  We propose and analyse a method that allows for the production of squeezed
states of the atomic center-of-mass motion that can be injected into an atom
interferometer. Our scheme employs dispersive probing in a ring resonator on a
narrow transition of strontium atoms in order to provide a collective
measurement of the relative population of two momentum states. We show that
this method is applicable to a Bragg diffraction-based atom interferometer with
large diffraction orders. The applicability of this technique can be extended
also to small diffraction orders and large atom numbers by inducing atomic
transparency at the frequency of the probe field, reaching an interferometer
phase resolution scaling $\Delta\phi\sim N^{-3/4}$, where $N$ is the atom
number. We show that for realistic parameters it is possible to obtain a 20 dB
gain in interferometer phase estimation compared to the Standard Quantum Limit.
