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Yuhang jice jishu

Laser Cooling and Optical Lattice Trapping of Strontium Atoms

Zhanjun Fang

Published Jan 1, 2011• 1 citation
OpenAlex

Abstract

The Strontium cooling experiment realized to transfer atoms from the first stage broadband cooling to the second stage narrow line cooling.The temperature of the atoms after the second stage cooling is about 2μK.The lattice laser is on during the cooling process,so the atoms are loaded into the lattice when their temperature is low enough.cooled by narrow line cooling.

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The Strontium cooling experiment realized to transfer atoms from the first stage broadband cooling to the second stage narrow line cooling.The temperature of the atoms after the second stage cooling is about 2μK.The lattice laser is on during the cooling process,so the atoms are loaded into the lattice when their temperature is low enough.cooled by narrow line cooling.
To decrease the Doppler broadening and frequency shift,reducing the atomic velocity by laser cooling is necessary.Noting that through the first stage cooling,atoms can only be slowed down to the temperature at the mK level,which is too high to be loaded to the optical lattice,We made further efforts to slow down the velocity of the atoms,and the second stage cooling was implemented.When the first stage cooling stopped,using the 689 nm laser corresponding to(5s2)1S0→(5s5p)3P1further cooling the atoms could attain the temperature at as low asμK level.Using the timing sequence accurately controlling the time of interaction between the electromagnetic field and atoms.calculated the temperature through timeof-flight method.Dropping time of cold atoms in the MOT was accurately controlled,and the atomic photos were taken at 0ms and 20 ms.By analyzing the data,the atomic temperature after the second stage cooling is 4.39μK,uncertainty is only 0.19μK and the number of the atoms 1.2×107.The temperature and number of the Sr atoms offer the important reference for estimating the SNR of signal,which is also the foundation for achieving the high-precision standard of time and frequency.
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%.
The presence of ultra-narrow inter-combination spectroscopic lines in alkaline earth elements places them as promising candidates for optical atomic clocks, quantum computation, and for probing fundmental physics. Doppler cooling of these atoms is typically achieved through two subsequent stages: the initial cooling is on the 1s0-1p1 transition followed by cooling using the narrow-line 1s0-3p1 transition. However, due to significantly lower linewidth of the second stage cooling transition, efficient transfer of atoms into the second stage becomes technically challenging. The velocity distribution of the atoms after the first stage of cooling is too broad for atoms to be captured efficiently in the second stage cooling. As a result, the capture efficiency of atoms into the second stage Magneto-Optical Trap is low, even if the linewidth of the second stage cooling laser is artificially broadened.
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.
A compact laser cooling system for transportable optical lattice clocks based on cold 171 Yb atoms has been presented. The system size has been reduced by eliminating the first cooling stage and using compact configurations for the main subsystems of the laser cooling system. A Zeeman slower has been modeled and developed, reducing the velocity of atoms from a thermal source to a level sufficient for their direct capture in a second-stage magneto-optical trap, avoiding the first cooling stage. A method for estimating the number of atoms and their temperature in the magneto-optical trap is presented. The performance of the compact laser cooling system with the developed Zeeman slower has been experimentally confirmed: ~10 6 171 Yb atoms with a temperature of ~35 μK have been collected in ~1 s. The achieved parameters allow for loading of ~4000 atoms into the optical lattice.