Passive states as optimal inputs for single-jump lossy quantum channels

The passive states of a quantum system minimize the average energy among all the states with a given spectrum. We prove that passive states are the optimal inputs of single-jump lossy quantum channels. These channels arise from a weak interaction of the quantum system of interest with a large Markovian bath in its ground state, such that the interaction Hamiltonian couples only consecutive energy eigenstates of the system. We prove that the output generated by any input state $ρ$ majorizes the output generated by the passive input state $ρ_0$ with the same spectrum of $ρ$. Then, the output generated by $ρ$ can be obtained applying a random unitary operation to the output generated by $ρ_0$. This is an extension of De Palma et al., IEEE Trans. Inf. Theory 62, 2895 (2016), where the same result is proved for one-mode bosonic Gaussian channels. We also prove that for finite temperature this optimality property can fail already in a two-level system, where the best input is a coherent superposition of the two energy eigenstates.