Ultracold atom-ion systems have been a topic of interest for atomic physicists studying chemical reactions and since recently, the cold ion community (ion trap quantum computation and simulation). They have been looking at the possibility of using an ultracold atom gas to sympathetically cool ions since intrinsic motional modulation i.e micromotion is an inherent cause of decoherence in coherent applications of cold ions. Interest is also piqued by the possibility of using this hybrid system for studying impurity physics and to better understand ion-neutral reactions aimed at creation of molecular ions. In this thesis, we aim to study the effect of ion micromotion in atom-ion collision. As a prelude, we treat the 1D collision of a particle in a harmonic trap (ion) and a free particle (atom) using different numerical schemes. This system is of interest in its own right due to the mixed 0D-1D dimensionality. Atom-ion potential is simplified to a zero range potential all through out the work. Next we deal with a similar problem but with the trapped particle in a time dependent harmonic trap identical to an ion Paul trap. Finally we extend the study of micromotion to a model system in 3D with an ion in a 3D spherical Paul trap and a heavy atom at the trap centre. We discuss the effect micromotion has on potential applications of such a system, like a quantum phase gate.
Ultracold atom-ion systems have been a topic of interest for atomic physicists studying chemical reactions and since recently, the cold ion community (ion trap quantum computation and simulation). They have been looking at the possibility of using an ultracold atom gas to sympathetically cool ions since intrinsic motional modulation i.e micromotion is an inherent cause of decoherence in coherent applications of cold ions. Interest is also piqued by the possibility of using this hybrid system for studying impurity physics and to better understand ion-neutral reactions aimed at creation of molecular ions. In this thesis, we aim to study the effect of ion micromotion in atom-ion collision. As a prelude, we treat the 1D collision of a particle in a harmonic trap (ion) and a free particle (atom) using different numerical schemes. This system is of interest in its own right due to the mixed 0D-1D dimensionality. Atom-ion potential is simplified to a zero range potential all through out the work. Next we deal with a similar problem but with the trapped particle in a time dependent harmonic trap identical to an ion Paul trap. Finally we extend the study of micromotion to a model system in 3D with an ion in a 3D spherical Paul trap and a heavy atom at the trap centre. We discuss the effect micromotion has on potential applications of such a system, like a quantum phase gate.
Over the recent years a new research field emerged that combines technologies for cooling and trapping of neutral atoms and charged ions in one experimental apparatus. These atom-ion hybrid traps enable the investigation of atom-ion interactions at unprecedentedly low collision energies targeting the ultracold s-wave regime. In this thesis I report on experiments performed in an hybrid atom-ion apparatus combining single Ba^+ or Rb^+ ions with ultracold neutral Rb atoms. The main focus of this work is studying the emerging three-body reaction dynamics in a cold charged gas. First, I will present measurements investigating cold reactions in the hetero-nuclear Ba^+ - Rb system. So far, most theoretical and experimental studies have been dealing with reactions between two particles. However, we find that three-body reactions already dominate over two-body ones at atomic densities down to 10^{−11} cm^{−3} once collision energies are in the low mK regime. We then investigate the energy scaling of inelastic ternary collisions between an ion and two atoms. A power-law scaling of the three-body rate coefficient is predicted by theory. We find agreement with the experiment if we include the ion’s kinetic energy distribution that we access numerically. The next study elaborates on the very first observation of ternary reactions in a hybrid cold atom-ion setup. For this we immerse a single Rb^+ ion into ultracold Rb atoms with sufficiently reduced excess micromotion. Occurring reactions are signaled by the creation of highly energetic Rb^+ ions. I will also showcase novel techniques to investigate the atom and the ion system by using the respective other one which were developed by us over the course of the last years. In order to reduce the total ion kinetic energy in a Paul trap one has to compensate the ion’s excess micromotion. This is routinely being done by analyzing the ion’s fluorescence. By observing collision energy dependent elastic atom-ion collisions in an ultracold atom cloud we directly infer the ion’s micromotion energy. This way we can perform atom-ion experiments with optically dark Rb^+ ions and good excess micromotion. Without the need of visible laser light for micromotion compensation we are able to investigate the evolution of quasi-static stray electric fields in a Paul trap over a period of several months. The last study I will present is targeted at neutral Rb only. There, we spectroscopically access the molecular state distribution of Rb_2 molecules formed in neutral three-body recombination by state-selective ionization of these molecules. Trapping and subsequent detection of these ions is being done with our Paul trap, which has the role of a very sensitive single ion detector in these measurements.
Abstract Reaching ultracold temperatures within hybrid atom–ion systems is a major limiting factor for control and exploration of the atom–ion interaction in the quantum regime. In this work, we present results on numerical simulations of trapped ion buffer gas cooling using an ultracold atomic gas in a large number of experimentally realistic scenarios. We explore the suppression of micromotion-induced heating effects through optimization of trap parameters for various radio-frequency (rf) traps and rf driving schemes including linear and octupole traps, digital Paul traps, rotating traps and hybrid optical/rf traps. We find that very similar ion energies can be reached in all of them even when considering experimental imperfections that cause so-called excess micromotion. Moreover we look into a quantum description of the system and show that quantum mechanics cannot save the ion from micromotion-induced heating in an atom–ion collision. The results suggest that buffer gas cooling can be used to reach close to the ion’s groundstate of motion and is even competitive when compared to some sub-Doppler cooling techniques such as Sisyphus cooling. Thus, buffer gas cooling is a viable alternative for ions that are not amenable to laser cooling, a result that may be of interest for studies into cold controlled quantum chemistry and charged impurity physics.
Hybrid quantum systems represent one of the most promising routes in the progress of experimental quantum physics and in the development of quantum technologies. In a hybrid quantum system two (or more) different quantum systems interact in the same experimental setup. Therefore, these composite systems benefit from both the properties of each single system and from the presence of an interaction term, leading to the emergence of new variables that can be experimentally manipulated. \nA promising hybrid quantum system is the one realized by the com- bination of an ultracold atomic gas and trapped ions. Ultracold atoms and trapped ions are two of the most studied physical systems for the implementation of several quantum technologies, like e.g. quantum simulation, quantum computa- tion, and quantum metrology. When trapped together, atoms and ions interact via an interaction potential that scales asymptotically with R^(−4), where R is the inter- particle distance, due to the electrostatic (attractive) force between the ion’s electric monopole and the atom’s induced dipole. Interestingly, this potential has a typical range on the order of hundreds of nm, i.e. approx. two orders of magnitude longer than the range of atom-atom interactions. Several studies have proposed to use this interaction to realize new quantum simulations, study few-body physics, and control atom-ion chemical reactions. \nElastic collisions between ions and atoms can be exploited to sympathetically cool the ions and try to reach the so-far elusive s-wave scattering regime, in which atom- ion collisions can lead to a quantum coherent evolution of the composite system. However, the ultracold atom-ion mixtures realized so far were not brought to the s-wave scattering regime because of the so-called “micromotion”, a driven motion affecting the dynamics of the ions trapped in Paul traps. Atom-ion collisions in the presence of micromotion cause a coupling of energy from the oscillating field of the Paul trap to the colliding particles, which can be heated up in the collision. In order to realize an atom-ion experiment in which the system could reach the s-wave scattering regime, the choice of the atomic species and the ion trapping strategy are crucial. \nWe decided to build a new experimental apparatus for the realization of an ultracold atom-ion quantum hybrid system made of a quantum gas of fermionic Lithium and trapped Barium ions. The choice for the elements ensures that atoms and ions in their electronic ground state will not undergo charge- exchange collisions, i.e. inelastic processes for which an electron is “exchanged” be- tween the two colliding particles. Additionally, the large mass ratio ensures an efficient cooling of the ion in the ultracold gas. \nFor what regards the ion trapping strategy, in order to remove the limitations set by micromotion, we conceived a new trap. This is formed by the superposition of an electric quadrupole static potential and an optical lattice along the untrapping direction of the electric quadrupole. The ions are moved into this electro-optical trap (EOT) from a standard Paul trap, in which the ions are first trapped after their production through photoionization. \nIn this thesis, I will describe how this new experimental apparatus for the real- ization of an ultracold atom-ion quantum hybrid system was conceived, designed and assembled. I will first describe the motivations for investigating atom-ion interactions in the ultracold regime. Then, I will describe the experimental techniques to trap and cool Barium ions and Lithium atoms, and how we plan to make them interact. The largest part of the thesis will be dedicated to the description of the parts of the experimental setup that I designed and realized, like the Lithium optical setup, the Barium imaging system and the electrical setup of the ion trap, including a compact RF drive based on interdependent resonant circuits that I developed for operating the Paul trap. The last chapter of the thesis is dedicated to this innovative drive.
Ultracold cold molecular ions promise new directions in various studies of fundamental physics, such as precision measurements, ultracold chemistry and quantum information sciences. All these exciting applications require the molecular ion to be prepared at ground state of motional and internal degrees of freedom. It has been proposed that this stringent goal could be achieved through sympa-thetic cooling via collisions with laser-cooled neutral atoms. Three fundamentalissues of this method are addressed in this thesis.First, an analytical model is established to accurately describe collision-induced heating of a single ion in contact with cold neutral atoms. This model reveals that micromotion interruption is the cause of heating, and gives results about steady-state temperature and sympathetic cooling rate verified by Monte-Carlo simulations. It also provides insight into the power-law tails observed in theenergy distribution of the trapped ion.Next, we consider the case of multiple ions, whose inter-particle Coulomb repulsion causes ions in the Coulomb crystal state to spontaneously melt into a gas phase ion cloud, due to the same micromotion interruption mechanism. The analysis of this problem with a plasma model leads to the experimental determination of a quantity central to plasma physics, Coulomb Logarithm, in anion trap.Finally, we demonstrate a molecular ion spectroscopy technique through the example of trap-depletion photodissociation of BaCl + . Although not sensitive to rotational structure, this method already reveals much about the fundamental quantum physics in the photodissociation process. The measured cross-section results paves the road toward state-selective spectroscopy currently going on in our lab.
The atom-ion hybrid system provides us with new platform to study ultracold collisions and chemical reactions, which play important role in quantum simulations, quantum information processing utilizing atom-ion interactions. To investigate chemical reactions in an ultracold regime, we start with neutral atoms trapped in an optical dipole trap and ions in a linear Paul trap, and combine them together to construct a hybrid system. Here we introduce our recent work on ultracold elastic/inelastic collisions at a single atom level with a state-by-state manner in our atom-ion hybrid system. From the quantitative discussion on the collision cross sections, the microscopic collisional mechanisms have been clarified.
The atom-ion hybrid system provides a new platform to study ultracold collisions and chemical reactions. To investigate chemical reactions in ultracold regimes, we combine neutral atoms in an optical trap and ions in a Paul trap to construct an atom-ion hybrid system. Here we introduce our recent work on ultracold elastic/inelastic collisions at a single atom level with a state-by-state manner. From quantitative discussions on the collision’s cross-sections, we clarified the microscopic collisional mechanisms.