Don Towsley

We study a quantum switch serving a set of users. The function of the switch is to convert bipartite entanglement generated over individual links connecting each user to the switch, into bipartite or tripartite entangled states among (pairs or groups of) users at the highest possible rates at a fixed ratio. Such entanglement can then be converted to quantum-secure shared secret bits among pairs or triples of users using E91-like Quantum Key Distribution (QKD) protocols. The switch can store a certain number of qubits in a quantum memory for a certain length of time, and can make two-qubit Bell-basis measurements or three-qubit GHZ-basis projective measurements on qubits held in the memory. We model a set of randomized switching policies. Discovering that some are better than others, we present analytical results for the case where the switch stores one qubit per user at a given time step, and find that the best policies outperform a time division multiplexing (TDM) policy for sharing the switch between bipartite and tripartite entanglement generation. This performance improvement decreases as the number of users grows. The model is easily augmented to study the capacity region in the presence of qubit decoherence, obtaining similar results. Moreover, decoherence appears to have little effect on capacity. We also study a smaller class of policies when the switch can store two qubits per user. The full manuscript can be found at https://arxiv.org/abs/1901.06786.

We develop a protocol that allows a pair of users to sift a secret key starting from shared variable-length Greenberger-Horne-Zeilinger (GHZ) states. It is an extension of the BBM’92 protocol which relies on measurements in the matching basis for entanglement witness. We then design an entanglement generation scheme over a quantum network that equips the quantum key generation protocol to achieve key rates that are independent of the distance between the two users. The key new insight in our protocol is to allow a repeater node to use n-qubit GHZ projective measurements that can fuse n successful entangled links, i.e., two-qubit entangled Bell pairs shared across network edges, incident at that node, into an n-qubit GHZ state shared by the far nodes of those edges. If we allow even 3-fusions at the nodes, we find by developing a connection to a modified version of the site-bond percolation problem that despite lossy (hence probabilistic) link-level entanglement generation, and probabilistic success of the fusion measurements at nodes, one can generate entanglement between end two parties at a rate that stays constant as the distance between them increases. This is not possible to attain with any (non-error-corrected) quantum networking protocol using Bell measurements alone.