Nicolas Gisin

We construct and classify entangled measurements with tetrahedral symmetry as group-covariant orbits of a fiducial state. For this class, localizability follows from the Clifford level of a diagonal phase in the fiducial, yielding an explicit hierarchy and efficient constructions. The framework recovers the two-qubit Elegant Joint Measurement (EJM) uniquely and extends it to multiqubit EJMs with identical local geometry but inequivalent global entanglement classes for n\ge3. A continuous family interpolating between the Bell measurement and the EJM realizes an infinite set of localizable tetrahedral bases.

Quantum Key Distribution (QKD), invented by Bennett and Brassard and Ekert can be considered the first application of quantum information science. We focus on the assumption that an arbitrarily large number M of signals can be exchanged between the legitimate parties (Alice and Bob) and subsequently used for the computation of the final key. Here, we apply a novel proof technique to the BB84 QKD protocol and derive almost tight bounds on the minimum value M required to achieve a given level of security. The technique is based on a formulation of the uncertainty relation in terms of smooth entropies. We demonstrate significant improvements of the finite-key rate over existing results.

There are two main Grand Challenges for academic research in quantum communication. The first one concerns "device independent QKD", that is an implementation of Quantum Key Distribution that exploits the nonlocal correlation observed in violations of Bell's inequality to realize "self testing QKD apparatuses". The second one aims at futuristic continental scale quantum networks. The latter requires, among others, multimode quantum memories with close to a second memory times, a fascinating challenge. Interestingly, quantum networks also lead us to a refreshing revisit of nonlocality.

We present the results of the project QCRYPT, a collaborate effort of eight research teams in Switzerland with the ambition to produce a complete and practical fiber based QKD and high speed encryption system. For the QKD part, we put the emphasis on continuous operation with a wavelength multiplexed service channel for synchronization and distillation, efficient hardware real-time distillation, finite key security analysis and frugal authentication. For the secure high-speed encryption of large data volumes, we present a system able to multiplex up to ten 10 Gbit/s Ethernet inputs, pass the 100 Gbit/s data stream through authenticated encryption before transmitting it over an optical fiber to the decryptor. The cipher cores apply and frequently refresh the quantum keys delivered by the QKD system.