We present efficient multi-flow entanglement routing in Quantum Tree Network (QTN) with sublinear overhead, congestion-free operations, and error correction, outperforming conventional mesh networks.
We introduce Quantum Tree Networks (QTN), an architecture for hierarchical multi-flow entanglement routing. The network design is a k-ary tree where end nodes are situated on the leaves and routers at the internal nodes, with each node connected to k nodes in the child layer. The channel length between nodes grows with a rate ak, increasing as one ascends from the leaf to the root node. This construction allows for congestion-free and error-corrected operation with qubit-per-node overhead to scale sublinearly with the number of end nodes, N. The overhead for a k-ary QTN scales as O(Nlogkak⋅logkN) and is sublinear for all k with minimal surface-covering end nodes. More specifically, the overhead of quarternary (k=4) QTN is ∼O(N0.25⋅log4N). Alternatively, when end nodes are distributed over a square lattice, the quaternary tree routing gives the overhead ∼O(N⋅log4N). Our network-level simulations demonstrate a size-independent threshold behavior of QTNs. Moreover, tree network routing avoids the necessity for intricate multi-path finding algorithms, streamlining the network operation. With these properties, the QTN architecture satisfies crucial requirements for scalable quantum networks.
We propose quantum tree network (QTN) designs that are 1) free from congestion in multiflow communications, 2) optimal routing with only local information (self-information) and 3) completely covers 2D surface, 4) with only logarithmic overhead. If the channel is dominated by the insertion loss and the communication is limited by the local gates, as in current state-of-the-art technologies, the overhead is log(N), while this is relaxed to the general cases with poly-log overhead by engaging repeater chains in the long range communication.
Dynamic entanglement distribution is a key requirement for scalable, multi-user and multi-protocol quantum networks. We demonstrate a metropolitan-scale entanglement-based quantum communication network enabled by a quantum reconfigurable optical add-drop multiplexer (q-ROADM), which dynamically distributes polarisation-entangled photon pairs from a broadband source to six users over deployed campus and metropolitan fibre. The network supports programmable full-mesh, partial-mesh and sliced sub-network configurations, enabling flexible allocation of entanglement resources according to link condition and service requirement. We demonstrate stable six-user full-mesh operation over more than 150 hours, compare full-mesh and time-shared partial-mesh strategies under different source and detector conditions, and realise quantum network slicing with optional/additional interconnection links. We also show that the same infrastructure can support different quantum protocols by showcasing Secure Inaugural Authentication-Transfer (SIAT) combined with Network flooding over multiple paths to improve the security of onboarding a new user. These results demonstrate a q-ROADM-enabled entanglement distribution architecture as a novel route towards reconfigurable, service-oriented quantum networking over optical fibre infrastructure.
Entanglement distribution will form the backbone of many future distributed quantum technologies, especially the quantum internet. The act of purifying multiple noisy entangled states into a single one of higher quality has no analogue in classical networking and as such, this transforms the way in which we will consider future algorithms for routing entanglement. We outline the differences that arise because of this, demonstrate some elementary formalisms for `multi-path entanglement routing', and discuss the philosophical differences that arise when comparing this regime to conventional digital network theory. We also present a software package, QuNet, that uses novel `quantum cost-vector analysis' to simulate and benchmark routing in multi-user entanglement networks in a way that is is highly scalable in network size and the number of competing users. Our software accommodates both ground- and space-based networks, and implements efficient multi-user time-optimisation for mitigating congestion when quantum memories are available.