Abstract
Remote quantum entanglement can enable numerous applications including distributed quantum computation, secure communication, and precision sensing. We consider how a quantum network—nodes equipped with limited quantum processing capabilities connected via lossy optical links—can distribute high-rate entanglement simultaneously between multiple pairs of users. We develop protocols for such quantum “repeater” nodes, which enable a pair of users to achieve large gains in entanglement rates over using a linear chain of quantum repeaters, by exploiting the diversity of multiple paths in the network. Additionally, we develop repeater protocols that enable multiple user pairs to generate entanglement simultaneously at rates that can far exceed what is possible with repeaters time sharing among assisting individual entanglement flows. Our results suggest that the early-stage development of quantum memories with short coherence times and implementations of probabilistic Bell-state measurements can have a much more profound impact on quantum networks than may be apparent from analyzing linear repeater chains. This framework should spur the development of a general quantum network theory, bringing together quantum memory physics, quantum information theory, quantum error correction, and computer network theory.
Highlights
A quantum network can generate, distribute, and process quantum information in addition to classical data.[1,2] The most important function of a quantum network is to generate long distance quantum entanglement, which serves a number of tasks including the generation of multiparty shared secrets whose security relies only on the laws of physics,[3,4] distributed quantum computing,[5] improved sensing,[6,7] blind quantum computing (quantum computing on encrypted data),[8] and secure private-bid auctions.[9]Recent experiments have demonstrated entanglement links, viz., entanglement established between quantum memories separated by a few kilometers using a point-to-point optical link,[10] and longer range entanglement with satellites.[11,12] Further near-term demonstrations of long-range terrestrial entanglement are expected.[13]The conceptually simplest measurement module at a quantum network node is the two-qubit Bell state measurement (BSM), known as entanglement swapping
We proposed and analyzed quantum repeater protocols for entanglement generation in a quantum network in an architecture that uses the same elements as in many theoretical proposals and analyses of linear repeater chains
We accounted for channel losses between repeater nodes and the probabilistic nature of entanglement swaps at each repeater stemming from device inefficiencies, as well as the probabilistic nature of Bell-state measurements
Summary
A quantum network can generate, distribute, and process quantum information in addition to classical data.[1,2] The most important function of a quantum network is to generate long distance quantum entanglement, which serves a number of tasks including the generation of multiparty shared secrets whose security relies only on the laws of physics,[3,4] distributed quantum computing,[5] improved sensing,[6,7] blind quantum computing (quantum computing on encrypted data),[8] and secure private-bid auctions.[9]Recent experiments have demonstrated entanglement links, viz., entanglement established between quantum memories separated by a few kilometers using a point-to-point optical link,[10] and longer range entanglement with satellites.[11,12] Further near-term demonstrations of long-range terrestrial entanglement are expected.[13]The conceptually simplest measurement module at a quantum network node is the two-qubit Bell state measurement (BSM), known as entanglement swapping. We describe a more realistic protocol in which knowledge of success and failure of an external link at each time slot is communicated only to the two repeater nodes connected by the link, as is the case in the analysis of many ‘second-generation’ linear repeater chains.[26,27,29] Repeater nodes need to decide on which pair(s) of memories BSMs should be attempted (i.e., which internal links to attempt), based only on information about the states of external links adjacent to them.
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