Natural characterization of input-state imperfections

Determine whether trace distance is the most natural characterization of imperfections in the quantum input states for experimentally implemented f-BB84 position-verification schemes.

Background

The robustness result measures deviations of the prepared states from ideal BB84 states using trace distance. The authors question whether this is the most appropriate experimentally meaningful parameter and suggest that an experimentally feasible test of the input states might instead yield a more natural security criterion. The problem is to identify the appropriate characterization and connect it to a corresponding security regime.