Per-basis monitoring under drifting BB84 noise

Evaluate the separation between adaptive and conservative eavesdropping strategies for BB84 under channel drift when the legitimate parties impose independent per-basis error-rate constraints rather than a basis-averaged constraint.

Background

The BB84 drift experiments in the paper use a basis-averaged fidelity constraint of at least 0.87. Under this convention, an adaptive attacker outperforms a conservative noise-indexed lookup rule by 0.024 in fidelity. However, the stationary-channel analysis shows that replacing the averaged constraint with separate constraints on the two basis error rates substantially changes the attainable attack information and can reverse the sign of the gain from basis asymmetry.

The authors therefore identify the effect of per-basis monitoring on the adaptive advantage under drift as an unresolved quantitative question. The requested comparison would determine whether the reported value of adaptation persists, increases, or decreases under the monitoring convention commonly used in security proofs.

References

Because per-basis monitoring substantially alters the attacker's attainable information (Sec.~\ref{sec:asymmetry}), evaluating this separation under per-basis constraints is left for future measurements.

— Learnt Attacks on Quantum Key Distribution under Channel Noise and Device Drift  (2610.01792 - Mordarski et al., 1 Oct 2026) in Section 5.4, subsection “Value of adaptation on BB84”

The impact of delayed noise estimation or finite-sample fluctuations on the adaptive advantage remains an open question for future work.

— Learnt Attacks on Quantum Key Distribution under Channel Noise and Device Drift  (2610.01792 - Mordarski et al., 1 Oct 2026) in Section 6, Discussion