  A persistent current qubit has two quantum states with opposite currents
flowing in a superconducting loop. Their magnetic field couple to nuclear
spins. The qubit state is not only perturbed by the spins but it also gets
entangled with the spins' state on a very short timescale. However, when the
same spins are exposed to a strong but less than critical external magnetic
field, then the qubit field is just a small perturbation on top of the external
field and the entanglement with each spin is negligible. For a qubit which is
more microscopic than certain threshold this partial entanglement results in
negligible partial decoherence.
