  By a small-size complex network of coupled chaotic Hindmarsh-Rose circuits,
we study experimentally the stability of network synchronization to the removal
of shortcut links. It is shown that the removal of a single shortcut link may
destroy either completely or partially the network synchronization.
Interestingly, when the network is partially desynchronized, it is found that
the oscillators can be organized into different groups, with oscillators within
each group being highly synchronized but are not for oscillators from different
groups, showing the intriguing phenomenon of cluster synchronization. The
experimental results are analyzed by the method of eigenvalue analysis, which
implies that the formation of cluster synchronization is crucially dependent on
the network symmetries. Our study demonstrates the observability of cluster
synchronization in realistic systems, and indicates the feasibility of
controlling network synchronization by adjusting network topology.
