Walter Krawec

Quantum key distribution offers unconditionally secure keys for communicating parties. Although using high-dimensional quantum systems in QKD protocols does offer considerable advantages, which has been extensively shown in different experiments, analytical security proofs for high-dimensional protocols are not abundant. This is partly because many QKD protocols lack certain ``symmetry'' in terms of the parties' capabilities and responsibilities, which complicates security proofs. In this work, we consider one such protocol and provide analytical security proof and compare our results against prior work showing an advantage of our method. We also develop a continuity bound for conditional quantum entropies which is pertinent to our analysis here and may have applications in other scenarios also.

In this work, we analyze the security of a QKD repeater chain where some, but not all, repeaters and fiber links are under the control of an adversary. We show how to bound the quantum min-entropy for this scenario, needed to compute key-rates in the finite-key scenario. Our proof methods may also have numerous applications in other areas of QKD and quantum cryptographic research. Finally we evaluate our new bound and show that positive key-rates are possible even in noisy scenarios. Since early quantum repeaters are bound to be noisy, yet also bound to be partially trustworthy in some scenarios, our work shows improved bit generation rates are possible for early QKD networks.