Universal two-way secret key distillation

Construct a universal two-way secret key distillation protocol that achieves optimal performance for fully quantum tripartite states without prior knowledge of which preprocessing by two-way local operations and public communication maximizes the distillable key.

Background

Two-way secret key distillation is described as consisting of state-dependent two-way local preprocessing and public communication followed by one-way secret key distillation. Although the optimal state-aware protocol is known in this structural form, a state-agnostic protocol would need to identify the appropriate preprocessing without knowing the input state. The paper explicitly leaves the design of such a universal protocol unresolved.

References

Moreover, designing a universal protocol for two-way secret key distillation remains open for a similar reason.

— 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, paragraph following Theorem “Universal one-way secret key distillation from the output of an unknown wiretap channel”