  It is possible to condense a macroscopic number of bosons into a single mode.
Adding interactions the question arises whether the condensate is stable. For
repulsive interaction the answer is positive with regard to the ground-state,
but what about a condensation in an excited mode? We discuss some results that
have been obtained for a 2-mode bosonic Josephson junction, and for a 3-mode
minimal-model of a superfluid circuit. Additionally we mention the possibility
to stabilize an unstable condensate by introducing periodic or noisy driving
into the system: this is due to the Kapitza and the Zeno effects.
