Cooperatively-enhanced precision of hybrid light-matter sensors.

We consider a hybrid system of matter and light as a sensing device and quantify the role of cooperative effects. The latter generically enhance the precision with which modifications of the effective light-matter coupling constant can be measured. In particular, considering a fundamental model of $N$ qubits coupled to a single electromagnetic mode, we show that the ultimate bound for the precision shows double-Heisenberg scaling: $\Delta\theta\propto1/(Nn)$, with $N$ and $n$ being the number of qubits and photons, respectively. Moreover, even using classical states and measuring only one subsystem, a Heisenberg-times-shot-noise scaling, i.e. $1/(N\sqrt{n})$ or $1/(n\sqrt{N})$, is reached. As an application, we show that a Bose-Einstein condensate trapped in a double-well potential within an optical cavity can detect the gravitational acceleration $g$ with the relative precision of $\Delta g/g\simeq10^{-9}\text{Hz}^{-1/2}$. The analytical approach presented in this study takes into account the leakage of photons through the cavity mirrors, and allows to determine the sensitivity when $g$ is inferred via measurements on atoms or photons.