Session E09: Trapped Ions

Creation and Characterization of Entanglement in Radial-2D Ion Crystals

One-dimensional ion chains in rf traps have seen remarkable success in engineering high-fidelity quantum gates and simulating 1D quantum spin systems.  A comparable ability to control and probe two-dimensional ion crystals in rf traps would significantly expand the class of systems accessible to quantum simulation. Coulomb crystals in the radial-2D phase have been established as a realistic platform for quantum simulation [1], and site-resolved imaging of large radial-2D crystals has been demonstrated for a blade-style rf trap [2]. An integral component of quantum simulation yet to be demonstrated for this platform is the creation and characterization of entangled states. Here, we present the first Mølmer-Sørensen (MS) interaction with radial-2D crystals. A global application of the MS interaction, an entangling mechanism native to trapped ion systems, allows for the creation of a wide variety of spin models. We describe our implementation of MS interactions and discuss progress in characterizing frustration and entanglement in small ion lattices. 

[1] D'Onofrio et al., PRL 127, 020503 (2021)

[2] Xie et al., Quantum Sci. Technol. 6, 044009 (2021)   

Rotational Decoherence of a Trapped-Ion Planar Quantum Rotor

Trapped ions have been used to study the decoherence of simple model quantum systems, such as the harmonic oscillator and simple two-level system, in experiments that feature careful control over both the system and the environment under study. The quantum rigid rotor is another simple model quantum system, and in recent years has been the subject of theoretical study, but the resulting predictions have not yet been directly tested by experiment. I will present measurements of the decoherence of a planar quantum rotor, comprised of two trapped ions separated by 6 um. By preparing the rotor in a coherent superposition of angular momentum states and introducing an environment that induces angular momentum diffusion, we find that rotational coherences decay according to a simple scaling law with the distance measure in orientation space, in accordance with the corresponding Markovian master equation.   

Structural Phase Transition of Trapped Ions in the Quantum Regime

We experimentally characterize the 1D linear to 2D zigzag structural transition for arrays of ions confined in a linear radio-frequency Paul trap and cooled to near their ground state of motion. The transition is controllably quenched by a ramp of the confining potential. Raman sideband spectroscopy is used to probe both the energy level structure and the motional population distribution of the zigzag vibrational mode, which softens near the transition critical point. A robust critical point is achieved through stabilization of the trap potential, and significant ground state occupation is demonstrated crossing the transition. Near the critical point we resolve biases arising from ion-trap asymmetries that change the nature of the transition, and show how they can be suppressed.   

Rotation sensing with a compact Penning trapped calcium ion crystal system

In traditional mechanics, harmonic oscillators could be utilized for force, acceleration or rotation measurement. The traditional harmonic oscillators usually composed of a mass block. Here we describe a quantum harmonic oscillator based on trapped ion crystal. These penning trapped ions can form a 2D ion crystal. The calcium ions are cooled through lasers and driven by RF electronic magnetic field. The spins of the ions and the harmonic motion are coupled through a laser. Just like the traditional oscillators, we try to figure out if the trapped ions could form a rotation sensor. We also will show the compact penning trap design in our lab. In the penning trap, the super conducting magnet will be replaced by permanent magnet. The cost and the volume of the trap are greatly reduced.   

Towards studying interactions between trapped CaH+ ions and K atoms

The complex internal structure of molecular ions poses challenges with employing direct laser cooling [1]. An alternative and more general approach is to sympathetically cool the external and internal degrees of freedom of the molecular ions using laser-cooled atomic ions and neutral atoms, respectively [2]. Our hybrid ion-atom trap is ideally suited for this method of ground-state cooling of a molecular ion [3], and our goal is to demonstrate this by sympathetically cooling calcium mono-hydride ions (CaH⁺) using co-trapped calcium (⁴⁰Ca⁺) ions and potassium (³⁹K) atoms. Moreover, the ability to simultaneously trap the three species, ⁴⁰Ca⁺, ³⁹K, and CaH⁺, allows for the study of various possible chemical reactions. Knowledge of and control over these reactions is paramount for our goal of cooling CaH⁺. We have already investigated the charge exchange between Ca⁺ and K and established the ability to quench the reaction rate [4]. In this talk, we expand on this work and present our progress towards studying the interactions between CaH⁺ ions and K atoms.

 

References:

[1] Nguyen, Jason HV, et al., New Journal of Physics 13.6 (2011): 063023

[2] Hudson, Eric R., EPJ Techniques and Instrumentation 3 (2016): 1-21

[3] Jyothi, S., et al., Review of Scientific Instruments 90.10 (2019): 103201

[4] Li, Hui, et al., Physical Chemistry Chemical Physics 22.19 (2020): 10870-10881   

Multi-mode Ground-state Cooling of a Trapped Ion Chain beyond the Lamb-Dicke Regime

Laser cooling of a trapped ion chain to the motional ground state is the starting point of quantum information processing with trapped ions. Traditionally, ground-state preparation is carried out by resolved sideband cooling, which requires reaching the Lamb-Dicke regime to work efficiently. However, this requirement is challenging for axial modes of Be+ in a Paul trap, due to the light mass and low motional frequency, leading to Lamb-Dicke factors approaching 1. Here, we report a novel method to cool multiple motional modes in an ion chain to the ground state in parallel. We simultaneously drive all the red sideband transitions, and add a weak optical pumping to constantly reset the spin-state. We demonstrate cooling a single ion to the ground-state for η up to 0.78 within 200us. We also show effective parallel cooling for the axial modes of 30 ions to the near ground state. Our method has a speeding effect compared to traditional resolved sideband cooling, works robustly far outside the Lamb-Dicke regime, and can apply to any atomic system with arbitrary motional spectrums.   

Ground state cooling of weakly-coupled trapped-ion motion using parametric coupling

Laser cooling of trapped-ion motional degrees of freedom to near their ground state is crucial for high-fidelity quantum information applications and high-precision quantum metrology. However certain motional degrees of freedom are weakly coupled to the available cooling light due to geometrical constraints or weak mode participation of the cooling ions. We overcome these obstacles by using parametric coupling to exchange motional energy between modes that are weakly and strongly coupled to the cooling light respectively. In this way we demonstrate cooling to near the ground state of some weakly coupled modes in Be+-Be+ and Be+-Mg+ two-ion crystals. In a Be+-Mg+-Be+ crystal, we cool all three axial modes to near the ground state using only the Mg+ ion, even though this ion does not participate in the stretch mode of this symmetric crystal.  This method may be useful for efficient cooling of Coulomb-crystals of many charged species, including molecular ions and highly charged ions.   

Comparing optimized pulsed sideband cooling and continuous sideband cooling for optical qubits

Trapped-ion quantum computers rely on two-qubit gates that achieve optimal performance when the ions are near the ground state of motion. Ion cooling, however, takes up a significant amount of the time required to prepare the qubit register, and as the computer scales, cooling the ions in the middle of the algorithm becomes more critical. Therefore, cooling can be a significant source of latency in the algorithm. Recently, a graph theoretic method of determining the optimal set of sideband cooling (SBC) pulses for a given trapped ion system has been proposed and experimentally demonstrated [1]. Here, we investigate the difference in efficiency of optimized pulsed SBC versus continuous SBC [2] for optical qubits using numerical simulation and experimental confirmation with ⁴⁰Ca⁺ ions. 

[1] A. J. Rasmusson, M. D'Onofrio, Y. Xie, J. Cui, & P. Richerme. Optimized pulsed sideband cooling and enhanced thermometry of trapped ions. Phys. Rev. A 104, 043108 (2021).

[2] F. Diedrich, J. C. Bergquist, W. M. Itano, & D. J. Wineland. Laser Cooling to the Zero-Point Energy of Motion. Phys. Rev. Lett. 62, 403 (1989).   

Adiabatic control of motional states of CaO+

Well-controlled motional states of molecular ions are a prerequisite for applications such as quantum information processing and precision measurements. In this talk, we'll report our experimental results on adiabatic control of motional states of a calcium oxide ion (CaO⁺) in a linear RF trap. In our experiment, we co-trap a CaO⁺ with a calcium ion (Ca⁺), and sympathetically cool CaO⁺'s axial motion to the ground motional states via sideband cooling Ca⁺. Then we adiabatically ramp the secular frequency and investigate the change of the motional states. This work is the first step towards observing the interaction of the CaO⁺ dipole with its motion^[1,2].

 

[1] W.C. Campbell and E.R. Hudson. Dipole-Phonon Quantum Logic with Trapped Polar Molecular Ions. Phys. Rev. Lett. 125, 120501 (2020).

[2] M. Mills, H. Wu, E.C. Reed, L. Qi, K.R. Brown, C. Schneider, M.C. Heaven, W.C. Campbell and E.R. Hudson. Dipole–phonon quantum logic with alkaline-earth monoxide and monosulfide cations. Phys. Chem. Chem. Phys. 22, 24964-24973 (2020).   

Isotope-Selective Laser Ablation Loading of 137Ba+

¹³⁷Ba⁺ ions are excellent contenders to be used as high-fidelity qudits (d > 2). Thus, reliably producing and trapping this isotope is very important in achieving this goal. We report on a pulsed laser ablation loading technique that allows us to dependably load ¹³⁷Ba⁺ from a BaCl₂ target. We have further characterized the reliability of this ablation technique, along with the spatial and temporal variations of the atomic flux across the barium-salt target. We present observations of neutral and ion ablation-fluence regimes, a detailed overview of the ablation spot preparation process and plume velocity distributions. Through this process, we demonstrate a ¹³⁷Ba⁺ loading selectivity of 32±5%, significantly higher than its natural abundance of 11.2%.