  The paper considers the problem of equalization of passive linear quantum
systems. While our previous work was concerned with the analysis and synthesis
of passive equalizers, in this paper we analyze coherent quantum equalizers
whose annihilation (respectively, creation) operator dynamics in the Heisenberg
picture are driven by both quadratures of the channel output field. We show
that the characteristics of the input field must be taken into consideration
when choosing the type of the equalizing filter. In particular, we show that
for thermal fields allowing the filter to process both quadratures of the
channel output may not improve mean square accuracy of the input field
estimate, in comparison with passive filters. This situation changes when the
input field is `squeezed'.
