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Le Journal de Physique Colloques

COHERENT SPECTROSCOPY ON SINGLE ATOMIC SYSTEM AT REST IN FREE SPACE II

HANS G. DEHMELT

Published Dec 1, 1981• 15 citations
Publisher

Abstract

An individual atomic ion localized in the center of a small Paul rf quadrupole trap is a promising object for ultimate resolution laser spectroscopy because, broadly speaking, the ion may be brought to "A State of Complete Rest in Free Space" by side band cooling. As a consequence, all Doppler shifts vanish. "Free Space" is approximated insofar 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 micro-Gauss range. Naturally, there is no transit time broadening. Minute laser powers provided by harmonic generators suffice for saturation of optical transitions as well focused beams may be used. Millionfold atomic amplification of the single-ion fluorescence from a metastable level may bring resolutions of 1 part in 1018 within reach. Furthermore, the close localization of an atomic particle in space is one of the most fundamental problems in physics and worthy of study on its own merit.

DOI
10.1051/jphyscol:1981837

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Aug 29, 2007•8 citations
An individual atomic ion localized in the center of a small Paul rf quadrupole trap is a promising object for ultimate resolution laser spectroscopy because, broadly speaking, the ion may be brought to "A State of Complete Rest in Free Space" by side band cooling. As a consequence, all Doppler shifts vanish. "Free Space" is approximated insofar 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 micro-Gauss range. Naturally, there is no transit time broadening. Minute laser powers provided by harmonic generators suffice for saturation of optical transitions as well focused beams may be used. Millionfold atomic amplification of the single-ion fluorescence from a metastable level may bring resolutions of 1 part in 1018 within reach. Furthermore, the close localization of an atomic particle in space is one of the most fundamental problems in physics and worthy of study on its own merit.
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.
Zero-point level confinement1 in a trap is the quantum-mechanical equivalent of the classical single particle at rest in free space. Such confinement has been demonstrated—by the continuous Stern-Gerlach effect—only for the 150-GHz cyclotron motion in geonium, a single electron permanently confined in a cold Penning trap. Localizing the electron to ≈ 60 μm in the node of a standing wave in the trap cavity approximated free space, but decreased spontaneous emission tenfold. Driving the 60-MHz axial motion on a sideband higher by the ≈ 12-kHz magnetron frequency forced the magnetron motion to absorb the excess photon energy, shrinking its radius to ~ 15μm. Analogous laser cooling has reduced the oscillation amplitude of a Ba+ ion in a Paul rf trap to < 120 nm. As in nuclear magnetic resonance, confinement is now much smaller than the wavelength and sidebands disappear. Quantum jumps to and from an electronic metastable level of Ba+ have been demonstrated. Using In+ or a similar ion, this may make a monoion oscillator optical frequency standard with a 1000-day reproducibility of 10-18 possible.
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.
A single laser-cooled indium ion is a promising candidate for an ultimate resolution optical time or frequency standard. It can be shown that single ions from group IIIA of the periodic table (indium, thallium, etc.) can have extremely small systematic errors. In addition to being free from Doppler, transit-time and collisional shifts, these ions are also quite insensitive to perturbations from ambient magnetic and electric fields (mainly due to the use of a J=0-0 transition for spectroscopy). Of all group IIIA ions, indium seems to be the most practical, since it is heavy enough to have a tolerable intercombination cooling transition rate and (unlike thallium) has transitions which are easily accessible with frequency multiplied continuous-wave lasers. A single indium ion standard has a potential inaccuracy of one part in 10(exp 18) for integration times of 10(exp 6) seconds. We have made substantial progress during the grant period in constructing a frequency standard based upon a single indium ion. At the beginning of the grant period, single indium ions were being successfully trapped, but the lasers and optical systems were inadequate to achieve the desired goal. We have considerably improved the stability of the dye laser used to cool the ions and locked it to a molecular resonance line, making it possible to observe stable cooling-line fluorescence from a single indium ion for reasonable periods of time, as required by the demands of precision spectroscopy. We have substantially improved the single-ion fluorescence signal with significant benefits for the detection efficiency of forbidden transitions using the 'shelving' technique. Finally, we have constructed a compact, efficient UV 'clock' laser and observed 'clock' transitions in single indium ions using this laser system. We will elaborate on these accomplishments.
The present decade has seen great advances in optical frequency standards following the development of femtosecond laser frequency combs. With this technology, combined with a stable and narrow probe laser source (50-100 Hz linewidth), we made a systematic study of the 5s 2S1/2-4d 2D5/2 (S-D) transition frequency of the 88Sr+ ion as a function of the quantization axis in the trap. This study revealed the presence of systematic shifts caused by residual micromotion in our rf Paul trap and led to an evaluation of those shifts. Also, a method was developed based on the measurement of the Zeeman spectrum of the clock transition to cancel the electric quadrupole shift caused by patch potentials on the trap electrodes. From these results, we also determined the absolute frequency of the S-D clock transition frequency at 674 nm with a fractional frequency uncertainty of 3.4 × 10-14. We have since made improvements to our probe laser system and have observed 5 Hz Fourier transform limited linewidths on a Zeeman component of the S-D transition of the 88Sr+ ion. To observe such a narrow transition it is essential to have a known and well behaved drift of the laser frequency during the few minutes it takes to measure the ion resonance using the quantum jump signal. Our approach to minimize the drift rate and its sensitivity to temperature fluctuations has been to stabilize the cavity at the temperature where its coefficient of thermal expansion is zero. We will present an overview of our recent progress made on the frequency standard based on the laser-cooled and trapped single ion of 88Sr+.