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

• Main Authors: De Palma, Giacomo (Author), Mari, Andrea (Author), Lloyd, Seth (Contributor), Giovannetti, Vittorio (Author)
• Other Authors: Massachusetts Institute of Technology. Department of Mechanical Engineering (Contributor), Massachusetts Institute of Technology. Research Laboratory of Electronics (Contributor)
• Format: Article
• Language: English
• Published: American Physical Society, 2017-07-10T19:12:08Z.
• Subjects: Article
• Online Access: Get fulltext

 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[subscript 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[superscript 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.