  In this paper, we calculate the equation of state of two-flavor finite
isospin chiral perturbation theory at next-to-leading order in the
pion-condensed phase at zero temperature. We show that the transition from the
vacuum phase to a Bose-condensed phase is of second order. While the tree-level
result has been known for some time, surprisingly quantum effects have not yet
been incorporated into the equation of state. We find that the corrections to
the quantities we compute, namely the isospin density, pressure, and equation
of state, increase with increasing isospin chemical potential. We compare our
results to recent lattice simulations of 2+1 flavor QCD with physical quark
masses. The agreement with the lattice results is generally good and improves
somewhat as we go from leading order to next-to-leading order in $\chi$PT.
