Svetlana Kotochigova

We propose to combine neutral atom and trapped ion qubits in one scalable modular architecture that uses shuttling of individual neutral atoms in optical tweezers to realize atomic interconnects between trapped ion quantum registers. These interconnects are deterministic, and thus may be performed on-demand. The proposed protocol is as follows: a tweezer-trapped neutral atom qubit is brought close to a trapped ion in an ion chain serving as a module of a larger quantum computer, and an entangling gate is performed between the two qubits. Then the neutral atom is quickly moved to another, nearby trapped ion chain in the same modular ion trap and entangled with an ion in that chain, thus entangling the two separate ion chains. The optical dipole potential of the tweezer beam for the neutral atom does not measurably affect the trapped ions, while the RF ion trap does not affect the neutral atom. With realistic tweezer trap parameters, the neutral atom can be moved over millimeter scale distance in a few tens of microseconds, thus enabling a remote entanglement generation rate of over 10^3/s even with very modest assumptions for the atom-ion quantum gate speed, and possibly up to 10^4/s, which is two orders of magnitude higher than the current state-of-the-art with photonic interconnects.

We present experimental evidence of charge exchange between laser-cooled potassium $^{39}$K atoms and calcium $^{40}$Ca$^+$ ions in a hybrid atom-ion trap and give quantitative theoretical explanations for the observations. The $^{39}$K atoms and $^{40}$Ca$^+$ ions are held in a magneto-optical (MOT) and a linear Paul trap, respectively. Fluorescence detection and high resolution time of flight mass spectra for both species are used to determine the remaining number of $^{40}$Ca$^+$ ions, the increasing number of $^{39}$K$^+$ ions, and $^{39}$K number density as functions of time. Simultaneous trap operation is guaranteed by alternating periods of MOT and $^{40}$Ca$^+$ cooling lights, thus avoiding direct ionization of $^{39}$K by the $^{40}$Ca$^+$ cooling light. We show that the K-Ca$^+$ charge-exchange rate coefficient increases linearly from zero with $^{39}$K number density and, surprisingly, the fraction of $^{40}$Ca$^+$ ions in the 4p\,$^2$P$_{1/2}$ electronically-excited state. Combined with our theoretical analysis, we conclude that these data can only be explained by a process that starts with a potassium atom in its electronic ground state and a calcium ion in its excited 4p\,$^2$P$_{1/2}$ state producing ground-state $^{39}$K$^+$ ions and metastable, neutral Ca\,(3d4p$^3$P$_1$) atoms, releasing only 150 cm$^{-1}$ equivalent relative kinetic energy. Charge-exchange between either ground- or excited-state $^{39}$K and ground-state $^{40}$Ca$^+$ is negligibly small as no energetically-favorable product states are available. Our experimental and theoretical rate coefficients of $9\times10^{-10}$ cm$^3$/s are in agreement given the uncertainty budgets.

An important physical process unique to neutral-ion systems is the charge-transfer (CT) reaction. Here, we present measurements of and models for CT processes between co-trapped ultracold Ca atoms and Yb ions under well-controlled conditions. The theoretical analysis reveals the existence of three reaction mechanisms when lasers from a magneto-optical trap (MOT) and an additional catalyst laser are present. Besides the direct CT involving existent excited Ca population in the MOT, the second pathway is controlled by MOT-induced CT, whereas the third one mostly involves the additional red-detuned laser.

We have performed a combined experimental and theoretical study of the photodissociation cross section of the molecular ion DyCl$^+$. The photodissociation cross section for the photon energy range 35,500 cm$^{-1}$ to 47,500 cm$^{-1}$ is measured using an integrated ion trap and time-of-flight mass spectrometer; we observe a broad, asymmetric profile that is peaked near 43,000 cm$^{-1}$. The theoretical cross section is determined from electronic potentials and transition dipole moments calculated using the relativistic configuration-interaction valence-bond and coupled-cluster methods. The electronic structure of DyCl$^+$ is extremely complex due to the presence of multiple open electronic shells, including the 4f$^{10}$ configuration. The molecule has nine attractive potentials with ionically-bonded electrons and 99 repulsive potentials dissociating to a ground state Dy$^+$ ion and Cl atom. We explain the lack of symmetry in the cross section as due to multiple contributions from one-electron-dominated transitions between the vibrational ground state and several resolved repulsive excited states.

The photodissociation cross-section of SrCl$^+$ is measured in the spectral range of 36000 -- 46000 cm$^{-1}$ using a modular time-of-flight mass spectrometer (TOF-MS). By irradiating a sample of trapped SrCl$^+$ molecular ions with a pulsed dye laser, X$^1Σ^+$ state molecular ions are electronically excited to the repulsive wall of the A$^1Π$ state, resulting in dissociation. Using the TOF-MS, the fragments are detected and the photodissociation cross-section is determined for a broad range of photon energies. Detailed $\textit{ab initio}$ calculations of the molecular potentials and spectroscopic constants are also performed and are found to be in good agreement with experiment. The spectroscopic constants for SrCl$^+$ are also compared to those of another alkaline earth chalcogen, BaCl$^+$, in order to highlight structural differences between the two molecular ions. This work represents the first spectroscopy and $\textit{ab initio}$ calculations of SrCl$^+$.

The role of electronic excitation in charge exchange chemical reactions between ultracold Ca atoms and Ba$^+$ ions, confined in a hybrid trap, is studied. This prototypical system is energetically precluded from reacting in its ground state, allowing a particularly simple interpretation of the influence of electronic excitation. It is found that while electronic excitation of the ion can critically influence the chemical reaction rate, electronic excitation of the neutral atom is less important. It is also experimentally demonstrated that with the correct choice of the atom-ion pair, it is possible to mitigate the unwanted effects of these chemical reactions in ultracold atom-ion environments, marking an important step towards the next generation of hybrid devices.

Interactions between cold ions and atoms have been proposed for use in implementing quantum gates\cite{Idziaszek2007}, probing quantum gases\cite{Sherkunov2009}, observing novel charge-transport dynamics\cite{Cote2000}, and sympathetically cooling atomic and molecular systems which cannot be laser cooled\cite{Smith2005,Hudson2009}. Furthermore, the chemistry between cold ions and atoms is foundational to issues in modern astrophysics, including the formation of stars, planets, and interstellar clouds\cite{Smith1992}, the diffuse interstellar bands\cite{Reddy2010}, and the post-recombination epoch of the early universe\cite{Stancil1996b}. However, as pointed out in refs 9 and 10, both experimental data and a theoretical description of the ion-atom interaction at low temperatures, reached in these modern atomic physics experiments and the interstellar environment, are still largely missing. Here we observe a chemical reaction between ultracold $^{174}$Yb$^+$ ions and $^{40}$Ca atoms held in a hybrid trap. We measure, and theoretically reproduce, a chemical reaction rate constant of $ \rm \bf K =(2\pm1.3)\times10^{-10} cm^{3}s^{-1}$ for $ \rm \bf 1 mK \leq T \leq 10 K$, four orders of magnitude higher than reported for other heteronuclear cases. We also offer a possible explanation for the apparent contradiction between typical theoretical predictions and measurements of the radiative association process in this and other systems.

The formation of $^{40}$Ca$_2^+$ molecular ions is observed in a hybrid $^{40}$Ca magneto-optical and ion trap system. The molecular ion formation process is determined to be two-photon photo-associative ionization of ultracold $^{40}$Ca atoms. A lower bound for the two-body, two-photon rate constant is found to be $\barβ \geq 2 \pm 1 \times 10^{-15}$ cm$^{3}$ Hz. $\textit{Ab initio}$ molecular potential curves are calculated for the neutral Ca$_2$ and ionic Ca$_2^+$ molecules and used in a model that identifies the photo-associative ionization pathway. As this technique does not require a separate photo-association laser, it could find use as a simple, robust method for producing ultracold, state-selected molecular ions.

We demonstrate a simple technique for molecular ion spectroscopy. BaCl$^+$ molecular ions are trapped in a linear Paul trap in the presence of a room-temperature He buffer gas and photodissociated by driving an electronic transition from the ground X$^1Σ^+$ state to the repulsive wall of the A$^1Π$ state. The photodissociation spectrum is recorded by monitoring the induced trap loss of BaCl$^+$ ions as a function of excitation wavelength. Accurate molecular potentials and spectroscopic constants are determined. Comparison of the theoretical photodissociation cross-sections with the measurement shows excellent agreement. This study represents the first spectroscopic data for BaCl$^+$ and an important step towards the production of ultracold ground-state molecular ions.