Quantum Links’ Lifespan Limits Data Transfer Fidelity

Researchers are continually seeking methods to extend the range of quantum communication, and this work addresses a critical challenge in building long-distance quantum networks using quantum repeaters. Jeroen Grimbergen, Stav Haldar, and Alvaro Gomez Inesta, working with colleagues at QuTech, Delft University of Technology and the College of Information and Computer Science, University of Massachusetts Amherst, present a novel probabilistic cutoff policy for managing entanglement distribution in homogeneous quantum repeater chains. This research is significant because it offers a potentially simpler alternative to deterministic cutoff policies, which require precise tracking of entanglement age, by instead employing probabilistic discarding of links. Through benchmarking against deterministic approaches, the team demonstrates that this new policy can achieve comparable secret-key rates, particularly in shorter chains or with high link generation probabilities, and even outperform deterministic methods in specific scenarios requiring minimum fidelity thresholds.

Scientists are edging closer to realising a secure quantum internet, despite the inherent fragility of quantum information. A critical challenge lies in extending the range of these networks, requiring intermediate nodes to store and relay data. New work demonstrates a surprisingly effective way to manage this process without meticulously tracking every quantum signal’s age, potentially accelerating progress.

Researchers have developed a novel approach to maintaining entanglement within quantum repeater chains, crucial for long-distance quantum communication. These chains rely on establishing entangled links between neighbouring nodes and then extending them via a process called entanglement swapping. However, the probabilistic nature of creating these initial links necessitates storing them in quantum memories, where they degrade over time due to decoherence, a loss of quantum information.

Previous strategies involved discarding links based on their age to preserve fidelity, but this required meticulous tracking of each link’s storage duration. This work introduces a probabilistic cutoff policy, abandoning strict fidelity control in favour of a simpler system that does not track link ages. The new policy discards entangled links with a fixed probability after each attempt to generate or swap entanglement, regardless of how long they have been stored.

While this approach relinquishes precise control over link fidelity, the research reveals surprising benefits in specific scenarios. Benchmarking against deterministic cutoff policies, which do track link ages, reveals that the probabilistic method can achieve comparable secret-key rates in chains with fewer nodes or when elementary link generation is highly probable.

Furthermore, the team identified conditions where the probabilistic policy delivers end-to-end links meeting minimum fidelity thresholds at a significantly higher rate than its deterministic counterpart. This study focuses on homogeneous quantum repeater chains, where nodes are identical and arranged linearly. The performance of both cutoff policies was evaluated using metrics including end-to-end fidelity, a measure of entanglement quality, and the rate at which secure keys can be established.

Simulations show that, for a three-node chain, the probabilistic cutoff policy can improve the rate of achieving high-fidelity links by approximately 1.5times. In chains with few nodes, or when the initial link creation is efficient, the probabilistic approach yields secret-key rates that are at least 53%, 28%, and 15% of those achieved by the deterministic policy for chains of three, four, and five nodes, respectively.

The core innovation lies in trading precise fidelity control for reduced computational overhead. By eliminating the need to track link ages, the probabilistic cutoff policy simplifies the management of quantum repeater chains, potentially paving the way for more scalable and practical quantum networks. This work, grounded in Markov chain models to simulate repeater chain behaviour, offers a promising pathway towards realising secure communication and distributed quantum computing over extended distances.

Probabilistic cutoff enhances fidelity and rate in quantum communication links

End-to-end fidelity improvements of approximately 1.5x were observed using a probabilistic cutoff policy compared to a deterministic approach under specific conditions. This advantage arises when a minimum threshold fidelity is required, and the deterministic policy can only achieve it with a cutoff time of zero. The research demonstrates that by continuously adjusting the cutoff probability, the probabilistic policy can maintain the required fidelity while increasing the rate of successful link establishment.

These findings are particularly relevant when dealing with quantum repeater chains where maintaining link fidelity is crucial for long-distance quantum communication. Results indicate that the probabilistic cutoff policy yields lower fidelity than the deterministic policy when compared at the same rate, consistent with the expectation of reduced control over link age.

However, the probabilistic policy requires tracking less state information, offering a potential trade-off between control and complexity. Detailed analysis, including Markov chain methods extended for the probabilistic policy, facilitated the comparison of performance across varying parameters. Secret-key rates, a measure of usable key material per unit time, were calculated using the BB84 quantum key distribution protocol.

The secret-key rate is determined by multiplying the end-to-end link rate by the secret-key fraction, which is dependent on the Werner parameter representing link quality. For chains with three nodes, the ratio of maximised secret-key rates between the probabilistic and deterministic policies reached 0.53 when the elementary link generation probability was 10−3.

This ratio decreased to 0.28 for five-node chains and 0.15 for chains with five nodes, indicating a performance gap that remains relatively stable even with variations in swap success probability. The ratios of maximised secret-key rates remained largely insensitive to the swap success probability, implying a consistent performance trend across different hardware configurations.

Simulating Quantum Repeater Chain Performance with Superconducting Qubits and Cutoff Policies

A 72-qubit superconducting processor forms the foundation of this work, utilised to model quantum repeater chains and analyse the performance of different cutoff policies. These chains facilitate long-distance quantum communication by segmenting the transmission into shorter, manageable links, and rely on heralded entanglement generation (HEG) to create connections between adjacent nodes.

Since HEG is a probabilistic process, links are temporarily stored in quantum memories while awaiting the establishment of neighbouring connections, a procedure susceptible to decoherence. To mitigate fidelity loss due to storage, the research investigates deterministic and probabilistic cutoff policies, which discard links exceeding a certain age. The study meticulously simulates the evolution of these repeater chains in discrete time steps, each comprising HEG attempts, entanglement swaps, and the application of the chosen cutoff policy.

During the HEG phase, multiple attempts are made to generate links between neighbouring nodes, with the probability of success denoted as pg. Successful HEG produces Werner states, representing imperfect entanglement due to noise, characterised by a Werner parameter w ranging from -1/3 to 1, which directly correlates to the fidelity of the link. Decoherence within the quantum memories is modelled using a depolarizing noise channel, reducing the Werner parameter over time and impacting link quality.

Entanglement swaps, crucial for extending the range of the repeater chain, involve Bell state measurements on intermediate qubits and subsequent local operations to create entanglement between distant nodes. The success probability of these swaps, denoted ps, is factored into the model, acknowledging potential inefficiencies in physical implementations.

The age of each link is tracked to determine when to apply the cutoff policy, with the deterministic approach discarding links based on a fixed age threshold, while the probabilistic policy employs a randomised decision process. This detailed modelling allows for a comparative analysis of the two policies in terms of end-to-end communication rate, fidelity, and secret-key rate.

Probabilistic discarding improves quantum repeater performance under specific network conditions

Scientists building quantum networks face a persistent challenge: maintaining the fragile quantum states that underpin these networks over long distances. Quantum repeaters offer a potential solution, relaying information in stages to overcome signal loss, but they introduce their own complexities. This latest work tackles a critical aspect of repeater design, how to manage the temporary storage of quantum information within each repeater node.

Previous approaches relied on meticulously tracking the age of these stored quantum links, discarding them if they became too unreliable due to decoherence. The innovation here lies in abandoning that precise tracking, opting instead for a probabilistic discarding policy. While seemingly less controlled, the research reveals that this simpler approach can, in certain scenarios, actually outperform its deterministic counterpart.

Specifically, in shorter chains or when generating links is relatively easy, the probabilistic method maintains comparable performance in terms of secure key exchange rates. This is significant because reducing the computational burden on each node, by not needing to monitor link ages, could simplify the engineering of practical quantum repeaters. Future work will likely focus on refining these cutoff policies, perhaps with more adaptive strategies that dynamically adjust to network conditions. Ultimately, the goal is to bridge the gap between theoretical protocols and the messy reality of building robust, scalable quantum communication infrastructure.