The swap transpose on couplings translates to Petz' recovery map on quantum channels

Abstract

In [Ann. Henri Poincaré, {\bf 22} (2021), 3199-3234], De Palma and Trevisan described a one-to-one correspondence between quantum couplings and quantum channels realizing transport between states. The aim of this short note is to demonstrate that taking the Petz recovery map for a given channel and initial state is precisely the counterpart of the swap transpose operation on couplings. That is, the swap transpose of the coupling ${\Pi }_{\Phi }$ corresponding to the channel $\Phi$ and initial state $\rho$ is the coupling ${\Pi }_{rec}$ corresponding to the Petz recovery map ${\Phi }_{rec}.$

11 pages. v2: References to the closely related works [Duvenhage et al., Quantum 9, 1743 (2025)] and [Parzygnat et al., PRX Quantum 4, 020334 (2023)] have been included