When properly engineered, simple quantum systems such as harmonic oscillators or spins can be excellent detectors of feeble forces and fields. Following a general introduction, I will focus on using optomechanical systems as sensors of weak acceleration and strain fields. Dark matter particles coupling to standard model fields and particles would produce a coherent strain or acceleration signal in an elastic solid. We discuss the feasibility of searching for dark matter consisting of ultralight scalar or vector fields in the 10^-12 eV--10^-6 eV/c^2 mass range using various optomechanical systems. I will also show that understanding and addressing issues around quantum control are essential steps towards building quantum noise limited detectors.