All-optical control of pendular qubit states with nonresonant two-color laser pulses.

Practical methodologies for quantum qubit controls are established by two prerequisites, i.e., preparation of a well-defined initial quantum state and coherent control of that quantum state. Here we propose a new type of quantum control method, realized by irradiating nonresonant nanosecond two-color ($\omega$ and 2$\omega$) laser pulses to molecules in the pendular (field-dressed) ground state. The two-color field nonadiabatically splits the initial pendular ground state $\vert\tilde{0},\tilde{0}\rangle$ to a superposition state of $\vert\tilde{0},\tilde{0}\rangle$ and $\vert\tilde{1},\tilde{0}\rangle$, whose relative probability amplitudes can be controlled by the peak intensity of one wavelength component ($\omega$) while the peak intensity of the other component (2$\omega$) is fixed. The splitting of the quantum paths is evidenced by observing degrees of orientation of ground-state-selected OCS molecules by the velocity map imaging technique. This quantum control method is highly advantageous in that any type of polar molecules can be controlled regardless of the molecular parameters, such as rotational energy, permanent dipole moment, polarizability, hyperpolarizability, and hyperfine energy structures.