Coherence window in the dynamics of quantum nanomagnets

Decoherence in many solid-state systems is anomalously high, frustrating efforts to make solid-state qubits. We show that in nanomagnetic insulators in large transverse fields, there can be a fairly narrow-field region in which both phonon- and nuclear-spin-mediated decoherence are drastically reduced. As examples we calculate decoherence rates for the Fe-8 nanomolecule, for Ni particles, and for Ho ions in LiHoxY1-xF4. The reduction in the decoherence, compared to low field rates, can exceed six orders of magnitude. The results also give limitations on the observability of macroscopic coherence effects in magnetic systems.