  A scheme is presented for realizing a quantum phase gate with three-level
atoms, solid-state qubits--often called artificial atoms, or ions that share a
quantum data bus such as a single mode field in cavity QED system or a
collective vibrational state of trapped ions. In this scheme, the conditional
phase shift is tunable and controllable via the total effective interaction
time. Furthermore, we show that the method can be used for effective
preparation of graph-state entanglement, which are important resources for
quantum computation, quantum error correction, studies of multiparticle
entanglement, fundamental tests of non-locality and decoherence.
