The transfer of quantum states of light and their mapping into stationary quantum nodes is one of the building blocks for future quantum networks and envisionned applications [1] . Central to this endeavor is the distribution of entanglement between the material nodes, which opens a variety of major applications [2] . For instance, to realize long-distance quantum communications one needs to divide the distance into shorter quantum repeater links connecting entangled memories. Subsequent entanglement swapping operations enable an exponential improvement in distribution time. In this context, the efficiency of the entanglement mapping is a key parameter.
The transfer of quantum states of light and their mapping into stationary quantum nodes is one of the building blocks for future quantum networks and envisionned applications [1] . Central to this endeavor is the distribution of entanglement between the material nodes, which opens a variety of major applications [2] . For instance, to realize long-distance quantum communications one needs to divide the distance into shorter quantum repeater links connecting entangled memories. Subsequent entanglement swapping operations enable an exponential improvement in distribution time. In this context, the efficiency of the entanglement mapping is a key parameter.
In order to build long-distance quantum communications, quantum memories are required as a synchronization tool which allows entanglement to be stored and retrieved on-demand, when the system is ready to perform entanglement swapping [1]. The storage-and-retrieval efficiency is a key parameter: an increase from 60% to 90% drastically decreases, typically by 2 orders of magnitude, the average time for entanglement distribution over a distance of 600 kilometers. Within this context, we reported quantum transfer between light and cold-atom-based quantum memory featuring a single mode storage-and-retrieval efficiency as high as 87%(±5%), in the single excitation regime [2]. Moreover, we demonstrated the storage of single-photon entanglement into two memories, with a ratio of the input and output concurrences reaching 88%, a more than three-fold increase compared to prior works (Fig. 1). The demonstrated capability required operating at a very large optical depth (OD) of the atomic ensembles on theD1line of cesium, and with a strong and preserved suppression of the two-photon component.
Quantum networks, integrating quantum communication, quantum metrology, and distributed quantum computing, could provide secure and efficient information transfer, high-resolution sensing, and an exponential speed-up in information processing. Deterministic entanglement distribution over long distances is a prerequisite for scalable quantum networks, enabling the utilization of device-independent quantum key distribution (DI-QKD) and quantum teleportation to achieve secure and efficient information transfer. However, the exponential photon loss in optical fibres prohibits efficient and deterministic entanglement distribution. Quantum repeaters, incorporating entanglement swapping and entanglement purification with quantum memories, offer the most promising means to overcome this limitation in fibre-based quantum networks. Despite numerous pioneering efforts toward realizing quantum repeaters, a critical bottleneck remains, as remote memory-memory entanglement suffers from decoherence more rapidly than it can be established and purified over long distances. We overcome this by developing long-lived trapped-ion memories, an efficient telecom interface, and a high-visibility single-photon entanglement protocol. This allows us to establish and maintain memory-memory entanglement over a 10 km fibre within the average entanglement establishment time for the same distance. As a direct application, we demonstrate metropolitan-scale DI-QKD, distilling 1,917 secret keys out of 4.05*10^5 Bell pairs over 10 km. We further report a positive key rate over 101 km in the asymptotic limit, extending the achievable distance by more than two orders of magnitude. Our work provides a critical building block for quantum repeaters and marks an important step toward scalable quantum networks.
Transfert efficace d’intrication entre photons et mémoires quantiques Cette thèse porte sur le développement de mémoires quantiques efficaces basées sur des ensembles d’atomes froids. Ces mémoires, qui ont la capacité de stocker un état quantique photonique pour le récupérer sur demande après un temps défini, constituent un élément central pour la construction de réseaux quantiques à grande échelle. Nous commençons par expliquer pourquoi une mémoire quantique est importante dans ce contexte, et nous expliquons de quelle façon l’efficacité d’une mémoire est un paramètre crucial, qui se doit d’être proche de l’unité pour mener à bien des applications réalistes. Nous nous concentrons ensuite sur l’optimisation de ce paramètre en expliquant notamment comment l’absorptivité des atomes et la transition choisie influe sur l’efficacité. Dans le but de préparer notre ensemble d’atomes à agir comme une mémoire quantique efficace, plusieurs techniques expérimentales sont aussi présentées. Nous démontrons finalement deux expériences de stockage quantique sur la ligne D1 du césium. La première est le stockage de photons uniques avec une efficacité record de η = (87±5)%. Ensuite, nous étendons ce résultat à un stockage d’intrication photonique et nous démontrons un transfert d’intrication avec une efficacité de transfert de λ = (80 ± 20)%. Ces résultats constituent une avancée essentielle vers la construction d’un réseau quantique de grande échelle.
The realization of a large-scale quantum network of interconnected quantum nodes is one of the most promising directions in quantum technology. Such a network distributes entanglement between remote matter qubits via single photons in optical fibers, enabling quantum repeaters and long-distance secure communication and supporting emerging applications such as distributed quantum computing and network-enhanced metrology. To realize this vision, proposed architectures require quantum nodes that provide an efficient quantum light-matter interface for entanglement generation, serve as a quantum memory allowing long-lived storage of quantum states, and have the possibility to connect to telecommunication fiber-based photonic channels. This thesis presents the development and experimental realization of a quantum network node based on a single Rb-87 atom. A single atom is confined in a tightly focused optical dipole trap, and entanglement is generated between the polarization of an emitted photon and the Zeeman state of the atom. To enable long-lived quantum storage, we coherently map the atomic qubit to a basis less sensitive to magnetic fields using Zeeman-state-selective Raman transfer, reducing magnetic field sensitivity by a factor of 500. Such hyperfine qubit control is implemented via a two-photon Raman process driven by a laser field that is amplitude modulated at the qubit splitting using a fiber-based Mach-Zehnder interferometer. With this approach, we achieve a 1/e storage lifetime of 10 ms, extending coherence by a factor of 30 compared with the initial qubit encoding. Residual dephasing arises primarily from position-dependent effective magnetic fields caused by the tightly focused optical dipole trap beam together with atomic motion, and the differential AC-Stark shifts come from the trap detuning. While the first can be fully canceled via a standing-wave dipole trap, which needs to change the trap setup. We mitigate the second effects with a spin-echo pulse, extending coherence by a factor of two. Nonetheless, the significantly longer coherence time already allows high-fidelity entanglement distribution over fiber links with a length of a few hundred kilometers. To reduce fiber attenuation for long-distance distribution, we convert the emitted 780 nm single photons to the low-loss telecom S band using difference-frequency generation in a periodically poled lithium niobate waveguide, pumped at 1607 nm to produce 1517 nm photons. By combining long-lived quantum memory with a telecom interface, we demonstrated long-distance atom-photon entanglement distribution through 50 km and 101 km of spooled fiber with fidelities of 85% and 71%, respectively, establishing a key component for scalable quantum communication. These results establish neutral atom nodes as promising building blocks for metropolitan-scale quantum networking and outline a scalable path toward repeater protocols based on multiplexing memories and Rydberg-mediated interactions.
Cette thèse porte sur le développement de mémoires quantiques efficaces basées sur des ensembles d’atomes froids. Ces mémoires, qui ont la capacité de stocker un état quantique photonique pour le récupérer sur demande après un temps défini, constituent un élément central pour la construction de réseaux quantiques à grande échelle. Nous commençons par expliquer pourquoi une mémoire quantique est importante dans ce contexte, et nous expliquons de quelle façon l’efficacité d’une mémoire est un paramètre crucial, qui se doit d’être proche de l’unité pour mener à bien des applications réalistes. Nous nous concentrons ensuite sur l’optimisation de ce paramètre en expliquant notamment comment l’absorptivité des atomes et la transition choisie influe sur l’efficacité. Dans le but de préparer notre ensemble d’atomes à agir comme une mémoire quantique efficace, plusieurs techniques expérimentales sont aussi présentées. Nous démontrons finalement deux expériences de stockage quantique sur la ligne D1 du césium. La première est le stockage de photons uniques avec une efficacité record de η = (87±5)%. Ensuite, nous étendons ce résultat à un stockage d’intrication photonique et nous démontrons un transfert d’intrication avec une efficacité de transfert de λ = (80 ± 20)%. Ces résultats constituent une avancée essentielle vers la construction d’un réseau quantique de grande échelle.