Generalized Fidelity and the Data Processing Inequality

Abstract: In this note, we show that the generalized Bures--Wasserstein distance induced by the generalized fidelity does not satisfy the data processing inequality in dimensions greater than two. In dimension two, although we do not settle the DPI in full generality, we derive two sufficient conditions guaranteeing it by using an explicit formula for the real part of the generalized fidelity. Furthermore, we find a necessary and sufficient condition under which the generalized fidelity reduces to the Uhlmann fidelity.