Continuous Distribution of Entanglement in Quantum Networks with Regular Topologies

Abstract

Quantum computers can solve certain problems faster than classical computers, but they require many qubits to solve valuable problems. Distributed quantum computing provides a scalable approach to increasing the number of qubits by interconnecting small-capacity quantum devices, or quantum nodes. Entangled states shared between nodes, so-called entangled links, can serve as a resource for implementing nonlocal operations. A better understanding of distributing links in a network of quantum nodes can guide the design of hardware and protocols for distributed quantum computing systems. We used two metrics to measure the performance of such entanglement distribution protocols considering the network’s objectives. Specifically, the virtual node degree reflects the requirement for many links to perform nonlocal operations, while the virtual neighbourhood size reflects the need for links between remote nodes to increase the number of qubits available for computation. Contrary to most prior research, these metrics explicitly consider the time-dependent fidelity of entangled links. We used discrete-time simulations to investigate the performance of a protocol that continuously distributes entanglement in a quantum network with a regular topology. The number of entangled links in the network evolves as quantum nodes create new links through entanglement generation and entanglement swaps, and remove low-fidelity links. The nodes probabilistically attempt swaps and can maximise the performance metrics by varying this probability.

We found that the performance metrics exhibit qualitatively similar behaviour for various network parameters, such as coherence time and entanglement generation fidelity. However, the network parameters shift the swap attempt probability that maximises the virtual neighbourhood size differently. The effect of network boundaries on performance metrics depends on the network topology.