Probabilistic Cutoffs in Homogeneous Quantum Repeater Chains

TL;DR

This paper introduces a probabilistic cutoff policy for quantum repeater chains that simplifies link management without tracking link ages, achieving comparable or better secret-key rates and fidelity in certain scenarios compared to deterministic policies.

Contribution

The paper proposes a novel probabilistic cutoff policy for quantum repeaters that reduces complexity while maintaining high fidelity and secret-key rates, outperforming deterministic policies in specific cases.

Findings

Probabilistic cutoff policy achieves similar secret-key rates as deterministic policies in certain regimes.

The new policy can deliver higher fidelity links at increased rates under specific conditions.

Probabilistic approach reduces the need for tracking link ages, simplifying quantum repeater management.

Abstract

We study quantum repeater chains in which entangled links between neighbouring nodes are created through heralded entanglement generation and adjacent links are swapped as soon as possible. Since heralded entanglement generation attempts succeed only probabilistically, some links will have to be stored in quantum memories at the nodes of the chain while waiting for adjacent links to be generated. The fidelity of these stored links decreases with time due to decoherence, and if they are stored for too long then this can lead to low end-to-end fidelity. Previous work has shown that the end-to-end fidelity can be improved by deterministically discarding links when their ages exceed some cutoff value. Such deterministic cutoff policies provide strict control of the fidelity of all links, but they come at the expense of having to track link ages. In this work, we introduce a probabilistic cutoff policy that does not require tracking link ages, at the cost of abandoning strict control of the fidelity. We benchmark this new probabilistic cutoff policy against a deterministic cutoff policy. We compare the policies in terms of the end-to-end rate and fidelity, and the secret-key rate. We find that even though the probabilistic cutoff policy keeps track of less state, it can provide secret-key rates of the same order of magnitude as the deterministic cutoff policy in chains with few nodes or high elementary link generation probabilities. Moreover, we identify a scenario in which the probabilistic cutoff policy can deliver end-to-end links that are required to have some minimum threshold fidelity at a higher rate than the deterministic cutoff policy.