Quantum Logic Control of a Single Molecular Ion

An amazing level of quantum control is routinely reached in modern experiments with atoms, but similar control over molecules has been an elusive goal. A method based on quantum logic spectroscopy [1] can address this problem for a wide class of molecular ions [2,3]. We have now realized the basic elements of these proposals.\\ \\ In our demonstration, we trap a calcium ion together with a calcium hydride ion (CaH$^+$) that is a convenient stand-in for more general molecular ions. We laser-cool the two-ion crystal to its motional ground state and then drive Raman-transitions in the molecular ion, where a transition in the molecule also deposits a single quantum of excitation in the motion of the ion pair (motional``sideband''). We can efficiently detect this single quantum of excitation with the calcium ion, which projects the molecule into the final state of the sideband transition, a known, pure quantum state.\\ \\ The molecule can be coherently manipulated after the projection, and its resulting state read out by another quantum logic state detection [4,5] or alternatively, an entangled state between the logic ion and the molecule can be created [6]. All transitions we address in the molecule are either driven by a single, far off-resonant continuous-wave laser or by a far-off-resonant frequency comb. This makes our approach suitable for quantum control and precision measurement of a large class of molecular ions.\\ \\ {[1]} P.O. Schmidt,{\it et al.} Science {\bf 309}, 749 (2005).\\ {[2]} S. Ding, S. and D. N. Matsukevich, New J. Phys. {\bf 14}, 023028 (2012).\\ {[3]} D. Leibfried, New J. Phys. {\bf 14}, 023029 (2012) .\\ {[4]} C.-W. Chou, {\it et al.}, Nature {\bf 545}, 203 (2017).\\ {[5]} C.W. Chou {\it et al.}, arXiv1911.12808 (2019).\\ {[6]} Y. Lin {\it et al.}, arXiv:1912.05866 (2019).