  Tsallis entropy is a useful one-parameter generalization of the standard von
Neumann entropy in information theory. We study the variance of Tsallis entropy
of bipartite quantum systems in a random pure state. The main result is an
exact variance formula of Tsallis entropy that involves finite sums of some
terminating hypergeometric functions. In the special cases of quadratic entropy
and small subsystem dimensions, the main result is further simplified to
explicit variance expressions. As a byproduct, we find an independent proof of
the recently proved variance formula of von Neumann entropy based on the
derived moment relation to the Tsallis entropy.
