Experimental Study On Loading ⁸⁸Sr Into An Optical Lattice And Probing Its Clock Transition

Atomic clocks have experienced the development worldwide for six decades since the invention of the first atomic clock in National Physical Laboratory in 1955. Clocks based on microwave oscillators referenced to atomic transitions now define the systéme International(SI) second, that is an unperturbed hyperfine transition of caesium(Cs) atoms which is interrogated by a radio frequency of 9,192,631,770 Hz. Cs fountain standard have demonstrated its best time and frequency measurements of a few parts in 1016. The progress of the laser cooling technique has enabled the progress of clocks operating on an optical frequency, and also the techniques both of super narrow line-width lasers and femtosecond optical frequency combs. The optical lattice clock with neutral atoms occupies an outstanding position in the research field of optical clocks, demonstrating the great potential on its performance. An optical lattice clock uses a stabilized laser referenced to many(about 104) alkaline earth atoms confined in an optical lattice(optical standing wave). The Doppler-as well as recoil-free spectrum of Ultra-narrow 1S0-3P0 clock transition is obtained and its extraordinary stability due to the larger number of absorbers that are simultaneously interrogated. State-of-art optical clocks have surpassed microwave clocks in this century. Optical clocks achieve accuracy and stabilities with 10-18, superior to primary microwave fountain standards, causing discussion to redefine the SI second based on an optical transition. Optical lattice Clocks employ various atomic species and reference transitions. Among these clocks, fermion 87 Sr is used to an optical reference and is investigated to our knowledge worldwide. The best clock is 87 Sr optical lattice clock until now. Besides, one of the largest proportion for 4 isotopes Sr, 88 Sr is also studied to develop the optical lattice clock, owing to its easier cooling process and higher cooling efficiency.An optical clock based on Strontium is being studied in the key Laboratory of time and frequency standards, located in National Time Service Center. We have prepared cold atomic samples, confined atoms in an optical lattice and probed the clock transition spectrum. This thesis emphasizes on the optimization in the first-stage cooling, the detail process of the second-stage cooling, the suppression of the intensity noise of the lattice laser, the confinment of atoms in an optical lattice and the precision measurement of the clock transition. The thesis are organized as follows:(1) Two stages of laser cooling. The first one is based on the strong 1S0-1P1 cycling transition at 461 nm introduced briefly and we focus on its optimization. That includes the second-dimensional cooling on the thermal atomic beam, resulting to improving atomic number and stabilizing frequencies of repumping lasers 707 nm and 679 nm, enabling our system more stable to operate for a long time. Followed by the weaker 1S0-3P1 intercombination transition at 689 nm, atoms are transferred from Blue MOT to Red MOT, reducing the atomic temperature from 5m K to 2μK. The transferring efficiency is about 15%.(2) The intensity noise suppression of the lattice laser resource. The laser resource for optical lattice here used in our experiment is diode laser. The output laser passes through an optical filter cavity with its characteristics of filtering high frequency noise. The cavity can be called a mode-cleaner too. By mode-matching between the cavity and the laser beam and by introducing the optical feedback to the cavity on the basis of the technology Pound-Drever-Hall(PDH), the cavity and the laser achieve to their resonance. Consequently, the intensity noise is measured by using a pair of balanced photodiode, reaching to Shot-Noise-Limit(SNL) about at 8MHz. Otherwise, the intensity noise of 813 nm laser not filtered by the mode-cleaner cannot reach to the SNL within the analysis frequency 30 MHz. The level of noise suppression can allow the 813 nm diode laser to be utilized in loading Sr cold atoms into the optical lattice without disturbance,which also be good at probing the signal of clock transition.(3) The confinement of atomic samples in one-dimensional optical lattice. The theory of optical lattices for optical clocks is analyzed and the trap depth of the lattice is calculated, also its longitude frequency and radial frequency. Besides, the Lamb-Dick parameter is evaluated for the trap. Cold atoms are loaded into an optical lattice with ~570ER trap depth formed by retroreflecting laser power. The power of the single beam laser is ~850m W and its waist is ~38μm, fixed at the magic wavelength 813 nm. After cold atoms trapped in the optical lattice, the lifetime of atoms in 1-D optical lattice is measured and the value is about 270 ms. The temperature and the number is approximately 3.5μK and 1.2×105 respectively. Besides, the effects on both of number and temperature by the power of the lattice laser 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 can provide a long interrogation time for probing the clock transition, furthermore making the foundation for developing the optical lattice clock of Sr atoms. The collision shift is one of the factors to limit the improvement of the clock’s uncertainty. Bose-Hubbard model is applied here to describe the bosonic system and to discuss the average number for per lattice site. According to control the trap depth, only one atom in per lattice can be realized to eliminate the collisional frequency shifts.(4) The precise measurement of the clock transition. A weak static magnetic field is applied to enable direct optical excitation of forbidden electric-dipole transitions, like alkaline-earth atoms, bosonic atoms 174 Yb and 88 Sr. In the thesis, the experimental implementation of this method and the clock laser 698 nm are combined to probe 88 Sr confined in 1-D optical lattice. In order to obtain the transition probability spectrum, the frequency of 698 nm laser is scanned. A 461 nm pulse laser is utilized to measure the atoms populated on the ground state. On the basis of several periodical scanning, the probability spectrum is obtained with the structure consisting of one carrier and two sidebands. According to the spectrum, the longitudinal frequency of 1-D optical lattice for our experiment is about 108 k Hz and its trap depth is 41μK. Recoil shift of 698 nm clock laser is about 4.73 k Hz, consequently the Lamb-Dicke parameter in our experiment is 0.2. The line-width of trapped 88 Sr clock transition spectrum is 187 Hz.