A Surprisingly Simple Way of Reversing Trace Distance via Entanglement

Trace distance (between two quantum states) can be viewed as quantum generalization of statistical difference (between two probability distributions). On input a pair of quantum states (represented by quantum circuits), how to construct another pair, such that their trace distance is large (resp. small) if the original trace distance is small (resp. large)? That is, how to reverse trace distance? This problem originally arose in the study of statistical zero-knowledge quantum interactive proof. We discover a surprisingly simple way to do this job. In particular, our construction has two interesting features: first, entanglement plays a key role underlying our construction; second, strictly speaking, our construction is non-black-box.