Entropic bounds between two thermal equilibrium states

Abstract

The positivity conditions of the relative entropy between two thermal equilibrium states $\hat{\rho}_1$ and $\hat{\rho}_2$ are used to obtain upper and lower bounds for the subtraction of their entropies, the Helmholtz potential and the Gibbs potential of the two systems. These limits are expressed in terms of the mean values of the Hamiltonians, number operator, and temperature of the different systems. In particular, we discuss these limits for molecules which can be represented in terms of the Franck--Condon coefficients. We emphasize the case where the Hamiltonians belong to the same system at two different times $t$ and $t'$. Finally, these bounds are obtained for a general qubit system and for the harmonic oscillator with a time dependent frequency at two different times.