A Control Architecture for Entanglement Generation Switches in Quantum Networks

Entanglement between quantum network nodes is often produced using intermediary devices - such as heralding stations - as a resource. When scaling quantum networks to many nodes, requiring a dedicated intermediary device for every pair of nodes introduces high costs. Here, we propose a cost-effective architecture to connect many quantum network nodes via a central quantum network hub called an Entanglement Generation Switch (EGS). The EGS allows multiple quantum nodes to be connected at a fixed resource cost, by sharing the resources needed to make entanglement. We propose an algorithm called the Rate Control Protocol (RCP) which moderates the level of competition for access to the hub's resources between sets of users. We proceed to prove a convergence theorem for rates yielded by the algorithm. To derive the algorithm we work in the framework of Network Utility Maximization (NUM) and make use of the theory of Lagrange multipliers and Lagrangian duality. Our EGS architecture lays the groundwork for developing control architectures compatible with other types of quantum network hubs as well as system models of greater complexity

Quantum interference of electromechanically stabilized emitters in nanophotonic devices

Photon-mediated coupling between distant matter qubits may enable secure communication over long distances, the implementation of distributed quantum computing schemes, and the exploration of new regimes of many-body quantum dynamics. Nanophotonic devices coupled to solid-state quantum emitters represent a promising approach towards realization of these goals, as they combine strong light-matter interaction and high photon collection efficiencies. However, the scalability of these approaches is limited by the frequency mismatch between solid-state emitters and the instability of their optical transitions. Here we present a nano-electromechanical platform for stabilization and tuning of optical transitions of silicon-vacancy (SiV) color centers in diamond nanophotonic devices by dynamically controlling their strain environments. This strain-based tuning scheme has sufficient range and bandwidth to alleviate the spectral mismatch between individual SiV centers. Using strain, we ensure overlap between color center optical transitions and observe an entangled superradiant state by measuring correlations of photons collected from the diamond waveguide. This platform for tuning spectrally stable color centers in nanophotonic waveguides and resonators constitutes an important step towards a scalable quantum network

Recurrence Based Purification of Qudit Graph States

Preparation of multi-partite entangled quantum states under realistic experimental conditions invariably results in states with non-unit fidelity to the target state. Purification protocols address the need for higher fidelity states than what can be directly prepared. These protocols consume several noisy input states and return an output state of higher fidelity, succeeding probabilistically. We introduce a recurrence based purification protocol for two-colorable graph states on $d$-dimensional quantum systems (qudits). We analyze the performance of the protocol in terms of the minimal required fidelity of input states as well as the expected number of attempts required to successfully reach a specific target fidelity. We find that not only is the purification regime larger for states of greater qudit dimension, but the expected number of attempts to successfully purify a state may be orders of magnitude lower. We develop error thresholds for the protocol with faulty two-qudit operations using a general uncorrelated error model and study the dependence on system dimension and state node number. We observe that the gate error threshold of the protocol improves with increasing dimension and moreover that the threshold depends on the degree of the graph but is otherwise independent of the number of nodes. The qualitative behaviour of the error threshold is captured by an analytically solvable model in which a restricted class of errors is considered. The error thresholds determined here may serve as one benchmark of assessing whether future experimental implementations of two qudit operations function well enough to realize a practical advantage of replacing qubit with qudit states in a multi-partite quantum information protocol

An Architecture for Control of Entanglement Generation Switches in Quantum Networks

Entanglement between quantum network nodes is often produced using intermediary devices—such as heralding stations—as a resource. When scaling quantum networks to many nodes, requiring a dedicated intermediary device for every pair of nodes introduces high costs. Here, we propose a cost-effective architecture to connect many quantum network nodes via a central quantum network hub called an entanglement generation switch (EGS). The EGS allows multiple quantum nodes to be connected at a fixed resource cost, by sharing the resources needed to make entanglement. We propose an algorithm called the rate control protocol, which moderates the level of competition for access to the hub's resources between sets of users. We proceed to prove a convergence theorem for rates yielded by the algorithm. To derive the algorithm we work in the framework of network utility maximization and make use of the theory of Lagrange multipliers and Lagrangian duality. Our EGS architecture lays the groundwork for developing control architectures compatible with other types of quantum network hubs as well as system models of greater complexity