A note on coherence power of N-dimensional unitary operators

Understand

The coherence power of a quantum channel, that is, its ability to increase the coherence of input states, is a fundamental concept within the framework of the resource theory of coherence.

• In this note we discuss various possible definitions of coherence power.
• Then we prove that the coherence power of a unitary operator acting on a qubit, computed with respect to the $l_1$-coherence measure, can be calculated by maximizing its coherence gain over pure incoherent states.
• We proceed to show that this result fails for dimensions N>2, that is, the maximal coherence gain is found when acting on a state with non-vanishing coherence.