  Mesoscopic quantum phase coherence is important because it improves the
prospects for handling quantum degrees of freedom in technology. Here we show
that the development of such coherence can be monitored using magnetic neutron
scattering from a one-dimensional spin chain Y2BaNiO5, a quantum spin fluid
where no classical, static magnetic order is present. In the cleanest samples,
the quantum coherence length is 20 nm, almost an order of magnitude larger than
the classical antiferromagnetic correlation length of 3 nm. We also demonstrate
that the coherence length can be modified by static and thermally activated
defects in a quantitatively predictable manner.
