Frozen condition of quantum coherence

Quantum coherence as an important physical resource plays the key role in implementing various quantum tasks, whereas quantum coherence is often deteriorated due to the noise. In this paper, we analyse under which dynamical conditions the $l_1$-norm or the relative entropy of coherence can remain unchanged during the whole evolution (freezing coherence). For single qubit systems, a nice formula is given to realize freezing coherence. Conversely, for a $d\ (d>2)$ dimensional system, we identify universal geometric conditions of freezing coherence. This offers an affirmative answer to the open question: how can one determine whether a unital quantum operation can be decomposed as a convex combination of unitary operations [M. A. Nielsen and I. L. Chuang, Quantum Computation and Quantum Information, (Cambridge University Press, Cambridge, 2000)]. Based on this analysis, we also give a complete classification of coherent states from operational coherence theory. This builds the counterpart of entanglement classification under LOCC.