Optically levitated nanosphere shows definitive signature of its quantum ground state of motion.

Picture a marble rolling around inside a bowl. The motion of the marble represents its center-of-mass temperature, a quantity distinct from the object’s physical temperature. Now replace the marble with a levitated nanosphere and the bowl with an optical trap, and you have the experiment used by Felix Tebbenjohanns and colleagues at the Swiss Federal Institute of Technology (ETH), Zurich, to reduce a levitated nanoparticle’s center-of-mass temperature to close to its quantum ground state. The experimental signature showing that the nanosphere had entered the quantum regime had, until now, been seen only in mechanically clamped systems coupled to optical cavities.