Characterizing error propagation in quantum circuits: the Isotropic   Index
  This paper presents a novel index in order to characterize error propagation in quantum circuits by separating the resultant mixed error state in two components: an isotropic component, that quantifies the lack of information, and a dis-alignment component, that represents the shift between the current state and the original pure quantum state. The Isotropic Triangle, a graphical representation that fits naturally with the proposed index, is also introduced. Finally, some examples with the analysis of well-known quantum algorithms degradation are given. 
Andr\'e L. Fonseca de Oliveira and Efrain Buksman and Ilan Cohn and   Jes\'us Garc\'ia L\'opez de Lacalle
quant-ph