Squeezed vacuum as a universal quantum probe
  We address local quantum estimation of bilinear Hamiltonians probed by Gaussian states. We evaluate the relevant quantum Fisher information (QFI) and derive the ultimate bound on precision. Upon maximizing the QFI we found that single- and two-mode squeezed vacuum represent an optimal and universal class of probe states, achieving the so-called Heisenberg limit to precision in terms of the overall energy of the probe. We explicitly obtain the optimal observable based on the symmetric logarithmic derivative and also found that homodyne detection assisted by Bayesian analysis may achieve estimation of squeezing with near-optimal sensitivity in any working regime. Besides, by comparison of our results with those coming from global optimization of the measurement we found that Gaussian states are effective resources, which allow to achieve the ultimate bound on precision imposed by quantum mechanics using measurement schemes feasible with current technology. 
Roberto Gaiba, Matteo G A Paris