A scalable entanglement distribution protocol in quantum networks

Quantum networks are pivotal in enabling quantum communication, allowing the exchange of quantum bits of information (qubits). One way to realize quantum networks is to distribute entanglement between every pair of nodes in a network so that they can exchange qubits via quantum teleportation. As the process suggests, such quantum networks are called entanglement distribution networks. 

One solution to establishing long-distance entanglement links in large-scale networks is to integrate a network of two-way quantum repeaters on intermediate distances. This approach divides a long link into several shorter, more manageable segments. These elementary entanglement links are then connected through a process known as entanglement swapping or Bell-state measurement. For more background information, check out the basics of two-way repeaters in this post and how to upgrade existing classical network infrastructure to enable entanglement distribution here. 

Different schemes have been proposed for orchestrating the end-to-end entanglement generation process starting from elementary links. This blog post focuses on the practical aspects of such protocols, specifically featuring an asynchronous approach—sequential scheme—and emphasizes its advantages as discussed in our recent paper.

The need for asynchronous protocols for entanglement swapping 

As quantum networks scale up, they encounter issues like multiple user requests, routing, and congestion. Traditional solutions, relying on synchronized timing and central control, become cumbersome at larger scales due to complex routing and classical communication latency between the central controller and the rest of the network. Asynchronous protocols, such as those explored in our paper, offer a solution by enabling entanglement distribution that does not require such stringent coordination and are adaptable to decentralized routing protocols.

Asynchronous protocols are divided into two categories: sequential and parallel. The figure below illustrates how these protocols run on a simple linear network. In the parallel scheme, each repeater tries to generate entanglement with its neighbors. Once it is connected to two neighbors on a given path, it performs entanglement swapping. In contrast, in the sequential protocol, each link from one end-user (sender) to the other (receiver) is generated sequentially. After creating two subsequent links, entanglement swapping is performed.

Sequential scheme for entanglement: A closer look 

The sequential scheme operates on a simple yet effective principle: entanglement is extended iteratively from one node to the next, building a chain of entangled nodes across the network. As mentioned, the process begins with a sender initiating a pair of entangled photons and sending one part to a neighboring node, continuing until it reaches the receiver. Compared to the parallel scheme (see our other paper on distributed routing for the parallel scheme), this step-by-step approach not only simplifies the process but also reduces the need for simultaneous operations for distributed routing across the network. The following figure shows how the sequential scheme can be implemented for two user pairs as a hop-by-hop protocol with distributed routing. 

Performance metrics favor sequential approaches 

Our extensive simulations indicate that the sequential scheme is on par with or superior to the parallel scheme in key performance metrics such as entanglement bit rate, end-to-end fidelity, and secret key rate for quantum key distribution (QKD). The sequential approach's simplicity, avoiding the complexities of the parallel method, makes it a more practical choice for actual deployment.

In addition, introducing a cutoff strategy in the sequential scheme—terminating attempts that exceed a certain duration to prevent degradation of quantum states—is quite simple and further enhances the quality of the established entanglement.

Real-world applications and future prospects for the quantum internet

Applying the sequential scheme to actual network topologies, like SURFnet, has yielded promising results. The flexibility and lower operational demands of the sequential approach make it an ideal candidate for the backbone of the emerging quantum internet. Its capability to operate effectively across diverse and dynamically changing network conditions underscores its potential as a scalable solution for future quantum communications infrastructure.

As quantum technology continues to evolve, the methodologies for quantum entanglement distribution must also advance. The sequential scheme represents a significant step forward. Its adaptability, simplicity, and performance advantages highlight its potential as the preferred method for entanglement distribution networks, promising a more connected and secure quantum future.

To learn more about quantum technology at Outshift by Cisco, read our collection of blogs here.