Hermann Kampermann

Networks of nodes connected by pairwise quantum key distribution (QKD) links are actively developing now. We consider the following problem: Given pairwise secret keys from QKD, how to agree on a common (conference) key for the whole network using classical communication? We propose an algorithm based on spanning tree packing from the graph theory and prove its optimality. The same algorithm can be applied for the GHZ distillation in pair-entangled networks.

The security of prepare-and-measure satellite-based quantum key distribution (QKD), under restricted eavesdropping scenarios, is addressed. We particularly consider cases where the eavesdropper, Eve, has limited access to the transmitted signal by Alice, and/or Bob’s receiver station. For instance, Eve can only receive an attenuated version of the transmitted signals. This results in settings where an uncharacterized bypass channel, inaccessible to Eve, can also carry signals to Bob. We obtain generic bounds on the key rate in the presence of bypass channels and apply them to continuous-variable QKD protocols with Gaussian encoding as well as to the family of BB84 protocols. We find regimes of operation in which the above restrictions on Eve can considerably improve system performance. Our work opens up new security frameworks for spaceborne quantum communications systems.

The characterization of quantum devices is crucial for their practical implementation but can be costly in experimental effort and classical post-processing. Therefore, it is desirable to measure only information that is relevant for specific applications and develop protocols that require little additional effort. In this work, we focus on the characterization of quantum computers in the context of stabilizer quantum error correction. We prove that (i) physical and (ii) logical error channels induced by Pauli noise can be estimated from syndrome data under minimal conditions. Essentially, any Pauli channel a code can correct can also be estimated from its syndrome measurements. We also provide a concrete estimation algorithm for this task.

We investigate the security of a quantum key distribution scheme, where Bob right after each detection announces publicly his choice of measurement basis, and the measurement results if the measurement is performed in the testing basis (used for parameter estimation). Such a scheme saves memory and communication time on both sides by not sending all the classical information only at the end of each block but immediately after each detection. We prove its security and show that this method will not reduce the key rate compared to conventional sifting.

We investigate the problem of conference key agreement in pair entangled networks (PEN) where the parties can share bipartite entangled states. In such networks, a source whose global state factorizes into bipartite “pair-entangled network” (PEN) states is distributed to honest parties which can perform local operations and public classical post-processing (LOSR+PP) to establish a shared secret key among each other. In this setting, we derive several new upper bounds on the achievable conference key rate. In particular for pure PEN states we show that the optimal key rate can be achieved by a bipartite QKD strategy.