  A closed-form expression for Wigner-Yanase skew information in mixed-state
quantum systems is derived. It is shown that limit values of the mixing
coefficients exist such that Wigner-Yanase information is equal to Helstrom
information. The latter constitutes an upper bound for the classical expected
Fisher information, hence the inverse Wigner-Yanase information provides an
approximate lower bound to the variance of an unbiased estimator of the
parameter of interest. The advantage of approximating Helstrom's sharp bound
lies in the fact that Wigner-Yanase information is straightforward to compute,
while it is often very difficult to obtain a feasible expression for Helstrom
information. In fact, the latter requires the solution of an implicit second
order matrix differential equation, while the former requires just scalar
differentiation.
