  We compute the backscattered current in a double point-contact geometry of a
Quantum Hall system at filling fraction $\nu=5/2$ as a function of bias voltage
in the weak backscattering regime. We assume that the system is in the
universality class of either the Pfaffian or anti-Pfaffian state. When the
number of charge $e/4$ quasiparticles in the interferometer is odd, there is no
interference pattern. However, the coupling between a charge $e/4$
quasiparticle and the edge causes it to be absorbed by the edge at low
energies. Consequently, an interference pattern appears at low bias voltages
and temperatures, as if there were an even number of quasiparticles in the
interferometer. We relate this problem to that of a semi-infinite Ising model
with a boundary magnetic field. Using the methods of perturbed boundary
conformal field theory, we give an exact expression for this crossover of the
interferometer as a function of bias voltage. Finally, we comment on the
possible relevance of our results to recent interference experiments.
