Bounds on the distance between a unital quantum channel and the convex   hull of unitary channels, with applications to the asymptotic quantum   Birkhoff conjecture
  Motivated by the recent resolution of Asymptotic Quantum Birkhoff Conjecture (AQBC), we attempt to estimate the distance between a given unital quantum channel and the convex hull of unitary channels. We provide two lower bounds on this distance by employing techniques from quantum information and operator algebras, respectively. We then show how to apply these results to construct some explicit counterexamples to AQBC. We also point out an interesting connection between the Grothendieck's inequality and AQBC. 
Nengkun Yu, Runyao Duan, and Quanhua Xu
quant-ph math.FA math.OA