In this talk, I will give an overview of two recent developments on sensing with nonlinear optomechanical systems and detecting entanglement in Hydrogenic systems. Optomechanical systems operating in the quantum regime one of the most promising emerging quantum sensing technologies. We are interested in computing the fundamental sensitivity that such a system can achieve, especially for challenging tasks such as gravity sensing. It is generally easier to detect a time-dependent signal experimentally, but it is significantly more difficult to model. In the first part of the talk, I will show how we solve the dynamics of a time-dependent optomechanical system and compute the quantum Fisher information for time-varying displacements of the mechanical element. I will also discuss some settings where these results apply. In the second part of my talk, I will discuss a more esoteric question, namely the detection of entanglement for a Hydrogenic system. While the Hydrogenic solutions to the Schrödinger equation have been known for almost a century, the question of whether the two subsystems are entanglement is not often addressed. I will show how entanglement can be computed for free and localised Hydrogenic systems.