Memory and thermal amplification in spin--cavity squared commutators
Squared commutators in the Holstein--Primakoff limit of a spin--cavity system provide a compact way to separate propagation from covariance growth in a finite-temperature reservoir with memory. In the finite-temperature NMQSD construction, the linear quadrature commutator is fixed by the retarded spin--cavity propagator, whereas a quadratic commutator carries the same retarded factor together with a covariance factor. For a zero-mean Gaussian state, \(C_{R_i^2,R_j}(t)=4|\kappa_{ij}(t)|^2V_{ii}(t)\); the symmetrized expression gives the spin-side and mixed channels. Since \(\bar n\) enters the covariance sector but not the homogeneous retarded kernel, raising \(\bar n\) from 0 to 1 leaves the linear transfer unchanged while increasing the quadratic signal. Varying the bath-memory rate and the counter-rotating coupling within the stable HP region then shows how stored cavity history changes both the transfer weight and its distribution in time. The calculation separates memory-dependent propagation from thermal covariance growth in collective spin--cavity dynamics.