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

Ultracold 88Sr atoms for an optical lattice clock

Thomas Legero, J. S. Raaj Vellore Winfred, F. Riehle, Uwe Sterr

Published May 1, 2007• 4 citations
Publisher

Abstract

This paper reports results of cooling and trapping of88Sr for future use in an optical lattice clock. Strontium atoms are cooled to ultra-cold temperatures using a two-stage cooling process. In the first stage, atoms are captured from a Zeeman-slowed atomic beam and cooled to 2 mK in a magneto optical trap operated on the1So-1P1transition at 461 nm. The second cooling stage utilizes the spin forbidden1So-3P1transition at 689 nm. To reach ultra-cold temperatures, 50 ms of broadband cooling is followed by 28 ms long single frequency cooling. The transfer efficiency from the first to the second cooling stage was measured to be 23%, leaving 7 x 106 atoms at a temperature of 1 muK. The atomic cloud is overlapped with a horizontally oriented 1-D far-off resonance optical dipole trap (FORT) at 813 nm with an effective potential depth of 25 muK. Due to equal light shifts of the1Soand the3P1state simultaneous Doppler cooling during the atom loading into the FORT is possible which leads to a transfer efficiency into the dipole trap of up to 60%.

DOI
10.1109/freq.2007.4319045

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European Frequency and Time Forum•Apr 1, 2010•0 citations
This paper reports results of cooling and trapping of88Sr for future use in an optical lattice clock. Strontium atoms are cooled to ultra-cold temperatures using a two-stage cooling process. In the first stage, atoms are captured from a Zeeman-slowed atomic beam and cooled to 2 mK in a magneto optical trap operated on the1So-1P1transition at 461 nm. The second cooling stage utilizes the spin forbidden1So-3P1transition at 689 nm. To reach ultra-cold temperatures, 50 ms of broadband cooling is followed by 28 ms long single frequency cooling. The transfer efficiency from the first to the second cooling stage was measured to be 23%, leaving 7 x 106 atoms at a temperature of 1 muK. The atomic cloud is overlapped with a horizontally oriented 1-D far-off resonance optical dipole trap (FORT) at 813 nm with an effective potential depth of 25 muK. Due to equal light shifts of the1Soand the3P1state simultaneous Doppler cooling during the atom loading into the FORT is possible which leads to a transfer efficiency into the dipole trap of up to 60%.
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.
The optical lattice clock with neutral atoms occupies an outstanding position in the research field of atomic clocks, demonstrating the great potential of its performance (like the uncertainty and the stability). At present, the optical lattice clock has realized a 10-18 level of its uncertainty. In this paper, we present the realization of loading bosonic atoms 88Sr (strontium, alkaline-earth metals) into a one-dimensional (1D) optical lattice in our laboratory. The optical lattice where the atoms are trapped can make the energy level shift, called Stark shift. But there is the special optical lattice operating at the “magic” wavelength for clock transitions (5s2) 1S0-(5s5p) 3P0, which can make the same Stark light-shift for both of them, indicating a zero light-shift relative to the clock. In our experiment, Sr atoms are cooled in a two-stage cooling and its temperature can be as low as 2 μK. Then these cold atoms are confined in the Lamb-Dicke region by the lattice laser output from an amplified diode laser operating at the “magic” wavelength, 813 nm. Experimentally, it is straightforward to provide 850 mW of lattice power focused to a 38 μm beam radius. After the cold atoms have trapped in the optical lattice, the lifetime of atoms in 1D optical lattice is measured to be 270 ms. The temperature and the number are about 3.5 μK and 1.2×105 respectively. Besides, effects of the power of the lattice laser on both the number and temperature are analyzed. The number changes linearly with the laser power, while there is no obvious influence on the temperature by the power. This original and special approach for atoms trapped in the optical lattice can provide a long interrogation time for probing the clock transition. Furthermore, it may be the foundation for developing our optical lattice clock of strontium atoms.
We report on our progress toward the realization of an optical frequency standard referenced to strontium intercombination lines. While first frequency measurement of the weakly allowed 1S 0- 3P 1 transition was done with saturation spectroscopy on a thermal beam, we are preparing the experimental setup for high resolution spectroscopy on ultracold atoms of the doubly forbidden 1S 0- 3P 0 line. Our current setup allows the capture more than 10 7 atoms at 1 μK in 500 ms in two stages of magneto-optical trapping. We also demonstrated the dipole trapping of ultracold strontium isotopes for accurate spectroscopy and collisional measurements. A semiconductor laser source at 698 nm has already been locked to a pre-stabilization cavity obtaining a fast linewidth of less than 1 kHz. Second stage stabilization of the clock laser to ultra-high finesse cavity is in development. Recent measurement of elastic and inelastic cross-sections of the two most abundant 88Sr and 86Sr isotopes are also presented.
We propose and demonstrate a novel far-off-resonance optical dipole trap (FORT) that is compatible with Doppler cooling. Strontium atoms are magneto-optically cooled and trapped by the spin-forbidden transition 1 S 0 - 3 P 1 at 689 nm, while the tight-focused infrared laser couples these two states to the respective dipole-allowed singlet and triplet states to generate the same amount of light shift. These designed light shifts for the cooling transition, enabling simultaneous Doppler cooling and dipole trapping, lead to the efficient loading of cooled atoms into the FORT with a few recoil energies. This scheme provides an important step towards the realization of quantum degenerate strontium atoms by purely optical means.
We report the experimental realization of88Sr magneto-optical trap (MOT) operating at the wavelength of 461 nm and 689nm. The blue MOT is loaded via a 32 cm long spin-flip type Zeeman slower which enhances the MOT population by a factor of 44. We have trapped 1.6×10888Sr atoms with the use of 679nm and 707nm repumping laser. The88Sr cloud has a temperature of about 2.3 mK, measured by recording the time evolution of the absorption signal. The 689nm laser used in our experiment has a line width of about 200Hz. To optimize the transfer efficiency, the laser spectrum is broadened by frequency modulation technique to about 2 MHz. The transfer efficiency from blue MOT to red MOT is about 20%.