Optimising the choice of entanglement purification policy for a noisy, dynamic quantum network is a challenging problem, because the global network performance depends on the purification policy in a complex way. We address this challenge by studying entanglement buffers: systems that maintain high-quality entanglement, ensuring that it is readily available for consumption when needed. We study the performance of a two-node buffer, where each node has one long-lived quantum memory for entanglement storage and multiple short-lived memories for generation. Freshly generated entanglement may be used to purify stored entanglement, which degrades over time. We derive analytical expressions for the entanglement availability and the average fidelity upon consumption. Our solutions are computationally efficient and provide fundamental bounds to the performance of purification-based entanglement buffers. We show that purification must be performed as frequently as possible to maximise the average fidelity of entanglement upon consumption, even if this often leads to the loss of high-quality entanglement due to purification failures. Moreover, we obtain heuristics for the design of good purification policies in practical systems.

Bethany Davies is a postdoctoral researcher in the Department of Physics at the University of Basel, Switzerland. She did her Ph.D. at TU Delft.