Sequential vs. Synchronized Entanglement Swapping: QKD Secret-Key Rate under Optimized Link-Layer Control
The study compares two methods for timing entanglement swapping in quantum repeater chains: sequential (swap-and-wait) and simultaneous (wait-and-swap). Using a simulated four-hop chain under noise, the sequential method fails below a coherence time ratio of Textc/τ = 25, while the simultaneous method maintains a constant rate. This suggests that hardware memory coherence time is the critical factor determining whether connectionless swapping protocols are viable.