CWI Cryptology Group

Abstract: The realization of a highly connected network of qubit registers is a central challenge for quantum information processing and secure long-distance quantum communication. Diamond spins associated with NV centers are promising building blocks for such a network as they combine a coherent optical interface (similar to that of trapped atomic qubits) [1] with a local register of robust, long-lived nuclear spin qubits [2,3]. Here we present our latest progress towards scalable quantum networks and particular towards a loophole-free Bell test. We have recently demonstrated unconditional teleportation between diamond spin qubits residing in independent setups separated by 3 meters [4,5]. Importantly, the entanglement and readout fidelities achieved in the teleportation experiment [4] are sufficient for a violation of a Bell inequality with the detection loophole closed. Separating the two NV centers by more than a kilometer will furthermore enable closing of the locality loophole and the freedom-of-choice loophole, thus opening the door to realizing a loophole-free Bell test and demonstrating device-independent quantum key distribution [6].
[1] L. Robledo, L. Childress, H. Bernien, B. Hensen, P.F. A. Alkemade, R. Hanson, Nature 477, 547 (2011).
[2] T. H. Taminiau, J. Cramer, T. van der Sar, V. V. Dobrovitski, R. Hanson, Nature Nanotechnology 9, 171 (2014).
[3] M.S. Blok, C. Bonato, M.L. Markham, D.J. Twitchen, V.V. Dobrovitski, R. Hanson, Nature Physics 10, 189 (2014).
[4] W. Pfaff, B.J. Hensen, H. Bernien, S.B. van Dam, M.S. Blok, T.H. Taminiau, M.J. Tiggelman, R.N. Schouten, M. Markham, D.J. Twitchen, R. Hanson, Science (2014); see also arXiv:1404.4369 (2014).
[5] H H. Bernien, B. Hensen, W. Pfaff, G. Koolstra, M. S. Blok, L. Robledo, T. H. Taminiau, M. Markham, D. J. Twitchen, L. Childress, R. Hanson, Nature 497, 86 (2013).
[6] N. Brunner, D. Cavalcanti, S. Pironio, V. Scarani, S. Wehner, Bell nonlocality, Rev. Mod. Phys. 86, 419 (2014).

Abstract: In my theoretical physics group in Delft we like to think about how one can implement ideas from quantum information theory using actual physical (nano)systems and predict the quantum dynamic behavior of the information processing in the system. In this talk I will give a brief overview of projects, past and present, that focus on e.g. the physical implementation of quantum switches and quantum random access memories in hybrid optical-solid-state systems.

Abstract: One of the peculiar features of quantum mechanics is entanglement. It is known that entanglement is monogamous in the sense that a quantum system can only be strongly entanlged to one other system. In this talk, I will show how this so-called monogamy of entanglement can be captured and quantified by a "game". As an application of our analysis, we show that - in theory - the standard BB84 quantum-key-distribution scheme is one-sided device-independent, meaning that one of the parties, say Bob, does not need to trust his quantum measurement device: security is guaranteed even if his device is completely malicious. For more information, see arXiv:1210.4359. (Joint work with M. Tomamichel, J. Kaniewski, and S. Wehner)

Abstract: In many cryptographic protocols, the main ingredient of the security proof involves showing that a dishonest party has a limited amount of information about a particular string or quantum system of interest. This bound on the adversary's information often comes from a physical constraint, such as a limited or noisy quantum memory, which must then be harnessed by the security proof. In this talk, I will present a general technique for making use of this type of constraint in security proofs, and will give concrete applications to cryptography in the bounded storage model and to bounds on random-access codes. For more information, see arXiv:1305.1316. (Joint work with Omar Fawzi and Stephanie Wehner)