Speaker: Jack Harris, Yale University
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 having 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 a certain class of nonlinear extensions of quantum mechanics. We find that our data places constraints at roughly the same level as experiments at the LHC. I will also describe potential applications of these device, including for the distribution of entanglement over kilometer-scale optical fiber networks.