Analysis of SM quantum information

TL;DR

This paper analyzes the SM quantum information, revealing its dependence on eigenvector phases, and introduces a new invariant, non-monotone metric C_L, as a lower bound for SM quantum information.

Contribution

It demonstrates that SM quantum information is not a well-defined metric and proposes a new invariant metric C_L as a lower bound.

Findings

SM quantum information depends on eigenvector phases

C_L is invariant but not monotone

C_L serves as a lower bound for SM quantum information

Abstract

Morozova and Chentsov (Morozova and Chentsov 90) studied Riemannian metrics on the set of probability measures. They showed that, up to a constant factor, the Fisher information is the only Riemannian metric which is monotone under stochastic transformation. Sarovar and Milburn (Sarovar and Milburn 06) computed an upper bound on the Fisher information for one-parameter channels. In (O'Loan 07) we extended their bound to an upper bound on the Fisher information of multi-parameter families of states; we call this the SM quantum information. Petz and Sud\'ar (Petz 95) characterized fully the set of monotone metrics on the space of all density matrices. We analyse the SM quantum information in light of their work. We show that the SM quantum information is not a well-defined metric on the space of density matrices: different choices of phase of the eigenvectors lead to different metrics. We define a new metric C_L as a lower bound among the SM quantum informations. We look at properties of C_L and show that it is invariant but not monotone.