Quantum Colloquium | Measuring high-amplitude quantum motion in a nanogram object

I will describe measurements of individual phonons in a 1 ng body of superfluid helium. When this body is in equilibrium, its phonon correlations are consistent (up to 4th order) with a thermal state of mean occupancy ~ 1. This purity is preserved even when the mode is driven to a coherent state with an amplitude corresponding to ~100,000 phonons. I will describe how these results can be used to constrain nonlinear extensions of quantum mechanics, and to distribute entanglement over kilometer-scale optical fiber networks.