Strongly pumped SU(1,1) interferometer: loss-tolerant sensing below the shot-noise limit
A nonlinear SU(1,1) interferometer is a sequence of two coherently pumped high-gain parametric amplifiers, realized through parametric down-conversion or four-wave mixing. The radiation emitted by the first amplifier can be amplified or deamplified in the second one, depending on the phase shifts acquired on the way. This makes the interferometer extremely sensitive to phase shifts, its sensitivity reaching the Heisenberg limit in the lossless case.
Losses certainly reduce the phase sensitivity; however, detection loss can be overcome by making the interferometer gain-unbalanced. As we have shown theoretically  and experimentally , phase sensitivity below the shot-noise level can be achieved even with very inefficient detection provided that the second amplifier is pumped sufficiently strong.
In my talk I will consider different constructions of an SU(1,1) interferometer based on high-gain parametric down-conversion. In particular, by imaging one parametric amplifier on the other one, the interferometer is made not sensitive to the radial mode content. Such a construction can be applied to sensing of orbital angular momentum perturbations.
 M. Manceau et al., New Journal of Physics 19, 013014 (2017).
 M. Manceau et al., Phys. Rev. Lett. 119, 223604 (2017).