As a direct consequence of the no-cloning theorem, the deterministic amplification as in classical communication is impossible for unknown quantum states. This calls for more advanced techniques in a future global quantum network, e.g., for cloud quantum computing. A unique solution is the teleportation of an entangled state, i.e., entanglement swapping, representing the central resource to relay entanglement between distant nodes. Together with entanglement purification and a quantum memory it constitutes a so-called quantum repeater. Since the aforementioned building blocks have been individually demonstrated in laboratory setups only, the applicability of the required technology in real-world scenarios remained to be proven. Here we present a free-space entanglement-swapping experiment between the Canary Islands of La Palma and Tenerife, verifying the presence of quantum entanglement between two previously independent photons separated by 143 km. We obtained an expectation value for the entanglement-witness operator, more than 6 SDs beyond the classical limit. By consecutive generation of the two required photon pairs and space-like separation of the relevant measurement events, we also showed the feasibility of the swapping protocol in a long-distance scenario, where the independence of the nodes is highly demanded. Because our results already allow for efficient implementation of entanglement purification, we anticipate our research to lay the ground for a fully fledged quantum repeater over a realistic high-loss and even turbulent quantum channel.
As a direct consequence of the no-cloning theorem, the deterministic amplification as in classical communication is impossible for unknown quantum states. This calls for more advanced techniques in a future global quantum network, e.g., for cloud quantum computing. A unique solution is the teleportation of an entangled state, i.e., entanglement swapping, representing the central resource to relay entanglement between distant nodes. Together with entanglement purification and a quantum memory it constitutes a so-called quantum repeater. Since the aforementioned building blocks have been individually demonstrated in laboratory setups only, the applicability of the required technology in real-world scenarios remained to be proven. Here we present a free-space entanglement-swapping experiment between the Canary Islands of La Palma and Tenerife, verifying the presence of quantum entanglement between two previously independent photons separated by 143 km. We obtained an expectation value for the entanglement-witness operator, more than 6 SDs beyond the classical limit. By consecutive generation of the two required photon pairs and space-like separation of the relevant measurement events, we also showed the feasibility of the swapping protocol in a long-distance scenario, where the independence of the nodes is highly demanded. Because our results already allow for efficient implementation of entanglement purification, we anticipate our research to lay the ground for a fully fledged quantum repeater over a realistic high-loss and even turbulent quantum channel.
In the field of quantum communication the teleportation1 of single quanta plays a fundamental role in numerous quantum information-processing protocols. Quantum teleportation allows to faithfully transfer unknown quantum states over arbitrary distances and constitutes a method to circumvent the no-cloning theorem2. Even formally completely independent particles can become entangled via the process of entanglement swapping3. In a future quantum communication network4 this will be of utmost importance, enabling quantum computers to become globally interconnected. In order to prove the feasibility of quantum teleportation under optical link attenuations that will arise in a future space-application scenario, we extended the communication distance to 143 km, employing an optical free-space link between the two Canary Islands of La Palma and Tenerife. This work proofs the feasibility of ground-based freespace quantum teleportation. With our setup we were able to achieve coincidence production rates and fidelities to cope with the optical link attenuation, resulting from various experimental and technical challenges, which will arise in a quantum transmission between a ground-based transmitter and a low-earth-orbiting satellite receiver5. In our experiment we gained an average state fidelity for the teleported quantum states of more than 6 standard deviations beyond the classical limit of 2/3 and a process fidelity of 0.710(42). We expect that many of the features implemented in this experiment will be key blocks for future investigations.
Quantum Entanglement is the essence of quantum physics and inspires fundamental questions about the principles of nature. Moreover it is also the basis for emerging technologies of quantum information processing such as quantum cryptography, quantum teleportation and quantum computation. Bell's discovery, that correlations measured on entangled quantum systems are at variance with a local realistic picture led to a flurry of experiments confirming the quantum predictions. However, it is still experimentally undecided whether quantum entanglement can survive global distances, as predicted by quantum theory. Here we report the violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality measured by two observers separated by 144 km between the Canary Islands of La Palma and Tenerife via an optical free-space link using the Optical Ground Station (OGS) of the European Space Agency (ESA). Furthermore we used the entangled pairs to generate a quantum cryptographic key under experimental conditions and constraints characteristic for a Space-to-ground experiment. The distance in our experiment exceeds all previous free-space experiments by more than one order of magnitude and exploits the limit for ground-based free-space communication; significantly longer distances can only be reached using air- or space-based platforms. The range achieved thereby demonstrates the feasibility of quantum communication in space, involving satellites or the International Space Station (ISS).
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.
Entanglement is an essential phenomenon of quantum mechanics. Two entangled particles, photons for example, will individually yield random results upon being measured, but these results will always be perfectly correlated, no matter how far the two particles are separated from each other. Entanglement has been proven to be at the heart of a wide range of fundamental quantum effects and it drives exciting practical applications, such as quantum cryptography, quantum teleportation and quantum computing [1], which would be impossible in a world limited to classical physics. A team of researchers from the Institute for Quantum Optics and Quantum Information (IQOQI) in Vienna and the University of Vienna, led by Anton Zeilinger, has now reported the successful transmission of entangled photon pairs between two Canary Islands, bridging a distance of 144 km and a two-photon attenuation of almost ten million to one. The result, published in Nature Physics [2], is so far the most convincing demonstration to perform experiments with entangled photons in space.
Abstract The transmission of quantum states over extended distances is constrained by photon losses, ruling out direct amplification akin to classical telecommunications due to the non-cloning theorem. Overcoming this challenge involves implementing quantum repeater protocols that leverage entanglement swapping to create long-distance entanglement from shorter distances. A novel multi-hop quantum teleportation scheme, blending concepts from quantum repeaters and teleportation, is under exploration. It aims to transfer arbitrary two-qubit states between two distant parties, even in the absence of a direct quantum channel. Intermediate nodes, connected via a four-qubit entangled cluster state as quantum channels, are introduced based on a more general routing protocol. Bell measurements are independently conducted by the source node (Alice) and all intermediate nodes, with simultaneous transmission of measurement results, significantly reducing time consumption. Determining the quantum state from Bell measurement results requires only the destination node (Bob) for a simple unitary transformation. Moreover, this protocol holds promise for implementation on the IBM Quantum Experience platform once the requisite quantum circuits are designed. This overview encompasses both the theoretical and simulated status of the proposed scheme, with simulated findings incorporated into quantum state tomography to verify the accuracy of the transmitted quantum state.
Long-distance, entanglement distribution over thousands of kilometers is necessary to build a global-scale quantum network. Directly transferring entangled, telecom photons through fiber is relatively straight forward in local networks where the separation between nodes is below 100 km, corresponding to a 1% transmission. However, the exponentially greater loss at larger distances (e.g. only 10−6 transmission at 300 km) leads to unacceptably low transmission rates, which drop below the noise rates of typical photon detectors. Alternatively, the Earth’s rarefied atmosphere at high altitudes reduces the absorption and scattering of infrared photons. Therefore, free-space links between satellite and ground stations have been shown to enable entangled photon transmission over 2000 km. While this demonstration showed the viability of satellite-enhanced entanglement distribution, the source used in this demonstration produced distinguishable photons, which cannot be used for many fundamental protocols like teleportation. We are building a testbed to demonstrate the feasibility of distributing entangled, indistinguishable photons over a space-to-ground link using entanglement swapping.