13th QSC General Assembly in Amsterdam

Serge Fehr (CWI, Univ Leiden) - On the (In)security of the BUFF Transform

Krystal Guo (KdVI, QuSoft, UvA) - Combinatorial constructions for quantum wires

Álvaro G Iñesta (QuTech, TU Delft) -

Entanglement Buffering for Scalable Networks: How Imperfect Memories Can Beat Perfect Storage

Mariagrazia Iuliano (QuTech, TU Delft) - A Quantum Internet testbed using NV centers in diamond

Jeremy Young (Institute of Physics, UvA) -

Symmetries in Open Quantum Systems: Autonomous Error Correction and Quantum Metrology

Serge Fehr (CWI, Univ Leiden)
Digital signatures play an indispensable role in modern cryptography, both in the theory and in practical applications. The gold standard security property for digital signatures is "unforgeability" (under
chosen message attacks), demanding that it is computationally infeasible to produce a valid signature without knowing the secret key. However, in applications, possibly designed by software engineers that don't have a
rigorous understanding of cryptographic security, digital signatures may be slightly abused and their security guarantee (unknowingly) overstretched. In the worst case, this results in insecure applications.

Motivated by such (potential) non-standard uses of digital signature schemes in complex applications, the latest NIST call for post-quantum secure signature schemes explicitly recommends additional security properties (beyond standard unforgeability). As a consequence, several of the NIST submissions (explicitly or implicitly) employ the BUFF-transform, which is a generic way to obtain these additional security properties - at least that was the common belief...

In a recent line of work, we show that the situation is actually more complicated, in that one of these additional security properties (called "non-resignability") turns out to be rather elusive. Concretely, we
observe that the original analysis of the BUFF transform is incorrect, and we argue that non-resignability in its original formulation is actually (almost) unachievable; in particular, it is not achieved by the BUFF transform, thus invalidating prior such claims. We then consider a slightly weaker variant of the formal definition of non-resignability,
which circumvents the negative result, and we show that whether the BUFF transform achieves this variant depends on subtle details.

Krystal Guo (Korteweg-de Vries Instituut, QuSoft, UvA)
Abstract: Perfect state transfer was proposed as type of "quantum wire"; as a part of a quantum algorithm, one may want to transfer a state from one qubit to another qubit. While this could be done using a series of swap gates, such an operation might be prone to noise. This motivated the idea that one could construct a time-evolution quantum process that would perform the state transfer. Since its introduction in 2003, state transfer has been studied extensively with tools from algebraic graph theory and has let to many interesting combinatorial problems.

In this talk, we introduce peak state transfer—a generalization of perfect state transfer -- in both continuous and discrete-time quantum walks, which quantifies the maximum state transfer achievable under unitary evolution even when perfect state transfer is out of reach. Using a spectral characterization, we can determine this completely for some families of graphs, including an infinite families where the amount of peak state transfer tends to 1 as the number of vertices grows. We will highlight the combinatorial connections and matrix techniques used.  This is based on joint work with Vincent Schmeits.

Álvaro G Iñesta (QuTech, TU Delft) -