  We consider quantum entanglement in strongly correlated quantum impurity
systems for states manifesting interesting properties such as multi-level Kondo
effect and dual nature between itineracy and localization etc.. For this
purpose, we set up a system consisting of one or two quantum impurities
arbitrarily selected from the system as a subsystem, and investigate quantum
entanglement with its environmental system. We reduce the pure state of
interest as described above to the subsystem, and formulate quantum informative
quantities such as entanglement entropy, mutual information and relative
entropy. We apply them to the single impurity Anderson model where the most
basic Kondo effect is manifested, and obtain new insights into the Kondo effect
there. The obtained results suggest that the method proposed here is promising
for elucidating the quantum entanglement of pure states in various quantum
impurity systems.
