Conclusive precision bounds for SU(1,1) interferometers

In this paper, we revisit the quantum Fisher information (QFI) calculation in SU(1,1) interferometer considering different phase configurations. When one of the input modes is a vacuum state, we show by using phase averaging, different phase configurations give the same QFI. In addition, by casting the phase estimation as a two-parameter estimation problem, we show that the calculation of the quantum Fisher information matrix (QFIM) is necessary in general. Particularly, within this setup, the phase-averaging method is equivalent to a two-parameter estimation problem. We also calculate the phase sensitivity for different input states using QFIM approach.