Sequential vs. Simultaneous Entanglement Swapping under Optimal Link-Layer Control
2026-05-05 • Networking and Internet Architecture
Networking and Internet Architecture
AI summaryⓘ
The authors studied two ways to connect quantum networks: one where connections are made all at once (simultaneous SWAP-ASAP) and another where connections happen step-by-step (sequential swapping). They found that when memory in the network nodes lasts a short time, the step-by-step method struggles and drops to zero performance. However, as memory lasts longer, the step-by-step method improves and eventually matches the all-at-once method. This suggests that current limitations with the step-by-step approach are mostly due to how long quantum memory lasts today, not a fundamental issue with the method itself.
quantum networkentanglement swappingsequential swappingsimultaneous SWAP-ASAPmemory decoherencesecret-key ratereinforcement learningsix-state protocolquantum memory coherenceentanglement heralding latency
Authors
Priyam Srivastava, Akshat R. Sabavat, Siddharth Jain, Alan Scheller-Wolf, Sridhar Tayur, David Tipper, Prashant Krishnamurthy, Amy Babay, Kaushik P. Seshadreesan
Abstract
Connection-less, packet-switched quantum network architectures distribute entanglement across multi-hop paths through sequential entanglement swapping, in which each node acts on purely local state information. The architectural advantages over the connection-oriented alternative -- simultaneous SWAP-ASAP -- are compelling, but sequential swapping holds partial chains in intermediate buffers between successive swaps, exposing them to memory decoherence in a way simultaneous SWAP-ASAP avoids by design. We present a proof-of-principle study at fixed chain length $n = 4$ in which each elementary link is governed by a fixed reinforcement-learning policy optimizing the secret-key rate of the six-state protocol, leaving the network-layer protocol as the sole independent variable. Sweeping the network-layer memory coherence time $T_c^{\mathrm{ext}}$ over four orders of magnitude reveals a clear regime structure governed by the dimensionless ratio $T_c^{\mathrm{ext}}/τ$, where $τ$ is the per-link entanglement heralding latency. Simultaneous SWAP-ASAP delivers a constant rate across the full sweep. Sequential swapping, by contrast, collapses to zero end-to-end deliveries below $T_c^{\mathrm{ext}}/τ= 25$, and begins recovering at $T_c^{\mathrm{ext}}/τ= 50$. It remains limited by the simultaneous rate, which it saturates only at the relaxed end of the sweep. These results suggest that the connection-less penalty is a near-term phenomenon tied to present-day memory coherence rather than a fundamental property of sequential swapping.