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Coherent Spectroscopy of Ultra-Cold Mercury for the UV to VUV

R. Jason Jones

Published Nov 20, 2015• 0 citations
OpenAlex

Abstract

Abstract : Narrow UV transitions in atomic Hg can be utilized for a high quality atomic frequency standard. The primary goal of the research supported by this grant was the development of a laser-cooled source of ultracold Hg atoms and the spectroscopic study of the doubly-forbidden clock transition to better understand its potential as a future atomic clock. During this funding period a novel UV laser system was developed to efficiently cool and trap atomic Hg to temperatures below 100 micro Kelvin. The narrow 1S_0 to 3P_0 clock transition was measured and used to determine the temperature of the Hg ensemble. The grant supported the PhD research of two students

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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.

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Physics-Uspekhi•Feb 14, 2016•22 citations
Abstract : Narrow UV transitions in atomic Hg can be utilized for a high quality atomic frequency standard. The primary goal of the research supported by this grant was the development of a laser-cooled source of ultracold Hg atoms and the spectroscopic study of the doubly-forbidden clock transition to better understand its potential as a future atomic clock. During this funding period a novel UV laser system was developed to efficiently cool and trap atomic Hg to temperatures below 100 micro Kelvin. The narrow 1S_0 to 3P_0 clock transition was measured and used to determine the temperature of the Hg ensemble. The grant supported the PhD research of two students
Abstract : Research on this AASERT grant was focussed on the optical spectroscopy of trapped cold hydrogen for use as a spectroscopic tool and as a means of studying the gas. The grant supported research by two graduate students. During the grant an optical signal on the 1S - 2S transition in cold trapped hydrogen was observed, opening the way to a new form of ultra high resolution spectroscopy. The optical signal will also provide a valuable tool for manipulating a Bose condensate in hydrogen. The condensate can absorb two photons form a single running wave. The recoil is enough to sweep the atoms out of the trap, creating a new type of atom laser beam.
Abstract : With the funding provided under this grant, we have procured equipments for creating a sample of 87Rb atoms with ultracold temperature (35 nano-Kelvin) and high density (10(exp 12 /sq cm), using Raman cooling via velocity selective coherent population trapping. We have already been able to create a magneto-optic trap (MOT) of 87Rb atoms, loaded from background rubidium vapor inside the trap chamber. We have achieved a density close to 10(exp 12) /sq cm, at a temperature of 80 mu-K, which is typical of such a trap. The next step is to load the trap from a chirp-slowed atomic beam, and use a repumping beam which is dark at the center. This is expected to increase the density to about 10(exp 12) /sq cm. We will then Raman- cool the sample to about 35 nano-Kelvin, thus yielding a sample for a rubidium fountain clock. The fountain clock loaded from this source is expected to have a short term accuracy that is more than two orders of magnitude better than existing cesium fountain, which are already a factor of two better than existing cesium beam clocks used as primary frequency standards.
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.
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.
We review the current research on precision spectroscopy and quantum optics applications of laser-cooled lanthanides. We discuss the specific electronic structure of hollow atoms, which determine prospects for application in optical frequency standards and in quantum simulators based on spin interactions in optical lattices. Using the example of the thulium atom, we describe the specifics of laser cooling, optical lattice trapping techniques, and clock transition spectroscopy using spectrally narrow lasers.