Building a Quantum Network: How to Optimize Security and Expenses

Quantum key distribution (QKD) is regarded as a key-technology for the upcoming decades. Its practicability has been demonstrated through various experimental implementations. Wide-area QKD networks are a natural next step and should inherit the selling point of provable security. However, most research in QKD focuses on point-to-point connections, leaving end-to-end security to the trustworthiness of intermediate repeater nodes, thus defeating any formal proof of security: why bother outwitting QKD, if the repeater node is an easy prey, and an equally valuable target? We discuss methods of designing QKD networks with provable end-to-end security at provably optimized efforts. We formulate two optimization problems, along with investigations of computational difficulty: First, what is the minimal cost for a desired security? Second, how much security is achievable under given (budget-)constraints? Both problems permit applications of commercial optimization software, so allow taking a step towards an economic implementation of a globally spanning QKD network