Some properties of the materials involved in optical technologies give rise to nonlinear effects, such as a phase shift that depends quadratically on the intensity of the field. It is important then to be able to characterize nonlinear phase shifts, and even better when we do so using simple tools such as Gaussian optics.

In this work, Jiandong Zhang and coauthors have shown how a simple two-mode squeezer with a coherent state input creates a probe state for a nonlinear phase shift, which they then interfere at a beamsplitter and measure with a homodyne detector, creating what they call a hybrid nonlinear-linear interferometer. This clever device can attain the Cramer-Rao bound for certain parameter ranges, meaning that the estimation of the nonlinear phase shift has the lowest variance that can be attained with a given number of measurements.

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