  The quantum theory of fields is largely based on studying perturbations
around non-interacting, or free, field theories, which correspond to a
collection of quantum-mechanical harmonic oscillators. The quantum theory of an
ordinary fluid is `freer', in the sense that the non-interacting theory also
contains an infinite collection of quantum-mechanical free particles,
corresponding to vortex modes. By computing a variety of correlation functions
at tree- and loop-level, we give evidence that a quantum perfect fluid can be
consistently formulated as a low-energy, effective field theory. We speculate
that the quantum behaviour is radically different to both classical fluids and
quantum fields, with interesting physical consequences for fluids in the low
temperature regime.
