  In its simplest form, decoherence occurs when a quantum state is entangled
with a second state, but the results of measurements made on the second state
are not accessible. As the second state has effectively "measured" the first,
in this paper we argue that the quantum Fisher information is the relevant
metric for predicting and quantifying this kind of decoherence. The quantum
Fisher information is usually used to determine an upper bound on how precisely
measurements on a state can be used to estimate a classical parameter, and as
such it is an important resource. Quantum enhanced metrology aims to create
non-classical states with large quantum Fisher information and utilise them in
precision measurements. In the process of doing this it is possible for states
to undergo decoherence, for instance atom-light interactions used to create
coherent superpositions of atomic states may result in atom-light entanglement.
Highly non-classical states, such as spin-cat states (Schr\"odinger cat states
constructed from superpositions of collective spins) are shown to be highly
susceptible to this kind of decoherence. We also investigate the required field
occupation of the second state, such that this decoherence is negligible.
