On the Operational Role of Purification in Quantum Information.

par Zoe Garcia (LIP6, Sorbonne Université)

jeudi 12 nov. 2026, 11:00 → 12:00 Europe/Paris

Salle R. Huron (1R1-106)

Purifications are ubiquitous in quantum information, where they are often regarded just as convenient mathematical representations of quantum states and channels. In this talk, we will explore instead their operational significance. We will first briefly discuss to what extent purifications and Stinespring dilations can be physically obtained from black-box access to states and channels, highlighting fundamental limitations on universal purification procedures.

It will then focus on learning and estimation tasks and show that access to the purifying degrees of freedom can constitute a genuine operational resource. While access to purifications does not bring advantages in state discrimination and parallel channel discrimination, this equivalence breaks down for adaptative strategies. In particular, we will present examples exhibiting a strict advantage with two queries and, more strikingly, channels whose purifications can be perfectly distinguished with only three sequential queries, whereas perfect discrimination of the corresponding bare channels is impossible with any finite number of queries. These results establish an unbounded query separation and illustrate how environmental degrees of freedom can fundamentally change the information-processing capabilities associated with a quantum channel.