  Optomechanical systems provide a means for studying and controlling quantum
effects in the motion of macroscopic objects. To date, quantum optomechanical
effects have been studied in objects made from solids and gases. Here we
describe measurements of quantum behavior in the vibrations of a liquid body.
Specifically, we monitor the fluctuations of an individual acoustic standing
wave in superfluid liquid helium, and find that it displays the characteristic
signatures of zero-point motion and measurement back-action. This opens the
possibility of exploiting the properties of liquids in general (and superfluid
helium in particular) to access qualitatively new regimes of quantum
optomechanics.
