  Cavity-based large-scale quantum information processing (QIP) needs a large
number of qubits and placing all of them in a single cavity quickly runs into
many fundamental and practical problems such as the increase of cavity decay
rate and decrease of qubit-cavity coupling strength. Therefore, future QIP most
likely will require quantum networks consisting of a large number of cavities,
each hosting and coupled to multiple qubits. In this work, we propose a way to
prepare a $W$-class entangled state of spatially-separated multiple qubits in
different cavities, which are connected to a coupler qubit. Because no cavity
photon is excited, decoherence caused by the cavity decay is greatly suppressed
during the entanglement preparation. This proposal needs only one coupler qubit
and one operational step, and does not require using a classical pulse, so that
the engineering complexity is much reduced and the operation is greatly
simplified. As an example of the experimental implementation, we further give a
numerical analysis, which shows that high-fidelity generation of the $W$ state
using three superconducting phase qubits each embedded in a one-dimensional
transmission line resonator is feasible within the present circuit QED
technique. The proposal is quite general and can be applied to accomplish the
same task with other types of qubits such as superconducting flux qubits,
charge qubits, quantum dots, nitrogen-vacancy centers and atoms.
