Equal-fidelity surface for a qubit state via equal-distance extended   Bloch vectors and derivatives
  We describe and characterize an equal-fidelity surface in Bloch space targeted for a qubit state by means of equal-distance concept. The distance is generalized and defined as the Euclidean distance between extended Bloch vectors for arbitrary dimensional states. The distance is a genuine distance according to the definition and is related to other distances between quantum states and super-fidelity. 
Sang Min Lee, Heonoh Kim, Han Seb Moon
quant-ph