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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Recent progress of laser cooling for neutral mercury atom

Kangkang Liu, Ru-Chen Zhao, Xiaohu Fu, Jinmeng Hu, Yan Feng, Zhen Xu, Yuzhu Wang

Published Nov 18, 2014• 0 citations
Publisher

Abstract

Mercury is the heaviest stable atom that could be laser cooled, and have a large nuclear charge number. So it has a distinct advantage in quantum precision measurement such as fine-structure constant α and permanent electric dipole moment. Due to its insensitivity of black body radiation, atomic mercury is a good candidate of optical clock. Here we report our recent development of laser cooling of neutral mercury atom. By cooling the mercury source to about -70°C, an ultra-high vacuum system was realized to produce ultracold mercury atoms. The commercial frequency quadrupled semiconductor laser is locked on the cooling transition (1S0-3P1 transition, wavelength of 253.7 nm) by sub-Doppler frequency modulation spectroscopy. By the modification with feed-forward method, the UV laser becomes faster tunable and more stable. A folded beam configuration was used to realize the magneto-optical trap (MOT) because of the shortage of cooling laser power, and the ultracold mercury atoms were observed by fluorescence detection. All of six rich abundant isotopes have been observed, and the atom number is about 1.5×106 with density of 3.5×109 /cm3 for 202Hg. With optical shutter and the programmable system to control the time sequence, the temperature of ultracold atoms can be measured by time of flight method. To enhance the laser power, a 1014.8 nm fiber laser amplifier was developed, which can work at room temperature. After two stages of frequency doubling, about 75 mW of 253.7 nm UV laser were generated, and the saturated absorption spectroscopy of mercury atom was also observed. More power of UV laser could help to trap more atoms in the future. These works laid a good foundation to realize the mercury lattice clock.

DOI
10.1117/12.2073328

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Mercury is the heaviest stable atom that could be laser cooled, and have a large nuclear charge number. So it has a distinct advantage in quantum precision measurement such as fine-structure constant α and permanent electric dipole moment. Due to its insensitivity of black body radiation, atomic mercury is a good candidate of optical clock. Here we report our recent development of laser cooling of neutral mercury atom. By cooling the mercury source to about -70°C, an ultra-high vacuum system was realized to produce ultracold mercury atoms. The commercial frequency quadrupled semiconductor laser is locked on the cooling transition (1S0-3P1 transition, wavelength of 253.7 nm) by sub-Doppler frequency modulation spectroscopy. By the modification with feed-forward method, the UV laser becomes faster tunable and more stable. A folded beam configuration was used to realize the magneto-optical trap (MOT) because of the shortage of cooling laser power, and the ultracold mercury atoms were observed by fluorescence detection. All of six rich abundant isotopes have been observed, and the atom number is about 1.5×106 with density of 3.5×109 /cm3 for 202Hg. With optical shutter and the programmable system to control the time sequence, the temperature of ultracold atoms can be measured by time of flight method. To enhance the laser power, a 1014.8 nm fiber laser amplifier was developed, which can work at room temperature. After two stages of frequency doubling, about 75 mW of 253.7 nm UV laser were generated, and the saturated absorption spectroscopy of mercury atom was also observed. More power of UV laser could help to trap more atoms in the future. These works laid a good foundation to realize the mercury lattice clock.
Neutral mercury atom is one of good candidates of optical lattice clock. Due to its large atomic number, mercury atom is insensitive to black body radiation, which is the severe limitation for the development of optical clocks. However, the challenge of neutral mercury lattice clock is the requirement of high power deep-UV lasers, especially for both the cooling laser and the lattice laser. Here, we report the recent progress of neutral mercury lattice clock in SIOM, including the development for laser cooling of mercury atom and the cooling laser system with fiber laser amplifier. We have realized the magneto-optical trap of mercury atoms and measured the parameters of cold mercury atoms. A home-made external cavity diode laser works as a seed laser for a room temperature 1014.8 nm fiber laser amplifier. A new efficient frequency-doubling cavity from 1015 nm to 507 nm has been developed.
This thesis explores the development and application of laser-cooled mercury atoms for precision measurements relevant to fundamental physics. A primary motivation is the search for a permanent electric dipole moment in mercury, that could shed light on the baryon asymmetry of the universe. We present the design and construction of a custom experimental apparatus for producing and trapping cold mercury atoms, detailing challenges associated with working with mercury in an ultra-high vacuum environment and considering deep-ultraviolet optics. A magneto-optical trap operating on the 1S0-3P1 intercombination line at 254 nm is realized for all seven stable isotopes, achieving atom numbers up to 5x107 and phase-space densities up to 1x10-6 suitable for optical dipole trap loading. Cooling efficiency for all bosonic and fermionic isotopes are characterized over a wide parameter space, and we show sub-Doppler cooling in 199Hg and 201Hg. The successful loading of mercury into a high-power optical dipole trap is demonstrated, forming the basis for future electric dipole moment experiments of laser cooled mercury atoms. Prospects of evaporatively cooling mercury down to quantum degeneracy could allow for a quantum-enhanced metrology tool kit for the planned measurements. Apart from this, we are exploring the use of ultracold fermionic gases made of mercury as a quantum simulator for impurity physics. As a promising versatile and tunable platform for studying impurity-bath interactions, we are assessing the feasibility of observing Friedel oscillations in ultracold quantum gases.Further, we perform precision isotope shift spectroscopy on several dipole-allowed atomic transitions in mercury. Via King plot analysis this allows investigations of the mercury nuclear structure – also relevant for electric dipole moment bounds – and lays the foundation for probing a potential fifth force carrier coupling electrons to neutrons. We also propose upgrades to the machine for improvements in laser cooling, extensions of the spectroscopy search and a proposed experimental setup for probing the atomic electric dipole moment in an optical dipole trap of cold mercury atoms.
Optics lattice clock is a hot topic in the researches of frequency standard and metrology. Neutral mercury atom is one of the most promising candidates for optical lattice clock. Due to its large atomic number, mercury atom is insensitive to black body radiation, which is the severe limitation for developing the optical lattice clocks. To realize the optical lattice clock of neutral mercury atoms, the first step is to implement laser-cooling and trapping of neutral mercury atoms. The cooling transition of mercury atom is 1S0-3P1 transition. The wavelength is 253.7 nm, the line width is 1.27 MHz, and the saturation intensity is 10.2 mW/cm2. Quantum projection noise (QPN) is an important parameter that affects optical lattice clock. Increasing the loading rate of magneto-optical trap (MOT) can help lower the QPN, thereby improving the performance of optical lattice clock. In this work, we calculate the scattering force of deep UV cooling laser, which is exerted on mercury atom in our single chamber MOT, and numerically simulate the one-dimensional motion of the atom in the MOT. It gives us the capture velocity under optimized parameters of the MOT. Then we calculate the loading rate of three-dimensional MOT by a high efficient random sampling method. According to the rate equation of MOT, the loading rate is proportional to the atom number of the steady state, which is the accessible parameter in the experiment. An experimental setup of MOT is established with a high vacuum system and a frequency quadrupled semiconductor laser system. The fluorescence imaging on an EMCCD gives the atom number in the MOT. We also calibrate the vapor density of background mercury gas in the vacuum, and measure the atom number in a steady MOT. We numerically simulate and experimentally study the influences on the atom number on the parameters of MOT, such as laser intensity, laser detuning and magnetic field gradient. The calculated results are in consistent with the experimental results. We also find the optimized parameters to maximize the loading rate. The numerical simulation also gives some results beyond current experimental condition, especially for laser intensity. From the simulations we obtain the optimized MOT parameter and relation for laser cooling of neutral mercury atom. Especially, current deep UV cooling laser with 20 mW of power does not have enough power to enhance the loading rate. These results are rather valuable for designing the next generation of optical lattice clocks for neutral mercury atoms.
Due to its low sensitivity to blackbody radiation, neutral mercury is a good candidate for the most accurate optical lattice clock. Here we report the observation of cold mercury atoms in a magneto-optical trap (MOT). Because of the high vapor pressure at room temperature, the mercury source and the cold pump were cooled down to −40 °C and −70 °C, respectively, to keep the science chamber in an ultra-high vacuum of 6 × 10 −9 Pa. Limited by the power of the UV cooling laser, the one beam folded MOT configuration was adopted, and 1.5 × 10 5 Hg-202 atoms were observed by fluorescence detection.
The optical lattice clock develops fast in recent years, and its stability and accuracy has already surpassed the microwave fountain clock and the optical clock based on ion trap. The mercury (Hg) lattice clock has the possibility to make a breakthrough in the limitation of blackbody radiation shift for strontium (Sr) and ytterbium (Yb) lattice clock, and reach the uncertainty in E-18 level at room temperature. For the purposes of laser cooling of mercury atoms and interrogating of the clock transition from1S0state to3P0state, a high-power cooling laser at 253.7 nm and an ultra-stable Hz-level linewidth laser at 265.6 nm are required, respectively. Here, we report our development of these laser sources for a neutral Hg lattice clock.