  We propose a quantum computer architecture which is robust against
decoherence and scalable. As a qubit, we adopt rotational states of a nonpolar
ionic molecule trapped in an ion-trap. It is revealed that the rotational-state
qubits are much more immune to decoherence than the conventional
electronic-state qubits of atomic ions. A complete method set for state
preparation, single-qubit gate, controlled-NOT gate, and qubit-readout suitable
for the rotational-state qubits is provided. Since the ionic molecules can be
transported in an array of ion traps, the rotational-state qubits are expected
to be a promising candidate to build a large-scale quantum computer.
