  The hallmark of quantum physics is Planck's constant $h$, whose finite value
entails the quantization that gave the theory its name. The finite value of $h$
gives rise to inevitable zero-point fluctuations even at vanishing temperature.
The zero-point fluctuation of mechanical motion becomes smaller with growing
mass of an object, making it challenging to observe at macroscopic scales.
Here, we transition a dielectric particle with a diameter of 136 nm from the
classical realm to the regime where its zero-point motion emerges as a sizeable
contribution to its energy. To this end, we optically trap the particle at
ambient temperature in ultrahigh vacuum and apply active feedback cooling to
its center-of-mass motion. We measure an asymmetry between the Stokes and
anti-Stokes sidebands of photons scattered by the levitated particle, which is
a signature of the particle's quantum ground state of motion.
