  The interferometric power of a bipartite quantum state quantifies the
precision, measured by quantum Fisher information, that such a state enables
for the estimation of a parameter embedded in a unitary dynamics applied to one
subsystem only, in the worst-case scenario where a full knowledge of the
generator of the dynamics is not available a priori. For finite-dimensional
systems, this quantity was proven to be a faithful measure of quantum
correlations beyond entanglement. Here we extend the notion of interferometric
power to the technologically relevant setting of optical interferometry with
continuous-variable probes. By restricting to Gaussian local dynamics, we
obtain a closed formula for the interferometric power of all two-mode Gaussian
states. We identify separable and entangled Gaussian states which maximize the
interferometric power at fixed mean photon number of the probes, and discuss
the associated metrological scaling. At fixed entanglement of the probes,
highly thermalized states can guarantee considerably larger precision than pure
two-mode squeezed states.
