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IEEE Transactions on Instrumentation and Measurement

Cold strontium atoms for an optical frequency standard

Irène Ventrillard, Audrey Quessada, Richard P. Kovacich, J.‐J. Zondy, Arnaud Landragin, Giorgio Santarelli, Andr  Clairon, P. Lemonde

Published Apr 1, 2003• 1 citation
Publisher

Abstract

We report the capture of cold strontium atoms in a magnetooptical trap at a rate of 3/spl times/10/sup 10/ atoms/s. This is achieved with high power laser light at 461 nm generated by sum frequency mixing in a potassium titanyl phosphate (KTP) crystal using diode lasers at 813 nm and a Nd:YAG laser at 1064 nm. Up to 115 mW of blue light is obtained. Tuned to the /sup 1/S/sub 0//spl rarr//sup 1/P/sub 1/ of /sup 88/Sr, this light is used both to slow down an atomic beam in a Zeeman slower and to capture the atoms in a magnetooptical trap. Trapping of /sup 87/Sr and /sup 86/Sr is also reported.

DOI
10.1109/tim.2003.811660

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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)•Jan 22, 2003•1 citation
We report the capture of cold strontium atoms in a magnetooptical trap at a rate of 3/spl times/10/sup 10/ atoms/s. This is achieved with high power laser light at 461 nm generated by sum frequency mixing in a potassium titanyl phosphate (KTP) crystal using diode lasers at 813 nm and a Nd:YAG laser at 1064 nm. Up to 115 mW of blue light is obtained. Tuned to the /sup 1/S/sub 0//spl rarr//sup 1/P/sub 1/ of /sup 88/Sr, this light is used both to slow down an atomic beam in a Zeeman slower and to capture the atoms in a magnetooptical trap. Trapping of /sup 87/Sr and /sup 86/Sr is also reported.
As a key component of a strontium based optical clock, we have developed an apparatus which allows laser cooling and trapping of more than 10/sup 7/ /sup 88/Sr atoms to temperatures below 1 /spl mu/K. Using this system, we have performed the first high accuracy measurement of the /sup 88/Sr /sup 1/S/sub 0/-/sup 3/P/sub 1/ clock transition using an ultra-cold sample. Detailed descriptions of both the measurement technique and corresponding systematic uncertainties is provided. In addition, by varying the density of the ultra-cold /sup 88/Sr sample over a range of three orders of magnitude we obtain the first definitive measurement of a density-related frequency shift and linewidth broadening of an optical clock transition in an ultra-cold alkaline earth system. Spectroscopy of the /sub 1/S/sub 0/-/sup 3/P/sub 0/ transition is currently underway using this narrow probe laser and an octave spanning femtosecond laser. Our most recent results are presented. The proposed measurement technique, accuracy, and stability of an optical clock based on the EIT (electromagnetically-induced-transparency) measurement in /sup 88/Sr are described.
Optical atomic clocks have proven to be the most stable clocks with the lowest systematic uncertainty which has now reached a record level of (9.4 X10/(^{19}/)). Thus optical clocks now surpass the best fountain Cs atomic clocks by three orders of magnitude which opens the prospect of using optical clocks as frequency standards for timekeeping in the near future. \n \nThis thesis describes the progress that has been made towards a stationary optical lattice clock based on strontium atoms, and reports the results of an ongoing characterization. \n \nAn experimental apparatus was designed and built to efficiently slow, cool, and trap the strontium atoms which represent the heart of the strontium optical lattice clock discussed in this thesis. The novel aspects of the experimental development described in the thesis comprise a frequency doubling cavity for producing 461 nm laser light employed for cooling and trapping the strontium atoms in the first stage cooling. This cavity provided sufficient power for cooling and trapping the strontium atoms for up to 300 mW of 461 nm laser light. Furthermore, a new design of Zeeman slower based on spherical permanent magnets was produced, built and characterized. \n \nThe slower is compact, tunable, easy to assemble and maintain, and cost-effective as the permanent magnets do not require any power supply or water cooling. It is suggested that this kind of Zeeman slower constitutes an ideal solution for portable optical clock systems. A blue magneto-optical trap was realized for the first time with such a Zeeman slower. With about 30 mW of a 461 nm laser, up to 6X10/(^8/) atoms were captured.
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.
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.