  For most kinds of already known Bose-Einstein condensates experimental
evidence for the existence of a macroscopic coherent quantum state has been
provided, e.g., from the observation of interference phenomena, the formation
vortices, the detection of Josephson effects, or even by the manifestation of
superfluid behaviour. However, none of these hallmarks for a true, macroscopic
phase-coherent state has ever been reported for any insulating spin system in a
solid in which magnetic bosonic quasiparticles are thought to condense close to
a quantum critical point. We describe an attempt to detect superfluid behaviour
in TlCuCl3, and discuss in more detail a proposal for an experiment in which
the a.c. Josephson effect can be probed by using a device composed of two
weakly coupled magnetic insulators with different critical fields. We conclude
that the detection of the a.c. Josephson effect is feasible for a proper choice
of compounds with realistic material parameters.
