Abstract When shared between remote locations, entanglement opens up fundamentally new capabilities for science and technology. Envisioned quantum networks use light to distribute entanglement between their remote matter-based quantum nodes. Here we report on the observation of entanglement between matter (a trapped ion) and light (a photon) over 50 km of optical fibre: two orders of magnitude further than the state of the art and a practical distance to start building large-scale quantum networks. Our methods include an efficient source of ion–photon entanglement via cavity-QED techniques (0.5 probability on-demand fibre-coupled photon from the ion) and a single photon entanglement-preserving quantum frequency converter to the 1550 nm telecom C band (0.25 device efficiency). Modestly optimising and duplicating our system would already allow for 100 km-spaced ion–ion heralded entanglement at rates of over 1 Hz. We show therefore a direct path to entangling 100 km-spaced registers of quantum-logic capable trapped-ion qubits, and the optical atomic clock transitions that they contain.
Abstract When shared between remote locations, entanglement opens up fundamentally new capabilities for science and technology. Envisioned quantum networks use light to distribute entanglement between their remote matter-based quantum nodes. Here we report on the observation of entanglement between matter (a trapped ion) and light (a photon) over 50 km of optical fibre: two orders of magnitude further than the state of the art and a practical distance to start building large-scale quantum networks. Our methods include an efficient source of ion–photon entanglement via cavity-QED techniques (0.5 probability on-demand fibre-coupled photon from the ion) and a single photon entanglement-preserving quantum frequency converter to the 1550 nm telecom C band (0.25 device efficiency). Modestly optimising and duplicating our system would already allow for 100 km-spaced ion–ion heralded entanglement at rates of over 1 Hz. We show therefore a direct path to entangling 100 km-spaced registers of quantum-logic capable trapped-ion qubits, and the optical atomic clock transitions that they contain.
Envisioned large-scale quantum networks demand the ability to rapidly distribute and store entanglement between remote matter-based nodes using photons [1] . The minimum time required to distribute entanglement between two nodes is the time it takes light to travel between them, which over 100 km distance already 0.33 ms. The speed of light thus sets a fundamental limit on the achievable entanglement distribution rate: with a single quantum emitter in a node, one has to wait for the photon travel time before trying again with a new photon for any application that would require also classical communication (e.g. to realize a quantum repeater) [2] . Over 100 km of fiber, this limits the entanglement distribution attempt rate to 2 kHz and, given 1.5% transmission in the best telecom fibres, a success rate of 30 Hz. This is a general problem for entanglement distribution over long distances as far higher rates are critical for the practical realisation of quantum networks and their applications. To overcome this limit, several entanglement distribution attempts have to be done in parallel. This requires multiple modes of the node (memory qubits) to couple selectively to the multiple modes of the link.
Abstract Quantum networks promise to provide the infrastructure for many disruptive applications, such as efficient long-distance quantum communication and distributed quantum computing1, 2. Central to these networks is the ability to distribute entanglement between distant nodes using photonic channels. Initially developed for quantum teleportation3, 4 and loophole-free tests of Bell’s inequality5, 6, recently, entanglement distribution has also been achieved over telecom fibres and analysed retrospectively7, 8. Yet, to fully use entanglement over long-distance quantum network links it is mandatory to know it is available at the nodes before the entangled state decays. Here we demonstrate heralded entanglement between two independently trapped single rubidium atoms generated over fibre links with a length up to 33 km. For this, we generate atom–photon entanglement in two nodes located in buildings 400 m line-of-sight apart and to overcome high-attenuation losses in the fibres convert the photons to telecom wavelength using polarization-preserving quantum frequency conversion9. The long fibres guide the photons to a Bell-state measurement setup in which a successful photonic projection measurement heralds the entanglement of the atoms10. Our results show the feasibility of entanglement distribution over telecom fibre links useful, for example, for device-independent quantum key distribution11–13 and quantum repeater protocols. The presented work represents an important step towards the realization of large-scale quantum network links.
Concatenation of quantum repeater nodes has been proposed as a means to distribute entanglement efficiently across future long-distance networks. Here, we demonstrate a trapped-ion repeater node that establishes photonic entanglement over a 17 km fiber traversing a city, moving beyond laboratory-only prototypes. The node comprises two atomic ions coupled to an optical cavity. First, a detailed characterization is performed of the long fiber channel and of ion-photon entanglement distributed across it. Next, a repeater-node protocol is implemented, achieving entanglement between a local photon and a remote photon with a Bell-state fidelity of 81.6−1.4+1.9%. These results demonstrate that trapped-ion repeater-node technology can operate over existing city-scale fiber infrastructure and represent a step toward distributed quantum repeater networks.
Quantum networking is an established and growing field of research, with applications in many other fields of quantum technology, as well as fundamental research in quantum mechanics. A quantum network consists of multiple quantum devices that can interchange quantum information or be entangled with each other. Trapped ions coupled to optical cavities are among the most promising platforms in quantum technology broadly and networking specifically, with the ability to initialise and store quantum states in the ions and, via the cavity, transfer those states onto photons, which can interact with distant systems. This thesis describes two experiments using an ion trap with integrated high finesse optical cavity as a source of single photons. In the first, the photons were converted to a standard telecommunications wavelength through quantum frequency conversion and transmitted over 10 km of optical fibre. The attenuation experienced by the photons in optical fibre without frequency conversion would make transmission over such a long distance impractical. The converted photons were shown to maintain their nonclassical statistics, meaning such a system could be used to create quantum networks over long distances. A novel scheme for producing single photons from the ion-cavity system was demonstrated. This scheme significantly reduces the impact of decoherence in the system compared to commonly used schemes through selection of the initial state of the ion. This scheme may be directly applied to ion-photon entanglement and would lead to improved fidelity in ion-ion entanglement or state transfer. Both experiments represent significant steps towards the building of quantum networks based on ions coupled to optical cavities. They lay the groundwork for future experiments using the same system to entangle ions with photons and eventually with ions in other traps.
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.
A three-qubit quantum network node based on trapped atomic ions is presented. The ability to establish entanglement between each of the qubits in the node and a separate photon that has travelled over a 101km-long optical fiber is demonstrated. By sending those photons through the fiber in close succession, a remote entanglement rate is achieved that is greater than when using only a single qubit in the node. Once extended to more qubits, this multimode approach can be a useful technique to boost entanglement distribution rates in future long-distance quantum networks of light and matter.