High-Fidelity Multipartite States Generated Across a Seven-Node Quantum Network

The efficient distribution of quantum states is crucial for realising future secure communication networks and distributed computing, yet generating and sharing complex quantum entanglement remains a significant challenge, particularly over long distances. Janka Memmen and Anna Pappa, both from Technische Universität Berlin, along with their colleagues, address this problem by investigating how to create and share multipartite entanglement within a realistic network connecting Berlin and Frankfurt. Leveraging the existing infrastructure of the Q-net-Q project, a seven-node link between the two cities, the researchers demonstrate a method for generating high-fidelity multipartite states despite the limitations of available resources and network losses. This work identifies the critical role of quantum memories in enhancing distribution and highlights the potential benefits of multipartite entanglement for advanced cryptographic applications, such as secure conference key agreement and secret sharing.

Yet its generation is experimentally challenging, especially in networks susceptible to signal loss. While much research focuses on distributing entanglement in simple, star-like networks, practical implementations often rely on linear structures constrained by existing infrastructure. In this work, researchers investigate the generation of high-fidelity multipartite entangled states in a realistic quantum network, utilizing the Q-net-Q project, a long-distance link connecting Berlin and Frankfurt via seven relay nodes.

GHZ State Generation Across a Quantum Network

Researchers developed a novel approach to generating multipartite entanglement within a real-world quantum network, leveraging an existing infrastructure connecting Berlin and Frankfurt. This network, comprising seven relay nodes along a 664. 4km fiber optic link, presented challenges due to its linear structure, limited resources, and signal loss, but offered a unique opportunity to test entanglement distribution beyond simple point-to-point connections. The team focused on creating GHZ states, a type of multipartite entanglement involving three network nodes, as a foundation for more complex communication protocols and extending the network’s capabilities beyond standard key exchange.

The methodology creatively addresses the network’s limitations by utilizing only bipartite entanglement sources, initially establishing entanglement between pairs of nodes, rather than directly creating entanglement among multiple parties. To overcome signal loss within the fiber optic cables, the researchers investigated the potential of quantum memories, devices capable of storing quantum information for a short period, to enhance the distribution of multipartite entanglement. This involved carefully analyzing the performance requirements for these memories, such as storage time and efficiency, to ensure successful entanglement generation despite the network’s lossy nature. A key innovation lies in adapting the methodology to the specific characteristics of the existing network infrastructure, including variations in link length and signal loss between nodes, and the use of different detector types at each station.

Rather than relying on idealized network models, the team accounted for these real-world parameters to optimize the entanglement generation process. This pragmatic approach allows for a more realistic assessment of the feasibility and performance of multipartite entanglement in a practical quantum communication system. Beyond state generation, the research explores the practical applications of this multipartite entanglement, specifically assessing the performance of cryptographic protocols like Conference Key Agreement, Anonymous Conference Key Agreement, and Quantum Secret Sharing. By comparing the performance of these protocols using multipartite entanglement versus traditional methods, the team aims to identify scenarios where the use of multipartite states offers significant advantages, paving the way for more secure and efficient quantum communication networks.

GHZ Entanglement Distributed Across a Fiber Network

Researchers have demonstrated a method for generating complex quantum states, known as multipartite entanglement, across a real-world network connecting Berlin and Frankfurt. This achievement is significant because distributing these states is crucial for advanced cryptographic applications and distributed quantum computing, tasks that demand reliable quantum communication. The team leveraged an existing fiber optic network with seven relay nodes, a practical constraint often absent in laboratory demonstrations, and addressed the challenges of signal loss and imperfect components. The core of their work focuses on creating a specific three-party entangled state, a ‘GHZ state’, despite only having access to simpler, two-party entangled states as a starting point.

They explored how the strategic use of quantum memories at the central node of the network dramatically improves the success rate of generating this complex state. Without memories, the probability of successfully establishing the GHZ state is limited by the simultaneous arrival of signals at distant nodes, a scenario made difficult by signal loss. By storing qubits locally using quantum memories, the team effectively decouples these events, allowing more time for successful entanglement distribution and significantly increasing the overall generation rate. The results demonstrate that incorporating quantum memories at the central node is a key factor in maximizing the probability of successful state generation.

The team’s modeling shows that this approach overcomes the limitations imposed by signal loss and imperfect detection, offering a substantial improvement over strategies that rely on simultaneous signal transmission and detection. Furthermore, they have accounted for realistic sources of noise, such as dark counts in detectors and depolarization of qubits during transmission through the fiber optic cable, providing a comprehensive assessment of the system’s performance. This research highlights the feasibility of building practical quantum communication networks using existing infrastructure. By addressing the challenges of loss and noise, and by strategically employing quantum memories, the team has paved the way for future advancements in secure communication and distributed quantum computing. The demonstrated improvements in state generation rate represent a crucial step towards realizing the full potential of these technologies in real-world scenarios.

Berlin-Frankfurt GHZ State Distribution Demonstrated

This work demonstrates the successful generation of a three-partite GHZ state within a real-world quantum network, specifically the Q-net-Q link connecting Berlin and Frankfurt. Researchers investigated the feasibility and efficiency of distributing this entangled state across three nodes, comparing performance with and without the use of quantum memories. The inclusion of quantum memories significantly increased the rate at which entanglement could be established, though this came at the cost of reduced fidelity due to added noise. The study highlights scenarios where multipartite entanglement offers advantages over schemes relying solely on bipartite entanglement, particularly for applications like Conference Key Agreement and Secret Sharing.

While current network limitations, specifically high loss, constrain the practical implementation of three-partite states, the results demonstrate the potential of this approach. The authors acknowledge that achieving practical implementations requires improvements in experimental parameters and suggest future research directions including scaling up to larger networks, exploring alternative entanglement merging strategies, and developing more detailed models of quantum memory implementations. Further investigation into different network configurations and the generation of other entangled states also represents a promising avenue for future work.