  The "half-quantum" vortices ($\sigma$) and quasiparticles ($\psi$) in a
two-dimensional $p_x+ip_y$ superconductor obey the Ising-like fusion rules
$\psi\times \psi=1$, $\sigma\times \psi=\sigma$, and $\sigma\times \sigma=
1+\psi$. We explain how the physical fusion of vortex-antivortex pairs allows
us to use these rules to read out the information encoded in the topologically
protected space of degenerate ground states. We comment on the potential
applicability of this fact to quantum computation.
  Modified 11/30/05 to reflect manuscript as accepted for publication. Includes
corrected last section.
