**Holevo capacitites for qubit channels** - Dominic Berry

This study considers a class of qubit channels for which three states are always sufficient to achieve the Holevo capacity. For these channels it is known that there are cases where two orthogonal states are sufficient, two non-orthogonal states are required, or three states are necessary. Here a systematic theory is given which provides criteria to distinguish cases where two states are sufficient, and determine whether these two states should be orthogonal or non-orthogonal. In addition, the form of the optimal three-state ensemble is derived, and efficient methods of calculating the Holevo capacity are presented.