Average versus maximal coherence

Highlights: • Some relations between average and maximal coherence. • Some exact quantum uncertainty relations. • The coherence of almost all state is nearly maximal with respect to almost all reference bases in high dimensions. -- Abstract: Coherence is a fundamental feature of quantum mechanics and plays a crucial role in the quantum realm. The issue of quantification of coherence has received considerable interests in recent years, and a variety of coherence quantifiers have been introduced. With these measures, one can investigate coherence in a more quantitative way. Based on the Wigner-Yanase skew information, we evaluate average coherence of a state with respect to arbitrary mutually unbiased bases as well as with respect to all orthonormal bases, and demonstrate their equivalence. We further evaluate the maximum coherence, and reveal a remarkable fact that in high dimensions, the coherence of a generic state is nearly maximal with respect to almost all reference bases. A similar concentration phenomenon also manifests itself when the coherence is quantified by the relative entropy of coherence. This observation concerning the almost equivalence between the average coherence and the maximal coherence for generic states may have theoretical implications for quantum information and quantum thermodynamics.