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Physical Review A

Experimental study of anomalous heating and trap instabilities in a microscopic 137Ba ion trap

Ralph G. DeVoe, Christian Kurtsiefer

Published Jun 13, 2002• 61 citations
Publisher

Abstract

A single ${}^{137}{\mathrm{Ba}}^{+}$ ion has been laser cooled to the first few vibrational states of an $80\ensuremath{-}\ensuremath{\mu}\mathrm{m}$ radius rf quadrupole trap. Initial measurements showed no anomalous heating of the vibrational phonon for observation times up to 1 ms, corresponding to a heating rate less than 3.3 phonons/ms at a 95% confidence level. Subsequently we observed the growth of large and unstable bias voltages that were correlated with exposure to the atomic beam. After loading over 250 ions over a period of 720 days the trap became unstable, with ion lifetimes $<1 \mathrm{min}.$ A possible connection between this trap instability and anomalous heating observed in quantum computation experiments is discussed. An isotopically selectively laser cooling method was used to refine ${}^{137}\mathrm{Ba}$ ions out of a naturally abundant cloud.

DOI
10.1103/physreva.65.063407

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A single 137Ba+{}^{137}{\mathrm{Ba}}^{+}137Ba+ ion has been laser cooled to the first few vibrational states of an 80\ensuremath−\ensuremathμm80\ensuremath{-}\ensuremath{\mu}\mathrm{m}80\ensuremath−\ensuremathμm radius rf quadrupole trap. Initial measurements showed no anomalous heating of the vibrational phonon for observation times up to 1 ms, corresponding to a heating rate less than 3.3 phonons/ms at a 95% confidence level. Subsequently we observed the growth of large and unstable bias voltages that were correlated with exposure to the atomic beam. After loading over 250 ions over a period of 720 days the trap became unstable, with ion lifetimes <1min.<1 \mathrm{min}.<1min. A possible connection between this trap instability and anomalous heating observed in quantum computation experiments is discussed. An isotopically selectively laser cooling method was used to refine 137Ba{}^{137}\mathrm{Ba}137Ba ions out of a naturally abundant cloud.
We measure and characterize anomalous motional heating of an atomic ion confined in the lowest quantum levels of a novel rf ion trap that features moveable electrodes. The scaling of heating with electrode proximity is measured, and when the electrodes are cooled from 300 to 150 K, the heating rate is suppressed by an order of magnitude. This provides direct evidence that anomalous motional heating of trapped ions stems from microscopic noisy potentials on the electrodes that are thermally driven. These observations are relevant to decoherence in quantum information processing schemes based on trapped ions and perhaps other charge-based quantum systems.
An investigation of the sympathetic cooling \nmethod for the studies of large ultra-cold molecular ions in a quadrupole ion trap has been conducted.Molecular dynamics simulations are performed to study the rf heating mechanisms in the ion trap. The dependence of \nrf heating rates on the ion temperature, trapping parameters, \nand the number of ions is obtained. New rf heating mechanism \naffecting ultra-cold ion clouds exposed to laser radiation is described.The saturation spectroscopy setup of the hyperfine spectra \nof the molecular iodine has been built to provide an accurate frequency reference for the laser wavelength. This reference is used to obtain the fluorescence lineshapes of \nthe laser cooled Mg+^++ ions under different trapping conditions.The ion temperatures are deduced from the measurements, and \nthe influence of the rf heating rates on the fluorescence lineshapes \nis also discussed. Cooling of the heavy (m=720m=720m=720a.u.) fullerene ions to under 10K by the means of the sympathetic cooling by the Mg+^++ ions(m=24m=24m=24a.u.) is demonstrated. The single-photon imaging system has been developed and used to obtain the images of the Mg+^++ ion crystal structures at mK temperatures.
Sympathetic laser cooling of ions stored within a linear-geometry, radio frequency, electric-quadrupole trap has been investigated using computational and theoretical techniques. The simulation, which allows five sample ions to interact with 35 laser-cooled atomic ions, revealed an instability heating mechanism, which can prevent ions below a certain critical mass from being sympathetically cooled. This critical mass can, however, be varied by changing the trapping field parameters, thus allowing ions with a very large range of masses to be sympathetically cooled using a single ion species. A theoretical explanation of this instability heating mechanism is presented which predicts that the cooling-heating boundary in trapping parameter space is a line of constant qu{q}_{u}qu​ (ion trap stability coefficient), a result supported by the computational results. The threshold value of qu{q}_{u}qu​ depends on the masses of the interacting ions. A functional form of this dependence is given.
Several groups have recently used laser cooling to produce ordered arrays of ions, ion crystals, in rf quadrupole ion traps (Paul traps). Laser cooling has been understood using previous theories which ignore ion micromotion and replace the oscillating rf trap potential with a static pseudopotential. We show that in recent experiments, ion micromotion can exceed one wavelength of the cooling radiation and that the resulting Doppler shifts can transform laser cooling into laser heating. Changes in trap voltage, ion position, and laser intensity can all reverse the sign of laser cooling. This, together with ideas of deterministic chaos, can lead to a reinterpretation of experiments on ion crystals. The recognition that effects previously attributed to an experimental artifact, rf heating, may actually be due to a failure of laser cooling opens the door, on the one hand, to precise theories of phase transitions and, on the other, to improved cooling techniques that can permit the creation of larger ion crystals.
The 133Ba+^{133}\mathrm{Ba}^{+}133Ba+ ion is a promising candidate as a high-fidelity qubit, and the 137Ba+^{137}\mathrm{Ba}^{+}137Ba+ isotope is promising as a high-fidelity qudit (d>2)(d>2)(d>2). Barium metal is very reactive, and 133Ba+^{133}\mathrm{Ba}^{+}133Ba+ is radioactive and can only be sourced in small quantities, so the most commonly used loading method, oven heating, is less suited for barium and is currently not possible for 133Ba+^{133}\mathrm{Ba}^{+}133Ba+. Pulsed laser ablation solves both of these problems by utilizing compound barium sources while also giving some distinct advantages, such as fast loading, less displaced material, and lower heat load near the ion trap. Because of the relatively low abundances of the isotopes of interest, a two-step photoionization technique is used, which gives us the ability to selectively load isotopes. Characterization of the ablation process for our BaCl2{\mathrm{BaCl}}_{2}BaCl2​ targets are presented, including observation of neutral and ion ablation-fluence regimes, preparation and conditioning, lifetimes of ablation spots, and plume velocity distributions. We show that by using laser ablation on BaCl2{\mathrm{BaCl}}_{2}BaCl2​ salt targets with a two-step photoionization method, we can produce and trap barium ions reliably. Furthermore, we demonstrate that with our photoionization method, we can trap 137Ba+^{137}\mathrm{Ba}^{+}137Ba+ with an enhanced selectivity compared to its natural abundance.
A single 88Sr+^{88}\mathrm{Sr}^{+}88Sr+ ion has been cooled to the zero point of its axial motion in a radio frequency endcap trap. A ground-state occupation probability of 98.6(8)% was achieved using resolved sideband laser cooling on the 674nm674\phantom{\rule{0.3em}{0ex}}\mathrm{nm}674nm $^{2}S_{1∕2}\text{\ensuremath{-}}^{2}D_{5∕2}$ quadrupole transition. The ion's heating rate was measured to be 0.054(4) quanta/ms, implying a spectral density of electric field noise comparable to the other reported values, summarized in the paper by Deslauriers et al. [Phys. Rev. A 70, 043048 (2004)]. Low heating rates were only observed using a trap that had minimal exposure to atomic flux.