  We define metrics and measures to characterize the ratio of accessible
quantum entanglement for complex network failures in the quantum Internet. A
complex network failure models a situation in the quantum Internet in which a
set of quantum nodes and a set of entangled connections become unavailable. A
complex failure can cover a quantum memory failure, a physical link failure, an
eavesdropping activity, or any other random physical failure scenario. Here, we
define the terms entanglement accessibility ratio, cumulative probability of
entanglement accessibility ratio, probabilistic reduction of entanglement
accessibility ratio, domain entanglement accessibility ratio, and occurrence
coefficient. The proposed methods can be applied to an arbitrary topology
quantum network to extract relevant statistics and to handle the quantum
network failure scenarios in the quantum Internet.
