Scalable entanglement distribution using encoded hybrid repeater chains

As quantum networks scale, the challenge of overcoming signal loss and decoherence becomes paramount. While various quantum memory technologies exist, they often trade off between entanglement generation speed and coherence time. This paper investigates whether a hybrid repeater architecture—combining different memory types within a single node—can leverage the complementary strengths of these platforms to improve the end-to-end entanglement distribution rate (EDR) in long-distance quantum networks.

To evaluate this, the authors model two types of quantum memories: Type-1 (fast, multiplexed, but error-prone) and Type-2 (slow, high-coherence, and precise). They propose a hybrid repeater chain where only a subset of nodes are hybrid (containing both memory types), while others are simpler Type-1 repeaters. The study employs detailed Monte Carlo simulations to analyze fault-tolerant protocols using the three-qubit phase-flip repetition code, the [[7, 1, 3]] Steane code, and the [[9, 1, 3]] Shor code. Crucially, they develop a modified swap-as-soon-as-possible (swap-ASAP) policy that allows for efficient entanglement swapping and remote CNOT operations while respecting the symmetry constraints of the chosen error-correction codes.

• Hybrid Superiority: Hybrid architectures consistently outperform pure Type-1 or pure Type-2 architectures in EDR across various distance scales, particularly in the near-term regime where resources are constrained.
• Resource Efficiency: By placing hybrid nodes sparsely (e.g., every two hops), the architecture achieves performance comparable to a pure Type-2 chain while using significantly fewer expensive Type-2 memories.
• Policy Impact: The proposed deferred-consumption swap-ASAP policy significantly reduces information storage time compared to 'generate-all-links-first' approaches, thereby minimizing decoherence and increasing the overall entanglement distribution rate.
• Code Performance: Larger codes like the Steane and Shor codes provide higher error thresholds, making them more robust for long-distance distribution, provided the swap policy is optimized to maintain multiplexing benefits.