Preparation of four-mode cluster states with distant atomic ensembles

We propose a scheme to generate four-mode cluster states with four atomic ensembles trapped in single-mode cavities connected by short fibers. With the aid of the cavity dissipation, we find that a four-mode cluster state can be unconditionally created by the simultaneous driving of the ensembles with laser pulses of suitably chosen Rabi frequencies and phases. The scheme could be easily extended to the case of $N$-mode cluster states.