Quantum marginal inequalities and the conjectured entropic inequalities

Abstract: A conjecture -- \emph{the modified super-additivity inequality} of relative entropy -- was proposed in \cite{Zhang2012}: There exist three unitary operators $U_A\in \unitary{\cH_A},U_B\in \unitary{\cH_B}$, and $U_{AB}\in \unitary{\cH_A\ot \cH_B}$ such that $$ \rS(U_{AB}\rho_{AB}U^\dagger_{AB}||\sigma_{AB}) \geqslant \rS(U_A\rho_AU^\dagger_A||\sigma_A) + \rS(U_B\rho_BU^\dagger_B||\sigma_B), $$ where the reference state $\sigma$ is required to be full-ranked. A numerical study on the conjectured inequality is conducted in this note. The results obtained indicate that the modified super-additivity inequality of relative entropy seems to hold for all qubit pairs.