13th QSC General Assembly in Amsterdam

Quantum network applications often rely on the availability of high-quality entanglement across distant parties. Purification-based entanglement buffers are systems that maintain high-quality entanglement, ensuring it is readily available for consumption when needed. The performance of these buffers is shaped by various parameters, including the purification strategies used to maintain entanglement fidelity. In our work, we derive general analytical expressions for the system performance [1,2]. Our solutions are computationally efficient and establish fundamental bounds on the performance of purification-based entanglement buffers, offering valuable insights for designing efficient long-distance quantum communication systems. Notably, we find that simple purification protocols, such as DEJMPS, often provide superior buffering performance compared to complex protocols that maximize output fidelity.

[1] Davies, B., Iñesta, Á. G., & Wehner, S. (2024). Entanglement buffering with two quantum memories. Quantum, 8, 1458.

[2] Iñesta, Á. G., Davies, B., Kar, S., & Wehner, S. (2025). Entanglement buffering with multiple quantum memories. arXiv:2502.20240.

Jeremy Young (IoP, UvA) - Symmetries in Open Quantum Systems: Autonomous Error Correction and Quantum Metrology

Quantum systems are crucial to developing new technologies and studying fundamental science. However, current systems cannot be fully isolated from the environment, which can introduce detrimental noise effects that
destroy useful quantum features in these open quantum systems. Nevertheless, in some circumstances it is possible to instead utilize the environment as a resource via symmetries. Although the implications
of symmetries in Hamiltonian systems are well-understood, in open quantum systems the situation is more complex.

In this talk, I will show how two classes of symmetry exist for open quantum systems, “weak” and “strong”, and I will demonstrate the utility of strong symmetries via two examples. First, I will identify connections between steady state phase transitions and autonomous error correction, a form of quantum error correction in which active measurement and feedback is no longer needed. Second, I will show how strong symmetries allow for the preparation of highly-entangled collective states known as spin-squeezed states, which can be experimentally realized in optical cavity systems for quantum-enhanced metrology.

 

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

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.