Information gain and information leak in quantum measurements

We discuss the relationships among various quantities of information during the process of an efficient quantum measurement, e.g., information gain, quantum loss, Holevo information, and coherent information. In particular, we give an uncertainty like relation between information gain and coherent information. We also investigate the information gain by local measurements and quantum correlations in bipartite quantum systems. Moreover, we discuss two cases of information leak according to whether the observer of the environment possesses extra information about the measured system.