Abstract Fidelity and rate of information (or the key rate in quantum key distribution (QKD)) decay exponentially with distance in long-distance quantum channels . The most common and well-known approach to address this fundamental problem is the so-called quantum repeater. We show how end nodes can participate in a repeater graph state (RGS) protocol link, and propose a steering repeater protocol. The protocol discusses the issue of clock synchronization between nodes and reduces the complexity of end node devices. We also employ measurement device independent (MDI) scheme to secure the repeater link. In addition, we show that the RGS protocol can transmit steering states, extending its applicability beyond quantum key distribution.
Abstract Fidelity and rate of information (or the key rate in quantum key distribution (QKD)) decay exponentially with distance in long-distance quantum channels . The most common and well-known approach to address this fundamental problem is the so-called quantum repeater. We show how end nodes can participate in a repeater graph state (RGS) protocol link, and propose a steering repeater protocol. The protocol discusses the issue of clock synchronization between nodes and reduces the complexity of end node devices. We also employ measurement device independent (MDI) scheme to secure the repeater link. In addition, we show that the RGS protocol can transmit steering states, extending its applicability beyond quantum key distribution.
The all-photonic quantum repeater scheme, utilizing a type of graph state called the repeater graph state (RGS), promises resilience to photon losses and operational errors, offering a fast Bell pair generation rate limited only by the RGS creation time (rather than enforced round-trip waits). While existing research has predominantly focused on RGS generation and secret key sharing rate analysis, there is a need to extend investigations to encompass broader applications, such as distributed computation and teleportation, the main tasks envisioned for the Quantum Internet. Here we propose a new emitter-photonic qubit building block and an RGS protocol that addresses several key considerations: end node involvement in connection establishment, decoding of logical qubits within the RGS, and computing the Pauli frame corrections at each participating node to ensure the desired correct end-to-end Bell pair state. Our proposed building block significantly reduces the total number of emissive quantum memories required for end nodes and seamlessly integrates all-photonic and memory-based repeaters under the same communication protocol. We also present an algorithm for decoding logical measurement results, employing graphical reasoning based on graph state manipulation rules.
All-photonic repeater scheme based on a repeater graph state (RGS) promises tolerance to photon losses as well as operational errors. An attractive promise of this scheme is that it offers a fast Bell pair generation rate, limited only by the RGS creation time. This is in stark contrast to the traditional quantum repeater with quantum memories, where the Bell pair generation time is limited by the round-trip wait time leading to poor Bell pair generation performance and quality due to the decoherence of quantum memories. Prior research has focused on lowering the cost of RGS generation and improving upon the probabilistic generation to a deterministic generation with required photon counts to be the same as the number of qubits in the RGS. The operations of creating and manipulating the RGS into an end-to-end Bell pair are complex and have not been fully figured out in detail. Here, we focus on improving the practicality of the RGS scheme in real-world implementation and address three open questions of the RGS scheme, namely; how do end nodes participate in the connection, what classical information needs to be exchanged and processed between nodes, and how to make RGS scheme work hand in hand with the traditional quantum repeaters with memories.
The all-photonic quantum repeater scheme based on repeater graph states (RGS) offers a promising approach for constructing quantum networks without relying on long-coherence-time quantum memories, which remain a significant technological challenge. Despite substantial progress in defining new schemes for generating RGSs, and in analyzing their performance for the task of secret key generation, the integration of all-photonic schemes with memory-equipped quantum repeaters remains underexplored. We propose an architecture that enables seamless interoperability between all-photonic and memory-based quantum repeaters through an emitter-photon qubit building block, significantly reducing the number of quantum memories required at end nodes from a multiplicative dependence on the trial rate and number of RGS arms to an additive scaling. The core idea of our architecture is to abstract the all-photonic sections of the network as link-level connections between memory-equipped nodes, enabling integration into existing network-level protocols. Additionally, we outline the content and semantics of the messages necessary for a communication protocol based on graph state manipulation rules for computing Pauli frame corrections for obtaining the correct Bell pair. Our approach also provides a simplified method for calculating state fidelity directly from graph state properties.
All-photonic quantum repeater is not yet commercially applicable due to difficulty in the generation of repeater graph states (RGS) and the probabilistic nature of Bell-state measurement. In recent years, several deterministic protocols have been proposed to generate RGS. However, they can only create bare RGS and a rate-distance analysis for the deterministic approach is currently missing. We present a deterministic generation scheme of encoded RGS and show that after optimization, the repeater using our scheme performs at least 4.9 times better than the traditional probabilistic generation scheme in terms of secret key rate, with a significant reduction in the total number photons by 2 orders of magnitude. The duration of our generation protocol is only half of the existing deterministic schemes. We also describe in detail an experimental method using cavity QED-enhanced resonance fluorescence and time-bin encoding to implement our protocol.
The feasibility of trust-free long-haul quantum key distribution (QKD) networks is addressed. We combine measurement-device-independent QKD (MDI-QKD), as an access technology, with a quantum repeater setup, at the core of future quantum communication networks. This will provide a quantum link none of whose intermediary nodes need to be trusted, or, in our terminology, a trust-free QKD link. As the main figure of merit, we calculate the secret key generation rate when a particular probabilistic quantum repeater protocol is in use. We assume the users are equipped with imperfect single photon sources, which can possibly emit two single photons, or laser sources to implement decoy-state techniques. We consider apparatus imperfection, such as quantum efficiency and dark count of photodetectors, path loss of the channel, and writing and reading efficiencies of quantum memories. By optimizing different system parameters, we estimate the maximum distance over which users can share secret keys when a finite number of memories are employed in the repeater setup.
The feasibility of trust-free long-haul quantum key distribution (QKD) is addressed. We combine measurement-device-independent QKD, as an access technology, with a quantum repeater setup, at the core of future quantum communication networks. This will provide a quantum link none of whose intermediary nodes need to be trusted, or, in our terminology, a trust-free QKD link. As the main figure of merit, we calculate the secret key generation rate when a particular probabilistic quantum repeater protocol is in use. We assume the users are equipped with imperfect single photon sources, which can possibly emit two single photons, or laser sources to implement decoy-state techniques. We consider apparatus imperfection, such as quantum efficiency and dark count of photodetectors, path loss of the channel, and writing and reading efficiencies of quantum memories. By optimizing different system parameters, we estimate the maximum distance over which users can share secret keys when a finite number of memories are employed in the repeater setup.