Catalysis always degrades external quantum correlations

Catalysts used in quantum resource theories need not be in isolation and, therefore, are possibly correlated with external systems, which the agent does not have access to. Do such correlations help or hinder catalysis, and does the classicality or quantumness of such correlations matter? To answer this question, we first focus on the existence of a noninvasively measurable observable that yields the same outcomes for repeated measurements since this signifies macrorealism, a key property distinguishing classical systems from quantum systems. We show that a system quantumly correlated with an external system so that the joint state is necessarily perturbed by any repeatable quantum measurement also has the same property against general quantum channels. Our full characterization of such systems called totally quantum systems, solves the open problem of characterizing tomographically sensitive systems raised in Phys. Rev. Lett. 130, 020802 (2023). An immediate consequence is that a totally quantum system cannot catalyze any quantum process, even when a measure of correlation with its environment is arbitrarily low. It generalizes to a stronger result, that the mutual information of totally quantum systems cannot be used as a catalyst either. These results culminate in the conclusion that, out of the correlations, that a generic quantum catalyst has with its environment, only classical correlations allow for catalysis, and, therefore, using a correlated catalyst is equivalent to using an ensemble of uncorrelated catalysts.