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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Near-recoil-limited temperatures obtained by laser trapping on the narrow /sup 1/S/sub 0/-/sup 3/P/sub 1/ intercombination transition of neutral strontium

Kurt R. Vogel, T. Dinneen, Alan C. Gallagher, J. L. Hall

Published Jan 22, 2003• 1 citation
Publisher

Abstract

About 2/spl times/10/sup 7/ neutral strontium atoms have been loaded into a magneto-optical trap (MOT) that uses the narrow (/spl Gamma/=2/spl pi/ 7 kHz) intercombination line at 689 nm as the cycling transition. The atoms are second-stage laser cooled to /spl sim/10 /spl mu/K from an initial 5 mK atomic sample (/spl sim/5/spl times/10/sup 7/ atoms), generated in a vapor cell MOT that employs the strong (/spl Gamma/=2/spl pi/ 32 MHz) cycling transition at 461 nm. To demonstrate the low temperatures of these atoms, optical Ramsey spectroscopy is performed upon the /sup 1/S/sub 0/-/sup 3/P/sub 1/ transition using only two excitation pulses; the time dependence of the fringe contrast indicates an RMS velocity of 3 cm/s for the 689 nm MOT-cooled sample.

DOI
10.1109/freq.1999.841399

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Physical Review Letters•Feb 8, 1999•376 citations
About 2/spl times/10/sup 7/ neutral strontium atoms have been loaded into a magneto-optical trap (MOT) that uses the narrow (/spl Gamma/=2/spl pi/ 7 kHz) intercombination line at 689 nm as the cycling transition. The atoms are second-stage laser cooled to /spl sim/10 /spl mu/K from an initial 5 mK atomic sample (/spl sim/5/spl times/10/sup 7/ atoms), generated in a vapor cell MOT that employs the strong (/spl Gamma/=2/spl pi/ 32 MHz) cycling transition at 461 nm. To demonstrate the low temperatures of these atoms, optical Ramsey spectroscopy is performed upon the /sup 1/S/sub 0/-/sup 3/P/sub 1/ transition using only two excitation pulses; the time dependence of the fringe contrast indicates an RMS velocity of 3 cm/s for the 689 nm MOT-cooled sample.
Summary form only given. All-optical production of quantum degenerate atomic gases has been one of the long-standing targets in laser cooling. We report a novel approach towards this direction based on a narrow-line laser cooling for strontium atoms. By employing two transitions with markedly different dipole moments, i.e., the allowed /sup 1/S/sub 0/-/sup 1/P/sub 1/ (/spl lambda/=461 nm, /spl gamma/=2/spl pi//spl times/32 MHz) and the spin-forbidden /sup 1/S/sub 0/-/sup 3/P/sub 1/ (/spl lambda/=689 nm, /spl gamma/=2/spl pi//spl times/7.6 kHz) transition, thermal strontium atoms were Doppler-cooled down to 400 nK or the photon recoil temperature, in a magneto-optical trap (MOT). Presents the corresponding energy levels for cooling and trapping. The use of a narrow spin-forbidden transition successfully reduced the radiation trapping effects at a high atom density over 10/sup 12//cm/sup 3/, thus enabling us to attain the hitherto high phase space density of 0.01 in a MOT.
We report the experimental realization of strontium magneto-optical trap (MOT) operating on the intercombination transition 1 S 0 − 3 P 1 at 689 nm, namely red MOT. A 689 nm laser used for cooling and trapping is injection locked to a master laser, whose linewidth is narrowed to 150 Hz by locking to a high finesse optical reference cavity. 88 Sr atoms pre-cooled and trapped by the broad 1 S 0 − 1 P 1 transition at 461 nm are transferred to the red MOT with the help of a time sequence controller. The transfer ratio is about 20% and the red MOT's temperature is estimated to be less than 20 μK by the time-of-flight (TOF) image analysis.
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 =
Direct observation and frequency measurement of /sup 1/S/sub 0//spl rarr//sup 3/P/sub 0 /line within an accuracy of 20 kHz is reported. The resonance frequency with an accuracy of /spl sim/ 100 kHz is reported by indirect measurement of the energy difference between /sup 1/S/sub 0/ and /sup 3/P/sub 0/ states. It is performed by successive frequency measurements of atomic transitions with higher linewidth, easier to observe: /sup 1/S/sub 0/ - /sup 3/P/sub 1/, /sup 3/P/sub 1/ - /sup 3/S/sub 1/ and /sup 3/S/sub 1/ - /sup 3/P/sub 0/ transitions. To perform direct observation of the clock transition, atoms are cooled to a temperature of /spl sim/ 2 mK in a magneto-optical trap (MOT). Although the saturation broadening of the transition is orders of magnitude smaller than the Doppler width (1.5 MHz), a detectable signal is achieved by inducing a small leak in the MOT with a laser tuned to resonance. Atoms accumulate in the metastable /sup 3/P/sub 0 /state where they escape the trap. The resonance is detected by a decrease in the MOT fluorescence signal with a contrast of 1%.
We report narrow-line laser cooling and trapping of strontium atoms down to the photon recoil temperature. 88Sr{}^{88}\mathrm{Sr}88Sr atoms precooled by the broad $^{1}S_{0}\ensuremath{-}^{1}P_{1}$ transition at 461 nm were further cooled in a magneto-optical trap using the spin-forbidden transition $^{1}S_{0}\ensuremath{-}^{3}P_{1}$ at 689 nm. We have thus obtained an atomic sample with a density over ${10}^{12}\mathrm{cm}{}^{\ensuremath{-}3}$ and a minimum temperature of 400 nK, corresponding to a maximum phase space density of ${10}^{\ensuremath{-}2}$ which is 3 orders of magnitude larger than the value that has been obtained by magneto-optical traps to date. This scheme provides us an opportunity and system to study quantum statistical properties of degenerate fermions as well as bosons.