Quantum computation with rotational states of nonpolar ionic molecules

We propose a quantum computer architecture that 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 that includes state preparation, a single-qubit gate, a 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.

Figure 1

Energy-level diagram of qubit states. A pair of linearly polarized laser beams with frequencies ${\omega }_1$ and ${\omega }_2$ induces a resonant two-photon Raman transition between the qubit levels for a single-qubit gate. The state $|\text{aux}\rangle$ and two circularly polarized lasers with frequencies ${\omega }_1^{\text{aux}}$ and ${\omega }_2^{\text{aux}}$ are used when the Cirac-Zoller controlled-not gate is adopted.

Figure 2

Qubit readout scheme using the state-transfer technique. Rotational-state qubits of molecular ions are read out by instead measuring electronic-state qubits of cotrapped atomic ions with fluorescence detection. Energy levels with dotted lines indicate the motional excited state $|n=1\rangle$ with corresponding internal states.