Purification enables an unbounded query separation in sequential quantum channel discrimination
Abstract
Purifications are often used as a convenient mathematical representation of quantum states and channels when constructing protocols for quantum information processing tasks. Although powerful, this perspective can suggest that the purifying degrees of freedom and its correlations with an environment are just a useful change of representation. Here we show how they can instead be exploited as an operational resource in sequential quantum channel discrimination, even when the environmental reference frame is unknown or averaged. In contrast to state discrimination and parallel channel discrimination, for which bare and averaged-purification access are equivalent, sequential access to purified resources exhibits a strict advantage in probability of success. We prove this strict separation via two main results. In the first, we construct a pair of families of qubit-qubit channels with a strict separation in the two-query case, and in the second, we show an example of a pair of qubit-qubit channels whose purifications are perfectly distinguishable with three queries while no finite number of bare (non-purified) queries suffices for perfect discrimination. These separations imply an advantage of sequential over parallel strategies for channel discrimination and establish the impossibility of converting bare channel queries into averaged-purification queries via transformations that allow arbitrary interventions between queries, strengthening previous results.
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BibTeX
@misc{toro2026purification,
title = {{Purification enables an unbounded query separation in sequential quantum channel discrimination}},
author = {Zoe García del Toro and Marco Túlio Quintino and Jessica Bavaresco},
howpublished = {HAL},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.40218},
eprint = {2609.40218},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05787618v1},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Purification enables an unbounded query separation in sequential quantum channel discrimination
AU - Zoe García del Toro
AU - Marco Túlio Quintino
AU - Jessica Bavaresco
PY - 2026
DA - 2026/09/30
PB - HAL
DO - 10.48550/arxiv.2609.40218
UR - https://hal.science/hal-05787618v1
AB - Purifications are often used as a convenient mathematical representation of quantum states and channels when constructing protocols for quantum information processing tasks. Although powerful, this perspective can suggest that the purifying degrees of freedom and its correlations with an environment are just a useful change of representation. Here we show how they can instead be exploited as an operational resource in sequential quantum channel discrimination, even when the environmental reference frame is unknown or averaged. In contrast to state discrimination and parallel channel discrimination, for which bare and averaged-purification access are equivalent, sequential access to purified resources exhibits a strict advantage in probability of success. We prove this strict separation via two main results. In the first, we construct a pair of families of qubit-qubit channels with a strict separation in the two-query case, and in the second, we show an example of a pair of qubit-qubit channels whose purifications are perfectly distinguishable with three queries while no finite number of bare (non-purified) queries suffices for perfect discrimination. These separations imply an advantage of sequential over parallel strategies for channel discrimination and establish the impossibility of converting bare channel queries into averaged-purification queries via transformations that allow arbitrary interventions between queries, strengthening previous results.
ER -