  Some strategies for reducing energy consumption in information processing
devices involve the use of spin rather than charge to carry information. This
idea is especially attractive when the spin current is a collective one carried
by the condensate of a magnetically ordered state, rather than a quasiparticle
current carried by electrons or magnons. In this Chapter we explain how
easy-plane magnets can be viewed as Bose-Einstein condensates of magnons,
defined in terms of quanta of the spin-component perpendicular to the easy
plane, and how they can carry dissipationless spin-currents that induce
non-local interactions between electrically isolated conducting channels. We
comment specifically on important differences between superconductivity in
normal/superconducting/normal circuits and spin-superfluidity in
normal/magnetic/normal circuits.
