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Journal of the Optical Society of America B

Doppler-free optical spectroscopy on the Ba^+ mono-ion oscillator

G. R. Janik, Warren Nagourney, HANS G. DEHMELT

Published Aug 1, 1985• 120 citations
Publisher

Abstract

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.

DOI
10.1364/josab.2.001251

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Optical lamb-dicke confinement of a Ba+ mono-ion oscillator

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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

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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.

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We experimentally demonstrate laser spectroscopy of the electric-quadrupole transition $6s{}^{2}{S}_{1/2}--5d{}^{2}{D}_{3/2}$ of a single trapped ${}^{171}\mathrm{Yb}$ ion. The probed transition has a natural linewidth of 3.1 Hz, and is at a wavelength of 435.5 nm. A ${}^{171}\mathrm{Yb}$ ion is laser cooled to the range of the Doppler limit in a radio-frequency Paul trap. The $F=0--{F}^{\ensuremath{'}}=2,$ ${m}_{{F}^{\ensuremath{'}}}=0$ hyperfine component of the ${}^{2}{S}_{1/2}{--}^{2}{D}_{3/2}$ transition is excited by a laser source whose frequency is controlled by an optical reference cavity of high intrinsic stability. The recoilless carrier component of the vibrational sideband spectrum is resolved with a minimum linewidth of approximately 80 Hz, corresponding to a fractional frequency resolution of $1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}.$ This resolution limit is attributed to laser frequency instability. The effective decoherence rates of the residual ion motion are inferred from the observation that the axial and two radial low-frequency first-order secular-vibration sidebands show minimum linewidths in the range of 0.2--1 kHz. Prospects for an optical frequency standard based on the investigated atomic system are discussed.

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Towards an optical frequency standard based on a single Ca/sup +/ ion in a miniature trap

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Summary form only given. Among the ions proposed for a frequency standard in the optical domain, Ca/sup +/ is one of the most favorable candidates due to its wavelengths accessible with all-solid-state laser systems. This allows to imagine an ultimate experimental setup being simple and compact. The 4S/sub 1/2/-3D/sub 5/2/ electric quadrupole transition at 729 nm with a natural linewidth below 1 Hz serves as clock transition, while laser cooling on one of the resonance lines is carried out at 397 nm. To obtain the small spectral linewidths necessary for a frequency standard performance, it is essential to reduce the residual Doppler broadening caused by the RF heating in the Paul trap. The first-order Doppler broadening of a single ion can be eliminated in the Lamb-Dicke regime where the ion's amplitude of motion is inferior to the emitted wavelength. The main condition for the access of this frequency modulation regime is a high motional frequency of the trapped ion. This is more easily obtained in a low capacitance device and thus traps with reduced dimensions are required. We have set up a miniature ion trap of the Paul-Straubel-type. The stored ions are observed by a photomultiplier in the photon-counting mode as well as by an intensified CCD camera, permitting the visualization of their spatial behavior. Laser cooling of the Ca/sup +/ ions is carried out on the 4S/sub 1/2/-4P/sup 1/2/ resonance line at 397 nm. Progress on the characterization of the trapped single ion as well as on the generation of the involved wavelengths exclusively by solid-state lasers is reported.

Conference on Lasers and Electro-Optics•Aug 29, 2005•0 citations
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.
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
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.
Single ions trapped and laser cooled provide ideal objects for high resolution spectroscopy and frequency standards. For the latter purpose we are investigating the 5s2 1S0 → 5s5p 3P0 transition of In+. With a natural linewidth of only 1.1 Hz this resonance offers very high resolution and is highly immune to frequency shifts due to external electromagnetic fields, because it connects two levels with vanishing electronic magnetic momenta [1]. The wavelength of this clock transition is 236.5 nm and is technically very convenient, since it coincides with the fourth harmonic of the 946 nm Nd:YAG laser line. So this intrinsically frequency stable solid-state laser can be used to excite the transition [2]. For laser cooling and fluorescence detection of the indium ion the 5s2 1So -> 5s5p 3P1 transition at 230.6 nm can be employed. The use of the relatively narrow intercombination line for laser cooling allows us to study optical sideband cooling in the strong binding regime, where the oscillation frequencies of the ion in the trap (around 1 MHz) are larger than the optical linewidth of 360 kHz. In this parameter range laser cooling is possible to the quantum ground state of the vibrational motion in the trap corresponding to a temperature of roughly 100 μK. The same low temperature was obtained with sideband cooling of a small Coulomb crystal consisting of two ions, creating an interesting new quantum few-particle system.
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.
We experimentally demonstrate laser spectroscopy of the electric-quadrupole transition 6s2S1/2−−5d2D3/26s{}^{2}{S}_{1/2}--5d{}^{2}{D}_{3/2}6s2S1/2​−−5d2D3/2​ of a single trapped 171Yb{}^{171}\mathrm{Yb}171Yb ion. The probed transition has a natural linewidth of 3.1 Hz, and is at a wavelength of 435.5 nm. A 171Yb{}^{171}\mathrm{Yb}171Yb ion is laser cooled to the range of the Doppler limit in a radio-frequency Paul trap. The $F=0--{F}^{\ensuremath{'}}=2,$ ${m}_{{F}^{\ensuremath{'}}}=0$ hyperfine component of the 2S1/2−−2D3/2{}^{2}{S}_{1/2}{--}^{2}{D}_{3/2}2S1/2​−−2D3/2​ transition is excited by a laser source whose frequency is controlled by an optical reference cavity of high intrinsic stability. The recoilless carrier component of the vibrational sideband spectrum is resolved with a minimum linewidth of approximately 80 Hz, corresponding to a fractional frequency resolution of $1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}.$ This resolution limit is attributed to laser frequency instability. The effective decoherence rates of the residual ion motion are inferred from the observation that the axial and two radial low-frequency first-order secular-vibration sidebands show minimum linewidths in the range of 0.2--1 kHz. Prospects for an optical frequency standard based on the investigated atomic system are discussed.
Summary form only given. Among the ions proposed for a frequency standard in the optical domain, Ca/sup +/ is one of the most favorable candidates due to its wavelengths accessible with all-solid-state laser systems. This allows to imagine an ultimate experimental setup being simple and compact. The 4S/sub 1/2/-3D/sub 5/2/ electric quadrupole transition at 729 nm with a natural linewidth below 1 Hz serves as clock transition, while laser cooling on one of the resonance lines is carried out at 397 nm. To obtain the small spectral linewidths necessary for a frequency standard performance, it is essential to reduce the residual Doppler broadening caused by the RF heating in the Paul trap. The first-order Doppler broadening of a single ion can be eliminated in the Lamb-Dicke regime where the ion's amplitude of motion is inferior to the emitted wavelength. The main condition for the access of this frequency modulation regime is a high motional frequency of the trapped ion. This is more easily obtained in a low capacitance device and thus traps with reduced dimensions are required. We have set up a miniature ion trap of the Paul-Straubel-type. The stored ions are observed by a photomultiplier in the photon-counting mode as well as by an intensified CCD camera, permitting the visualization of their spatial behavior. Laser cooling of the Ca/sup +/ ions is carried out on the 4S/sub 1/2/-4P/sup 1/2/ resonance line at 397 nm. Progress on the characterization of the trapped single ion as well as on the generation of the involved wavelengths exclusively by solid-state lasers is reported.