Preparation and coherent manipulation of pure quantum states of a single molecular ion
Chin-Wen Chou, Christoph Kurz, David B. Hume, Philipp N. Plessow, David R. Leibrandt, Dietrich G. Leibfried
Laser cooling and trapping of atoms and atomic ions has led to breakthroughs in understanding of exotic phases of matter [1-3], development of exquisite sensors  and state-of-the-art atomic clocks . For molecules, with their more complicated internal structure, this level of measurement precision and quantum-state control has been reached only in a very limited number of settings [6,7] and never for individual molecular ions. Here we experimentally demonstrate a general method for pure quantum state preparation and coherent manipulation applicable to a broad range of molecular ions. We trap a single molecular ion in a linear Paul trap together with a single atomic ion. Using two laser beams far off-resonance, we drive Raman transitions that simultaneously change the state of the molecular ion and excite the shared motion of the two ions in the trap. The motional excitation can then be detected on the atomic ion thereby projecting the molecule with high probability into the final state of the Raman transition . Similar pulses can be used for pumping the molecule into fewer initial states. In this way, we solve both the problems of quantum state preparation and detection in an isolated molecular system, enabling the observation of coherent phenomena including Rabi flopping and Ramsey fringes in a single molecule. These are critical steps toward recent proposals for precision tests of fundamental physics [9-11] and quantum information processing [12,13] with trapped molecules.