Ultimate quantum bounds on mass measurements with a nano-mechanical oscillator

Nano-mechanical resonators have a large potential as sensors of very small adsorbed masses, down to the atomic level and beyond. Here I establish the fundamental lower bound on the mass that can be measured with a nano-mechanical oscillator in a given quantum state based on the quantum Cram\'er--Rao bound, limited only by the laws of quantum mechanics, and identify the quantum states which will allow the largest sensitivity for a given maximum energy.