  Continuous-variable Gaussian cluster states are a potential resource for
universal quantum computation. They can be efficiently and unconditionally
built from sources of squeezed light using beam splitters. Here we report on
the generation of three different kinds of continuous-variable four-mode
cluster states. In our realization, the resulting cluster-type correlations are
such that no corrections other than simple phase-space displacements would be
needed when quantum information propagates through these states. At the same
time, the inevitable imperfections from the finitely squeezed resource states
and from additional thermal noise are minimized, as no antisqueezing components
are left in the cluster states.
