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Japanese Journal of Applied Physics

Laser Cooling and Trapping of Alkaline Earth Atoms

Takayuki Kurosu, Fujio Shimizu Fujio Shimizu

Published Mar 1, 1992• 40 citations
Publisher

Abstract

Characteristics of laser trapping of Ca and Sr are experimentally studied and discussed. The lifetime of the trap is found to be limited by accumulation of the population in the 3P2 state, not by the decay 1P1→1D2. This result allows us to generate a continuous free-falling flow of ultra-cold ground state atoms which is useful for ultra-high resolution spectroscopy. Effects of an optical pumping laser which recycles the 1D2 population in Sr are investigated.

DOI
10.1143/jjap.31.908

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Characteristics of laser trapping of Ca and Sr are experimentally studied and discussed. The lifetime of the trap is found to be limited by accumulation of the population in the 3P2 state, not by the decay 1P1→1D2. This result allows us to generate a continuous free-falling flow of ultra-cold ground state atoms which is useful for ultra-high resolution spectroscopy. Effects of an optical pumping laser which recycles the 1D2 population in Sr are investigated.
ABSTRACT In a ceramic vapor cell we have created a robust Sr magneto-optical trap that stores about 108 atoms with lifetimes>200 ms. We eliminate the 5p 1P1—4d 1D2—*5p 3P2 leak and achieve a 10-fold improvement in trap lifetime byrepumping the 5p 3Po,2 dark states with 679 nm and 707 nm light. The observed lifetime is now limited by coldcollision losses, and we have preliminary measurements of the 2-body loss rate. Direct readout of the trap velocity distribution is possible using the narrow (7 kHz) 52 'S0----5p 3P1 intercombination line at 689 nm. We can also cool with this narrow transition and have achieved a 40-fold 1-D velocity compression for about 5% of the trapped atoms by applying this second-stage cooling. Keywords: Laser cooling, laser trapping, alkaline earths, cold collisions, frequency metrology 1. INTRODUCTION Laser cooling and trapping techniques have advanced to become an important tool of the atomic physics community, and in recent years there has been considerable interest in expanding the range of trapped elements. The alkalineearths, in particular, possess unique characteristics which make them interesting to trap. Studies of long-rangeatom-atom interactions are simplified because they have no hyperfine structure, and their narrow intercombinationtransitions make them excellent candidates for optical frequency standards. Furthermore, Doppler cooling with thesenarrow transitions can reduce temperatures to near the recoil limit.The most abundant alkaline earth isotopes have I =
Laser cooling and trapping of heavy alkaline-earth element barium have been demonstrated for the first time ever. For any possible cycling transition in barium that could provide strong cooling forces, the excited state has a very large branching probability to metastable states. Additional lasers are thus needed to bring the atoms back in to the cooling cycle. In this work barium atoms were efficiently collected in a magneto-optical trap (MOT) by large optical forces from the strong 6s2 1S0 – 6s6p 1P1 transition. Trapping of barium has been achieved using up to seven lasers simultaneously. The properties of the barium MOT were characterized. The efficiency of capturing an atom from a thermal atomic beam into the MOT is 0.4(1) %. The developed scheme is very efficient for collecting atoms. Loss rate mechanisms from the trap were studied by observing the decay of the trap population. Typical lifetimes of the MOT cloud are on the order of one second and are limited mainly due to insufficient repumping of metastable states. The range of velocities from which barium atoms can be captured into the MOT is about 30 m/s. Different laser transitions were employed for repumping barium atoms from the 6s5d 3D1 and the 6s5d 3D2 states. They lead to similar trap populations and lifetimes. The temperature of the cold atomic barium cloud was determined to about 5 mK. Atomic properties of the 5d2 3F2 state were studied with trapped atoms, in particular it's lifetime was determined as 160(10) s. This work has shown a possible scheme to trap atoms with a leaky cooling cycle. Since atoms with such properties are the majority of the elements in the periodic table, the number of optically trapable elements can be significantly enlarged. Of particular high interest is radium, which is chemical homologue to barium. The interest in radium stems from it's high sensitivity to possible nuclear and electron permanent electric dipole moments (EDM's). An EDM violates parity as well as time reversal and is therefore of fundamental importance. Searches for such EDM's are among the main research goals of the recently commissioned TRImP facility at KVI.
We consider laser cooling and trapping of alkaline-earth and similar atoms in a bichromatic field resonant to a closed optical transition 1S0→1P1 or 1S0→3P1. It is shown that new kinetic effects emerge compared to monochromatic fields, enabling the formation of a deep macroscopic trap capable of capturing and cooling neutral atoms to sub-Doppler temperatures. Such a purely optical macroscopic trap can serve as an alternative to the well-known magneto-optical trap and can be used in applications requiring minimization of the magnetic field in the cold atom cloud region. The obtained results are of interest for the new generation of quantum sensors and optical frequency standards.
We describe repumping and spectroscopy of laser-cooled strontium (Sr) atoms using the (5s5p) 3 P 2 –(5s4d) 3 D 2 transition. Atom number in a magneto-optical trap is enhanced by driving this transition because Sr atoms that have decayed into the (5s5p) 3 P 2 dark state are repumped back into the (5s 2 ) 1 S 0 ground state. Spectroscopy of 84 Sr, 86 Sr, 87 Sr and 88 Sr improves the value of the (5s5p) 3 P 2 –(5s4d) 3 D 2 transition frequency and determines the isotope shifts for the transition accurately enough to guide laser-cooling experiments with less abundant isotopes.
An ultra-stable optical clock based on coherent population trapping effect of alkaline-earth ions, such as Ca+^++, Sr+^++, Ba+^++, is analyzed here. The proposed transitions use the odd isotopes, so that the frequency shift is insensitive to the applied magnetic field. To enhance the signal, a large number of ions are trapped in a linear Paul trap, and laser cooled to crystallize such that the ions are in the Lamb-Dicke regime to avoid first order Doppler shift. Other relevant frequency shifts are also analyzed. Further techniques to improve the stability of the clock signal are discussed.
Abstract This study presents an achievement of laser cooling of alkaline-earth atoms in the Chinese Space Station’s strontium (Sr) atomic space optical clock. The system’s core components, physical unit, optical unit, and electrical unit, have a total volume of 306 L and a total mass of 163.8 kg. These compact and robust units can overcome mechanical vibrations and temperature fluctuations during space launch. The laser sources of the optical unit are composed of diode lasers, and the injection locking of slave lasers is automatically performed by a program. In the experiment, a blue magneto-optical trap of cold atoms was achieved, with the atom numbers estimated to be approximately (1.50 ± 0.13) × 10 6 for 87 Sr and (8.00 ± 0.56) × 10 6 for 88 Sr. This work establishes a foundation for atomic confinement and high-precision interrogation in space-based optical clocks and expands the frontiers of cold atom physics in microgravity.