Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Light-matter entanglement for hybrid quantum networks

Rego Falagán, Nuria

Published Sep 9, 2024• 0 citations
OpenAlex

Abstract

This research aims to establish a key element of hybrid quantum networks by demonstrating remote entanglement between disparate quantum systems, specifically a cold atomic ensemble and a rare-earth ion-doped crystal, on a long-term project. In this work, we focused on two important aspects towards this goal: the development of a qubit converter and quantum frequency conversion techniques. The qubit converter is built to transform polarization qubits from the cold atoms system into time-bin qubits to ensure compatibility with photons from the solid-state system. Then, quantum frequency conversion based on non-linear effects is studied to shift the 780nm entangled photon of the atomic system to the telecom C-band, aligning it with photons from the rare-earth ion-doped crystal experiment. This process is essential for conducting the Bell-state measurement required for remote entanglement generation.

Related papers

Quantum frecuency conversion for hybrid quantum networks

Nicolas Maring

The ability to control the optical frequency of quantum state carriers (i.e. photons) is an important functionality for future quantum networks. It allows all matter quantum systems - nodes of the network - to be compatible with the telecommunication C-band, therefore enabling long distance fiber quantum communication between them. It also allows dissimilar nodes to be connected with each other, thus resulting in heterogeneous networks that can take advantage of the different capabilities offered by the diversity of its constituents. Quantum memories are one of the building blocks of a quantum network, enabling the storage of quantum states of light and the entanglement distribution over long distances. In our group, two different types of memories are investigated: a cold atomic ensemble and an ion-doped crystal. In this thesis I investigate the quantum frequency conversion of narrow-band photons, emitted or absorbed by optical quantum memories, with two different objectives: the first one is to connect quantum memories emitting or absorbing visible single photons with the telecommunication wavelengths, where fiber transmission loss is minimum. The second and main goal is to study the compatibility between disparate quantum nodes, emitting or absorbing photons at different wavelengths. More precisely the objective is to achieve a quantum connection between the two optical memories studied using quantum frequency conversion techniques. The main core of this work is the quantum frequency conversion interface that bridges the gap between the cold ensemble of Rubidium atoms, emitting photons at 780nm, and the Praseodymium ion doped crystal, absorbing photons at 606nm. This interface is composed of two different frequency conversion devices, where a cascaded conversions takes place: the first one converts 780nm photons to the telecommunication C-band, and the second one converts them back to visible, at 606nm. This comes with several challenges such as conversion efficiency, phase stability and parasitic noise reduction, which are important considerations to show the conservation of quantum behaviors through the conversion process. This work can be divided in three parts. In a first one, we built a quantum frequency conversion interface between 606nm and the C-band wavelength, capable of both up and down-conversion of single photon level light. We also characterized the noise processes involved in this specific conversion. In the down-conversion case we showed that memory compatible heralded single photons emitted from a photon pair source preserve their non-classical properties through the conversion process. In the up-conversion case, we showed the storage of converted telecom photons in the praseodymium doped crystal, and their retrieval with high signal to noise ratio. The second part of the work was devoted to the conversion of photons from an emissive Rubidium atomic quantum memory to the telecom C band. In this work we converted the heralding photons from the atomic ensemble and measured non-classical correlations between a stored excitation and a C-band photon, necessary for quantum repeater applications. In the last part of the thesis, we setup the full frequency conversion interface and showed that heralded photons emitted by the atomic ensemble are converted, stored in the solid state memory and retrieved with high signal to noise ratio. We demonstrated that a single collective excitation stored in the atomic ensemble is transfered to the crystal by mean of a single photon at telecom wavelength. We also showed time-bin qubit transfer between the two quantum memories. This work represents the first proof of principle of a photonic quantum connection between disparate quantum memory nodes. The results presented in this thesis pave the way towards the realization of modular and hybrid quantum networks.

•Sep 13, 2018•1 citation

Telecom-compatible polarization-to-time-bin conversion of atom-photon entanglement for heterogeneous quantum networks

Christian Haen, Julian Groß-Funk, Max Bergerhoff +5 more

A key enabling feature of future quantum networks is interoperability between platforms that operate at different wavelengths and with different qubit encodings. We demonstrate an interface that converts atom-photon entanglement from polarization encoding at an atomic wavelength to time-bin encoding in the telecom C-band. Atom-entangled photons at 854 nm are generated from a single $^{40}$Ca$^+$ ion. After quantum frequency conversion to 1550 nm, the photonic polarization qubit is converted into a time-bin qubit using a fiber-based Mach--Zehnder-like encoder. Full quantum tomography of the final state verifies that the process preserves entanglement with 96.3(4.2)% fidelity. Together with the independent work of Ferrari et al. [arXiv:2607.07805 (2026)], this is the first demonstration of polarization-to-time-bin conversion of photons entangled with a single atomic quantum memory. The telecom-compatible interface enables robust qubit transmission over optical fibers and provides a key building block for heterogeneous quantum networking architectures.

quant-pharXiv:2607.29609•Aug 3, 2026•0 citations

Quantum frequency down-conversion of 40Ca+-resonant polarization-entangled photons to the telecom O-band

Matthias Bock, Stephan Kucera, Elena Arenskötter +5 more

In a typical quantum repeater scenario, remote quantum memories have to be interconnected by flying qubits transmitted in quantum channels. e.g. single photons in photonic fiber links. To establish long-distance communication, photons in the low-loss telecom wavlength regime are desirable. However, the majority of the candidates for quantum memories such as trapped atoms or ions, rare earth ensembles, color centers in diamond, atomic ensembles etc. possess optical transitions merely in the visible or near-infrared spectral regime below 1000 nm. One possible solution is the transduction of these wavelengths to the telecom region via quantum frequency conversion (QFC) based on three-wave-mixing processes in a nonlinear χ(2)-medium. Technical achievements in recent years enabled high conversion efficiencies and low conversion-induced noise contributions resulting in signal-to-background ratios high enough to demonstrate the preservation of several classical and quantum properties of the photons, e.g. first- and second-order coherence [1], time-bin entanglement [2], polarization entanglement [3] or indistinguishability [4]. QFC thus provides a powerful toolbox to interface quantum systems to the telecom bands and beyond that yet to interconnect dissimilar quantum systems.

Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•Jun 1, 2017•0 citations

Engineering mesoscopic coherence between photons and atoms

Mahdi Hosseini, Zongfeng Li, Trevor Kling

Rare earth ions in solids provide a versatile platform for quantum optical information control. Our research focuses on light storage at telecom wavelengths in Erbium-doped crystals, aiming to develop efficient quantum optical memories for applications in quantum communication and computation. Additionally, we design solid-state devices relying on collective atom-atom interactions to enhance light-matter interactions, based on rare-earth photonic devices.

Quantum Sensing and Nano Electronics and Photonics•Mar 21, 2025•0 citations

Storing Quantum Correlations in Different Rare Earth Ion-doped Solids. A Tale of Two Memories.

Jacob H. Davidson

Quantum networks of the future will be required to span the globe and interconnect many different real world systems employed by different commercial, academic, and government users. Thus, an important measure of such a network's capacity will be its ability to interact with and distribute entanglement among different mediums and devices with potentially disparate properties. Few experiments have to date shown this sort of interconnectivity between different systems with different atomic species. Here I will present our demonstration of a system in which we store two correlated photons in solid state quantum memories implemented in different rare earth ion-doped materials using the atomic frequency comb protocol. The photon pairs are generated through a spontaneous parametric down-conversion process with their wavelengths chosen to be compatible with quantum repeater implementations. One member, with a wavelength at 795 nm, is stored in an Tm:LiNbO3 crystal while, simultaneously, the other member, generated at 1535 nm wavelength, is stored in an amorphous erbium doped optical fiber. The measured cross-correlation function between the recalled photons is g(2) = 16 +/- 2, which verifies that the non-classical correlations are preserved and were present during the storage of both photons. The two storage mediums are diverse not only in terms of their different photon acceptance wavelengths, but also with regards to the material properties as hosts for the dopant rare-earth ions. Our results show that quantum nodes and networks based on fundamentally different systems can be interfaced to distribute entanglement.

•Jun 4, 2018•0 citations

Solid-state light-matter interfaces on the quantum test bench

Christoph Clausen

Solid-state light-matter interfaces based on crystals doped with rare-earth ions have shown great progress in recent years, heading towards quantum technology applications. In this thesis we present three experiments testing the suitability of an atomic frequency comb light-matter interface, implemented in an Nd:YSO crystal, for the storage of quantum information. A dedicated source of entangled photons has been developed, and the storage and retrieval of quantum correlations and entanglement been studied. We show the successful transfer of photonic entanglement to the crystal, and the generation of a matter-matter entangled state between two crystals. These are two of the main steps in a quantum repeater protocol. We also demonstrate faithful storage and retrieval of quantum information encoded in the polarization of heralded single photons. These experiments prove the strong potential of solid-state light-matter interfaces based on atomic frequency combs for quantum communication.

•Jan 1, 2013•0 citations
This research aims to establish a key element of hybrid quantum networks by demonstrating remote entanglement between disparate quantum systems, specifically a cold atomic ensemble and a rare-earth ion-doped crystal, on a long-term project. In this work, we focused on two important aspects towards this goal: the development of a qubit converter and quantum frequency conversion techniques. The qubit converter is built to transform polarization qubits from the cold atoms system into time-bin qubits to ensure compatibility with photons from the solid-state system. Then, quantum frequency conversion based on non-linear effects is studied to shift the 780nm entangled photon of the atomic system to the telecom C-band, aligning it with photons from the rare-earth ion-doped crystal experiment. This process is essential for conducting the Bell-state measurement required for remote entanglement generation.
The ability to control the optical frequency of quantum state carriers (i.e. photons) is an important functionality for future quantum networks. It allows all matter quantum systems - nodes of the network - to be compatible with the telecommunication C-band, therefore enabling long distance fiber quantum communication between them. It also allows dissimilar nodes to be connected with each other, thus resulting in heterogeneous networks that can take advantage of the different capabilities offered by the diversity of its constituents. Quantum memories are one of the building blocks of a quantum network, enabling the storage of quantum states of light and the entanglement distribution over long distances. In our group, two different types of memories are investigated: a cold atomic ensemble and an ion-doped crystal. In this thesis I investigate the quantum frequency conversion of narrow-band photons, emitted or absorbed by optical quantum memories, with two different objectives: the first one is to connect quantum memories emitting or absorbing visible single photons with the telecommunication wavelengths, where fiber transmission loss is minimum. The second and main goal is to study the compatibility between disparate quantum nodes, emitting or absorbing photons at different wavelengths. More precisely the objective is to achieve a quantum connection between the two optical memories studied using quantum frequency conversion techniques. The main core of this work is the quantum frequency conversion interface that bridges the gap between the cold ensemble of Rubidium atoms, emitting photons at 780nm, and the Praseodymium ion doped crystal, absorbing photons at 606nm. This interface is composed of two different frequency conversion devices, where a cascaded conversions takes place: the first one converts 780nm photons to the telecommunication C-band, and the second one converts them back to visible, at 606nm. This comes with several challenges such as conversion efficiency, phase stability and parasitic noise reduction, which are important considerations to show the conservation of quantum behaviors through the conversion process. This work can be divided in three parts. In a first one, we built a quantum frequency conversion interface between 606nm and the C-band wavelength, capable of both up and down-conversion of single photon level light. We also characterized the noise processes involved in this specific conversion. In the down-conversion case we showed that memory compatible heralded single photons emitted from a photon pair source preserve their non-classical properties through the conversion process. In the up-conversion case, we showed the storage of converted telecom photons in the praseodymium doped crystal, and their retrieval with high signal to noise ratio. The second part of the work was devoted to the conversion of photons from an emissive Rubidium atomic quantum memory to the telecom C band. In this work we converted the heralding photons from the atomic ensemble and measured non-classical correlations between a stored excitation and a C-band photon, necessary for quantum repeater applications. In the last part of the thesis, we setup the full frequency conversion interface and showed that heralded photons emitted by the atomic ensemble are converted, stored in the solid state memory and retrieved with high signal to noise ratio. We demonstrated that a single collective excitation stored in the atomic ensemble is transfered to the crystal by mean of a single photon at telecom wavelength. We also showed time-bin qubit transfer between the two quantum memories. This work represents the first proof of principle of a photonic quantum connection between disparate quantum memory nodes. The results presented in this thesis pave the way towards the realization of modular and hybrid quantum networks.
A key enabling feature of future quantum networks is interoperability between platforms that operate at different wavelengths and with different qubit encodings. We demonstrate an interface that converts atom-photon entanglement from polarization encoding at an atomic wavelength to time-bin encoding in the telecom C-band. Atom-entangled photons at 854 nm are generated from a single 40^{40}40Ca+^++ ion. After quantum frequency conversion to 1550 nm, the photonic polarization qubit is converted into a time-bin qubit using a fiber-based Mach--Zehnder-like encoder. Full quantum tomography of the final state verifies that the process preserves entanglement with 96.3(4.2)% fidelity. Together with the independent work of Ferrari et al. [arXiv:2607.07805 (2026)], this is the first demonstration of polarization-to-time-bin conversion of photons entangled with a single atomic quantum memory. The telecom-compatible interface enables robust qubit transmission over optical fibers and provides a key building block for heterogeneous quantum networking architectures.
In a typical quantum repeater scenario, remote quantum memories have to be interconnected by flying qubits transmitted in quantum channels. e.g. single photons in photonic fiber links. To establish long-distance communication, photons in the low-loss telecom wavlength regime are desirable. However, the majority of the candidates for quantum memories such as trapped atoms or ions, rare earth ensembles, color centers in diamond, atomic ensembles etc. possess optical transitions merely in the visible or near-infrared spectral regime below 1000 nm. One possible solution is the transduction of these wavelengths to the telecom region via quantum frequency conversion (QFC) based on three-wave-mixing processes in a nonlinear χ(2)-medium. Technical achievements in recent years enabled high conversion efficiencies and low conversion-induced noise contributions resulting in signal-to-background ratios high enough to demonstrate the preservation of several classical and quantum properties of the photons, e.g. first- and second-order coherence [1], time-bin entanglement [2], polarization entanglement [3] or indistinguishability [4]. QFC thus provides a powerful toolbox to interface quantum systems to the telecom bands and beyond that yet to interconnect dissimilar quantum systems.
Quantum networks of the future will be required to span the globe and interconnect many different real world systems employed by different commercial, academic, and government users. Thus, an important measure of such a network's capacity will be its ability to interact with and distribute entanglement among different mediums and devices with potentially disparate properties. Few experiments have to date shown this sort of interconnectivity between different systems with different atomic species. Here I will present our demonstration of a system in which we store two correlated photons in solid state quantum memories implemented in different rare earth ion-doped materials using the atomic frequency comb protocol. The photon pairs are generated through a spontaneous parametric down-conversion process with their wavelengths chosen to be compatible with quantum repeater implementations. One member, with a wavelength at 795 nm, is stored in an Tm:LiNbO3 crystal while, simultaneously, the other member, generated at 1535 nm wavelength, is stored in an amorphous erbium doped optical fiber. The measured cross-correlation function between the recalled photons is g(2) = 16 +/- 2, which verifies that the non-classical correlations are preserved and were present during the storage of both photons. The two storage mediums are diverse not only in terms of their different photon acceptance wavelengths, but also with regards to the material properties as hosts for the dopant rare-earth ions. Our results show that quantum nodes and networks based on fundamentally different systems can be interfaced to distribute entanglement.
Solid-state light-matter interfaces based on crystals doped with rare-earth ions have shown great progress in recent years, heading towards quantum technology applications. In this thesis we present three experiments testing the suitability of an atomic frequency comb light-matter interface, implemented in an Nd:YSO crystal, for the storage of quantum information. A dedicated source of entangled photons has been developed, and the storage and retrieval of quantum correlations and entanglement been studied. We show the successful transfer of photonic entanglement to the crystal, and the generation of a matter-matter entangled state between two crystals. These are two of the main steps in a quantum repeater protocol. We also demonstrate faithful storage and retrieval of quantum information encoded in the polarization of heralded single photons. These experiments prove the strong potential of solid-state light-matter interfaces based on atomic frequency combs for quantum communication.