  A general theory of thermal magnetic fluctuations near conductive materials
is developed; such fluctuations are the magnetic analog of Johnson noise. For
realistic experiments in quantum computing and magnetic resonance force
microscopy, the predicted relaxation can be rapid enough that substantial
experimental care should be taken to minimize it. The same Hamiltonian matrix
elements that govern fluctuation and dissipation are shown to also govern
entanglement and renormalization, and a specific example of a
fluctuation-dissipation-entanglement theorem is constructed.
