  It is usually argued that the presence of gapless quasiparticle excitations
at the nodes of the d-wave superconducting gap should strongly decohere the
quantum states of a d-wave qubit, making quantum effects practically
unobservable. Using a self-consistent linear response non-equilibrium
quasiclassical formalism, we show that this is not necessarily true. We find
quasiparticle conductance of a d-wave grain boundary junction to be strongly
phase dependent. Midgap states as well as nodal quasiparticles contribute to
the conductance and therefore decoherence. Quantum behavior is estimated to be
detectable in a qubit containing a d-wave junction with appropriate parameters.
