Quantum limited measurement of space-time curvature with scaling beyond the conventional Heisenberg limit

We study the problem of estimating the phase shift due to the general relativistic time dilation in the interference of photons using a non-linear Mach-Zender interferometer setup. By introducing two non-linear Kerr materials, one in the bottom and one in the top arm, we can measure the non-linear phase $\phi_{NL}$ produced by the space-time curvature and achieve a scaling of the standard deviation with photon number ($N$) of $1/N^{\beta}$ where $\beta > 1$, which exceeds the conventional Heisenberg limit of a linear interferometer ($1/N$). The non-linear phase shift is an effect that is amplified by the intensity of the probe field. In a regime of high number of photons, this effect can dominate over the linear phase shift.