Study On The Key Techniques Of The Absolute Frequency Measurement In Cold Ytterbium Optical Clocks

The "second",the unit of time,is one of the seven base units of the International System of Units?SI?with the highest measurement precision up to now.The definition of the "second"went through a series of changes.From 1895 to 1960,the "second"was considered to be the fraction 1/86400 of the solar day.However,the rotational changes of the earth cannot satisfy the higher precision.So in 1960,the 11^thh General Conference Weights and Measures?CGPM?adopted the definition based on the tropical year of 1900 as 1 s=1 year/31556925.9747.The development of the atomic standard of time and frequency realized and reproduced much more accurate and precise time.In 1967,the 13^th CGPM replaced the definition of the "second"by the atomic time,where the "second"is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the unperturbed Caesium 133 atom.The "second"has been defined as a fixed numerical value,which is realized by the Caesium fountain clocks?primary clocks?around the world.Subsequently,the ComitéInternational des Poids et Mesures?CIPM?has adopted various secondary representations of the "second",based on a selected number of spectral lines of atoms,ions.These unperturbed frequencies can be determined with a relative uncertainty not lower than that of the realization of the "second".For about 50 years,the microwave frequency standards keep on developing,whose uncertainty and long-term instability are at the level of 10^-16.In the last decade,the atomic clocks based on optical transition has experienced a rapid development.The uncertainty and instability of the optical clocks could reach to the level of 10^-18 in the fractional frequency units,which have surpassed the microwave clocks.These atomic frequency standards are expected as the definition of the next SI "second".There are two types of optical clocks,one is the ion clock based on electromagnetic traps,while the other is the neutral atomic clock based on an optical lattice.The optical lattice clock interrogates a number of atoms simultaneously and has a higher signal-to noise ratio leading to a lower instability.The absolute frequency measurement is an important part of the research in the optical clocks.The worldwide comparisons can be realized throught the absolute frequency measurement,which can test the repeatability and reliability of the clocks.The frequency of an optical clock is five order of magnitude higher than that of a microwave clock.The invention and applications of the frequency comb linked the microwave and optical frequency.The absolute frequency measurement of an optical clock is to measure the optical frequency against the SI "second",which normally needs to lock the optical frequency to the atomic transition.The comb links the optical and microwave frequency standards.In this way,the optical frequency standards that is referenced to the atomic transition can be traced to the SI "second".The development of optical clocks not only improves the precision of the next generation time and frequency standards,but also applies to many frontier science researches such as the gravitational redshift measurement,the variation of the fundamental constants with time,tests of relativity and so on.The cold Yb optical clock is a neutral atomic clock that is similar to the Sr,Hg and Mg optical lattice clock.The frequency of the ¹S₀-³P₀ transition of ¹⁷¹Yb trapped in an optical lattice is a secondary representation of the SI "second".In the last decade,the State Key Laboratory of Precision Spectroscopy in East China Normal University?ECNU?is dedicated to build two cold Yb optical clocks and has finished the closed-loop locking and systematic evaluation.The fractional frequency instability is better than 4×10^-17 after 5000 s averaging time and the systematic uncertainty is1.7×10^-16.Nowadays,the absolute frequency measurement of the cold Yb optical clocks is in the state of study,which is the main part of this thesis.This thesis is an experimental and theoretical study on the key techniques of the absolute frequency measurement.The cold atom system is the key part of optical clock.Reliably cooling,trapping and detecting the cold atom system are important to the stable operation of optical clock.Firstly,the deep study on the laser cooling and trapping of Yb atoms is present.Specially,the dynamics of cold Yb atoms in the one-dimensional optical lattice is present experimentally and theoretically.The cooling,trapping and state preparation of the Yb atoms are experimentally realized.And the clock transition spectrum,closed-loop locking and synchronous comparison between two clocks are experimentally realized.Frequencies of repumping lasers at649 nm and 770 nm and lattice laser at 759 nm are synchronously locked to a single cavity by the PDH technique,which can reduce the complexity of optical clocks,improve the integration and operation robustness,which is helpful to build small and transportable optical clocks.Secondly,the new ultra-stable clock laser is biult.The stability of the clock laser has a direct influence on the clock short-term stability.The non-continuous interrogation of the atomic system in the optical clock results in the Dick effect and the clock laser noise limits the clock stability.An improvement of the clock laser stability could realize a better short-term clock stability,which speeds up for evaluating the systematic shifts and measuring the other qualities.Locking the fundamental laser at 1156 nm of the clock laser at 578 nm to a 30cm horizontal ULE cavity is realized and the technical noises are reduced by using the vibration and acoustic isolation room,the fiber phase noise cancellation,the power stability and the temperature control of EOM.Lastly,the preliminary study on the absolute frequency measurement of cold Yb optical clock is present.We measure the optical frequency by using the fiber comb and build a local time and frequency standard.The offset frequency of the fiber comb is locked to a 35 MHz synthesizer referenced to the H-maser and the repetition frequency could be locked to a 250 MHz synthesizer.To acquire the narrow-linewidth comb and improve the measurement precision,the beat signal between the comb and an ultrastable laser at 578nm is locked to a 35 MHz synthesizer referenced to the H-maser.Based on the satellite link,we compare the local East China Normal University H-maser with the National Institute of Metrology of China time and frequency standard and build the local time and frequency standard.The experimental approach of the absolute frequency measurement is present according to the experimental condition and we will finish this work in the future.