Zoe
G. del Toro

02 / Featured · 2026

Purification enables an unbounded query separation in sequential quantum channel discrimination

A quantum device can lose information to its surroundings. What changes if we can access that environment as well as the device’s usual output? We show that this access can help distinguish quantum channels when their uses are connected sequentially, even without knowing the environment’s reference frame.

For a particular pair of qubit channels, three uses of their purifications are enough to tell them apart with certainty. Using the original channels alone, no finite number of uses allows perfect discrimination. This advantage is absent for parallel strategies with averaged purification access.