Cryptographic Insights into Quantum Foundations

by Prof. Renato Renner (ETH Zurich)

Thursday 30 Oct 2025, 16:15 → 17:30 Europe/Berlin

ENC-D114 (ENC)

Imagine being invited to play a game — the Complementarity Game. You prepare a qubit Q and send it to a referee, along with a pair of predictions (z, x). The referee then randomly measures Q in either the computational or diagonal basis, yielding an outcome Z or X, respectively. You win if Z = z or X = x.

Quantum theory, via Heisenberg’s uncertainty principle, tells us that winning this game with certainty is impossible. This limitation is essential to quantum cryptography: an adversary capable of winning the Complementarity Game could also break quantum key distribution.

However, in my talk, I will show that when multiple agents collaborate, they can, in fact, win the Complementarity Game with certainty. This relies on an additional assumption about how agents combine their knowledge, which is not part of standard quantum theory, but is implicit in communication scenarios.

The result challenges fundamental assumptions, not only in quantum cryptography but also in the foundations of quantum theory.