Fabian Wiesner

Quantum state verification plays a vital role in many quantum cryptographic protocols, as it allows using quantum states from an untrusted source. While some progress has been made in this direction, the question of whether the most prevalent type of quantum state verification, namely cut-and-choose verification, can be efficient and secure, is still not answered in full generality. In this work, we show a fundamental limit for quantum state verification for all cut-and-choose approaches used to verify arbitrary quantum states. We provide a no-go result showing that the cut-and-choose techniques cannot lead to quantum state verification protocols that are both efficient and secure. We show this trade-off for stand-alone and composable security, where the scaling of the lower bound for the security parameters renders cut-and-choose quantum state verification effectively useless.

Quantum repeaters have long been established to be essential for distributing entanglement over longdistances. Consequently, their experimental realization constitutes a core challenge of quantum communi-cation. However, there are numerous open questions about implementation details for realistic near-termexperimental setups. In order to assess the performance of realistic repeater protocols, here we presentReQuSim, a comprehensive Monte Carlo–based simulation platform for quantum repeaters that faithfullyincludes loss and models a wide range of imperfections such as memories with time-dependent noise. Ourplatform allows us to perform an analysis for quantum repeater setups and strategies that go far beyondknown analytical results: This refers to being able to both capture more realistic noise models and analyzemore complex repeater strategies. We present a number of findings centered around the combination ofstrategies for improving performance, such as entanglement purification and the use of multiple repeaterstations, and demonstrate that there exist complex relationships between them. We stress that numericaltools such as ours are essential to model complex quantum communication protocols aimed at contributingto the quantum Internet.