  We have recently shown that there is a limit to quantum coherence in
many-particle spin qubits due to spontaneous symmetry breaking. These results
were derived for the Lieb-Mattis spin model. Here we will show that the
underlying mechanism of decoherence in systems with spontaneous symmetry
breaking is in fact more general. We present here a generic route to finding
the decoherence time associated with spontaneous symmetry breaking in many
particle qubits, and subsequently we apply this approach to two model systems,
indicating how the continuous symmetries in these models are spontaneously
broken and discussing the relation of this symmetry breaking to the thin
spectrum. We then present in detail the calculations that lead to the limit to
quantum coherence, which is due to energy shifts in the thin spectrum.
