters.