Trapped atoms and ions, which are among the best-controlled quantum systems, find widespread applications in quantum science. For molecules, a similar degree of control is currently lacking owing to their complex energy-level structure. Quantum-logic protocols in which atomic ions serve as probes for molecular ions are a promising route for achieving this level of control, especially for homonuclear species that decouple from blackbody radiation. Here, a quantum-nondemolition protocol on single trapped [Formula: see text] molecules is demonstrated. The spin-rovibronic state of the molecule is detected with >99% fidelity, and a spectroscopic transition is measured without destroying the quantum state. This method lays the foundations for new approaches to molecular spectroscopy, state-to-state chemistry, and the implementation of molecular qubits.
This dissertation describes experiments performed in the MOTion trap, a hybrid atom--ion trap comprised of a magneto-optical trap (MOT) and a linear Paul trap with an integrated time-of-flight mass spectrometer (ToF-MS).We aim to sympathetically cool the vibrational and rotational degrees of freedom of a BaCl
+ molecular ion by overlapping a cold (
sim4~mK) Ca MOT.\nThis sympathetic cooling can prepare a molecular ion in its rovibrational quantum ground state, providing a method for state preparation for applications in molecular ion quantum logic.\nWhile collisions between the molecular ion and the cold atoms can cool the internal degrees of freedom, chemical reactions are also possible for electronically excited atoms.\nAs such, we study these excited-state atom--ion reactions at low temperature
<1~K and develop tools to control these collisions.\nWe make the first experimental observation of reaction blockading, an effect that suppresses excited-state reactions at low temperature, and develop a method to reverse this suppression if desired with the addition of a catalyst laser, providing a means to optically control such reactions.Beyond its intended purpose of studying and achieving sympathetic cooling, we find that this apparatus is a powerful tool for chemical studies.By imaging the ions, we can observe reactions occurring one atom at a time.\nWe can precisely tune the collision energy or temperature down to the mK regime to as high as $\\sim$10~K using techniques described in this dissertation, allowing controlled interrogation of these low-temperature reactions.\nFurther, using lasers to address electronic transitions, we can precisely control the quantum state of the reactants, and we can measure the reaction products with the ToF-MS.\nA testament to the value of this apparatus is the study of BaOCa$^+$ described in this dissertation.\nUsing the tools of the MOTion trap, we were able to identify the production of BaOCa
+, the first observation of a mixed hypermetallic oxide, and determine the path of reaction as BaOCH
3+ + Ca(
4s4p~
3PJ)
rightarrow BaOCa
+ + CH
3.In addition to developing control of these atom--ion interactions towards sympathetic cooling of molecular ions, we also work towards further developing new quantum logic schemes for molecular ions, such as dipolar quantum logic, dipole--phonon quantum logic (DPQL), and electric-field gradient gates (EGGs).Specifically, we propose a class of molecules particularly well-suited for DPQL, the alkaline earth monoxide cations, that can also be added to existing atomic ion experiments with minimal additional experimental complexity.\nWe further develop the techniques for DPQL and propose methods for single- and two-qubit gates, outlining a path towards universal quantum computation with molecular ions.
We present theoretical and experimental progress towards a new approach for the precision spectroscopy, coherent manipulation and state-to-state chemistry of single isolated molecular ions in the gas phase. Our method uses a molecular beam for creating packets of rotationally cold neutrals from which a single molecule is state-selectively ionized and trapped inside a radiofrequency ion trap. In addition to the molecular ion, a single co-trapped atomic ion is used to cool the molecular external degrees of freedom to the ground state of the trap and to detect the molecular state using state-selective coherent motional excitation from a modulated optical-dipole force acting on the molecule. We present a detailed discussion and theoretical characterization of the present approach. We simulate the molecular signal experimentally using a single atomic ion, indicating that different rovibronic molecular states can be resolved and individually detected with our method. The present approach for the coherent control and non-destructive detection of the quantum state of a single molecular ion opens up new perspectives for precision spectroscopies relevant for, e.g., tests of fundamental physical theories and the development of new types of clocks based on molecular vibrational transitions. It will also enable the observation and control of chemical reactions of single particles on the quantum level. While focusing on N2+ as a prototypical example in the present work, our method is applicable to a wide range of diatomic and polyatomic molecules.