  We prove the quantum conditional Entropy Power Inequality for quantum
additive noise channels. This inequality lower bounds the quantum conditional
entropy of the output of an additive noise channel in terms of the quantum
conditional entropies of the input state and the noise when they are
conditionally independent given the memory. We also show that this conditional
Entropy Power Inequality is optimal in the sense that we can achieve equality
asymptotically by choosing a suitable sequence of Gaussian input states. We
apply the conditional Entropy Power Inequality to find an array of
information-theoretic inequalities for conditional entropies which are the
analogues of inequalities which have already been established in the
unconditioned setting. Furthermore, we give a simple proof of the convergence
rate of the quantum Ornstein-Uhlenbeck semigroup based on Entropy Power
Inequalities.
