HI-QDC: An Isometric Modular Scaling for Hop-Independent Quantum Data Center

As quantum data centers scale, increasing the number of intermediate nodes (hops) leads to significant fidelity degradation due to repeated entanglement swapping. This paper asks whether the inherent path diversity of server-centric network topologies can be leveraged to restore end-to-end entanglement fidelity to the quality of a single elementary link, effectively making the network's performance independent of its internal hop count.

The authors propose the Hop-Independent Quantum Data Center (HI-QDC), a modular architecture based on the BCube topology. The research proceeds in two stages:

1. Black-Box Analysis: The authors model the network as a provider of multiple raw end-to-end entangled states (Werner states) and calculate the minimum number of copies required by different purification protocols (recursive 2-to-1 vs. nested r-to-1) to restore fidelity to a target link-level threshold.
2. Architectural Instantiation: They map these purification requirements onto a BCube topology. Because BCube provides multiple edge-disjoint paths, the network can generate the necessary raw copies for purification. The authors define a modular structure where each module acts as a fidelity-restoration block, hiding its internal hop count from the rest of the network.

The study identifies specific operating regimes where a BCube module can preserve both the fidelity and the yield (expected number of copies) of an elementary link. By using topology to supply the path multiplicity required for purification, the HI-QDC architecture successfully converts physical path redundancy into fidelity recovery. This allows the network to treat each module as an effective elementary link, enabling recursive scalability without the typical fidelity decay associated with larger network diameters.