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Physical Review A

Localized visible Ba+ mono-ion oscillator

Werner Neuhauser, M. Hohenstatt, P. E. Toschek, HANS G. DEHMELT

Published Sep 1, 1980• 406 citations
Publisher

Abstract

An individual barium ion, continuously observed by laser fluorescence, has been isolated in a Paul rf quadrupole trap at room temperature. By optical sideband cooling its microscopically measured image has been reduced in thickness to \ensuremath{\sim} 2 \ensuremath{\mu}m in the object plane, the diffraction limit. Estimated ion temperatures reached are ${T}_{i}\ensuremath{\simeq}10 \mathrm{to} <36$ mK. With cooling, the ion could be held indefinitely, without cooling \ensuremath{\sim} 30 s. In the future the technique seems capable of attaining kinetic temperatures \ensuremath{\sim} ${10}^{\ensuremath{-}8}$ K, much lower than realized so far by other means, with corresponding far-reaching implications.

DOI
10.1103/physreva.22.1137

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An individual barium ion, continuously observed by laser fluorescence, has been isolated in a Paul rf quadrupole trap at room temperature. By optical sideband cooling its microscopically measured image has been reduced in thickness to \ensuremath{\sim} 2 \ensuremath{\mu}m in the object plane, the diffraction limit. Estimated ion temperatures reached are Ti\ensuremath≃10to<36{T}_{i}\ensuremath{\simeq}10 \mathrm{to} <36Ti​\ensuremath≃10to<36 mK. With cooling, the ion could be held indefinitely, without cooling \ensuremath{\sim} 30 s. In the future the technique seems capable of attaining kinetic temperatures \ensuremath{\sim} 10\ensuremath−8{10}^{\ensuremath{-}8}10\ensuremath−8 K, much lower than realized so far by other means, with corresponding far-reaching implications.
Laser fluorescence spectroscopy has been performed on the two-photon 62S1/2 to 52D3/2 transition in Ba+. Using signals from an individual laser-cooled ion in a rf trap, a laser limited linewidth of less than 3 MHz has been achieved with an effective wavelength of 2.07 μm. The absence of any sidebands in the spectra spaced at the 5.5-MHz oscillation frequency of the ion in the rf quadrupole trap indicates that the ion vibrational motion must be well into the Lamb–Dicke regime. An upper limit of 165 nm is given to the vibrational amplitude.
An individual atomic ion localized in the center of a small Paul RF quadrupole trap has potential as an ultimate laser frequency standard because the ion may be brought to `a state of complete rest in free space' by sideband cooling. As a consequence, all Doppler shifts vanish, `Free Space' is approximated in so far as the electric trapping field vanishes in the center of the trap and there is no Stark effect. Neither need there be a Zeeman effect as magnetic fields may be controlled down to the microgauss range. There is also no transit time broadening. Minute laser powers provided by harmonic generators suffice for saturation of optical transitions as strongly focused beams may be used. A million-fold atomic amplification of the single-ion fluorescence from a metastable level may bring resolutions of 1 part in 1018within reach.
A single Hg+ ion that is confined in an rf (Paul) trap can be laser cooled so that the amplitude of its motion is much less than an optical wavelength (the Dicke limit for optical transitions).1 More recently, we used the technique of sideband cooling to reach the zero point of motion.2 This realizes for the first time, we believe, the fundamental limit of laser cooling and the ideal of an isolated atomic particle at rest in free space to within the quantum mechanical limits imposed by the surrounding apparatus. In both limits, Doppler effects to all orders become negligible, the interrogation time is arbitrarily long, and the fundamental shot noise limit of a single atom is readily attained. We report these results and our progress toward the 2S1/2—2D5/2 282-nm transition with a resolution approaching the 2-Hz natural linewidth.
We report the laser cooling and trapping of barium ions in a novel planar Paul trap. Unlike other rf quadrupole traps, the electrodes of a planar Paul trap are confined to one or more parallel planes. They may therefore be constructed on a micrometer scale by the same photolithographic techniques used for semiconductor devices. Such devices are under construction in our laboratory. Micro-traps permit entirely new studies, for example, superradiance of a few ions, which occurs as ion-ion distances approach one wavelength of light. Photolithography can also generate an array of traps on a single substrate for an atomic clock. Our traps consist of one or more parallel planes, one above the other, each plane having either a conducting ring or a hole in a conducting sheet. Thus, a ring or a hole results in a potential well for trapping. A three-ring design produces a quadrupole potential that is harmonic up to eighth-order for a particular choice of ring diameters and spacings. Three-hole traps have been constructed with inner hole radii ranging from 50 to 400 μm, and both single barium ions and clouds have been observed.
We demonstrate the confinement of a single laser cooled Ba+ ion, held in a radiofrequency trap, to a region of space whose linear dimensions are much less than the wavelength of the interrogating light divided by 2π (Lamb‐Dicke regime1). This is the condition for the complete freedom from first order Doppler shifts. From the absence of observable Doppler sidebands on a narrow, two‐photon feature in the spectrum of Ba+, we infer an upper bound on the ion orbit radius of 165 nm. This is the first clear demonstration of Lamb‐Dicke confinement in the visible.2
An experimental study of sideband laser cooling of trapped ions in miniature radio-frequency traps is presented. In+{\mathrm{In}}^{+}In+ ions are laser cooled by exciting the narrow intercombination line ${5s}^{2}{}^{1}{S}_{0}\ensuremath{\rightarrow}5s5p{}^{3}{P}_{1}$, which shows resolved sidebands due to the vibrational motion of the ions in the trap. The influence of the micromotion at the frequency of the oscillating trapping field is investigated. Heating processes through coupling with the trapping field occur for certain regions of negative laser detuning. A method of bichromatic sideband cooling is introduced that combines efficient cooling from high temperatures with the possibility of reaching low mean vibrational quantum numbers 〈n〉<1〈n〉<1〈n〉<1 by using narrow optical transitions. This technique allows recording of high-resolution excitation spectra of the cooling transition in the Lamb-Dicke regime and thus determination of 〈n〉〈n〉〈n〉. Single ions as well as two-ion Coulomb crystals have been cooled to temperatures below $100 \ensuremath{\mu}\mathrm{K}$, and population of the ground state of motion to more than 50% over long time scales is demonstrated. These values are close to the lower limit for the detection of vibrational excitation with the present method.