  We address parameter estimation for complex/structured systems and suggest an
effective estimation scheme based on continuous-variables quantum probes. In
particular, we investigate the use of a single bosonic mode as a probe for
Ohmic reservoirs, and obtain the ultimate quantum limits to the precise
estimation of their cutoff frequency. We assume the probe prepared in a
Gaussian state and determine the optimal working regime, i.e. the conditions
for the maximization of the quantum Fisher information in terms of the initial
preparation, the reservoir temperature and the interaction time. Upon
investigating the Fisher information of feasible measurements we arrive at a
remarkable simple result: homodyne detection of canonical variables allows one
to achieve the ultimate quantum limit to precision under suitable, mild,
conditions. Finally, upon exploiting a perturbative approach, we find the
invariant sweet spots of the (tunable) characteristic frequency of the probe,
able to drive the probe towards the optimal working regime.
