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Acta Sinica Quantum Optica

Achieving the Second Doppler Cooling and Measuring the Temperature of Strontium Atoms

Yuli Xie

Published Jan 1, 2015• 0 citations
OpenAlex

Abstract

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.

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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.
After the realization of Doppler cooling based on(5s2)1S0→(5s5p)1P1 in alkaline-earth metal strontium,the temperature of two isotopes of strontium which is 88Sr(bosons)and 87Sr(fermions) are measured using time of flight and expansion measurement.And the result of the two isotopes are～9 mK(88Sr) and～2 mK(87Sr).According the theory of Doppler cooling limited temperature,the experimental results are analyzed.This can be the well basis for the next step,1S0→3P1 narrow line cooling of strontium.
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 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.
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 advent of the laser opened the way to the development of powerful new methods for manipulating and cooling atoms which were exploited in the realization of Bose–Einstein condensation in alkali atom vapours. To set the stage we describe a typical experiment, which is shown schematically in Fig. 4.1. A beam of sodium atoms emerges from an oven at a temperature of about 600 K, corresponding to a speed of about 800 m s –1 , and is then passed through a so-called Zeeman slower, in which the velocity of the atoms is reduced to about 30 m s –1 , corresponding to a temperature of about 1 K. In the Zeeman slower, a laser beam propagates in the direction opposite that of the atomic beam, and the radiation force produced by absorption of photons retards the atoms. Due to the Doppler effect, the frequency of the atomic transition in the laboratory frame is not generally constant, since the atomic velocity varies. However, by applying an inhomogeneous magnetic field designed so that the Doppler and Zeeman effects cancel, the frequency of the transition in the rest frame of the atom may be held fixed. On emerging from the Zeeman slower the atoms are slow enough to be captured by a magneto-optical trap (MOT), where they are further cooled by interactions with laser light to temperatures of order 100 μK. Another way of compensating for the changing Doppler shift is to increase the laser frequency in time, which is referred to as ‘chirping’. In other experiments the MOT is filled by transferring atoms from a second MOT where atoms are captured directly from the vapour.