Universal one-way secret key distillation from fully quantum states

Construct a universal one-way secret key distillation protocol for fully quantum tripartite states that optimizes the distillable key without prior knowledge of the state and without relying on a state-tailored measurement.

Background

The paper gives a universal one-way secret key distillation protocol for classical–quantum–quantum states when the classical distribution is known but the remaining state description is unavailable. For a fully quantum tripartite state, the optimal procedure begins with a state-dependent POVM by Alice, followed by one-way distillation on the resulting classical–quantum–quantum state. The authors identify the state-agnostic optimization of this measurement and the resulting distillation protocol as unresolved.

References

Several problems along this line remain open: starting from a fully quantum tripartite state, the optimal protocol for one-way secret key distillation is for Alice to perform a POVM and then apply the optimal protocol for the resulting classical-quantum-quantum post-measurement state.

— All you need is the universal correlation detector: A unified approach to universalize communication protocols over quantum channels  (2609.29954 - Watanabe et al., 24 Sep 2026) in Section 6, immediately following Theorem “Universal one-way secret key distillation from the output of an unknown wiretap channel”