| As the candidate of the next generation frequency standard, optical clock is superior in uncertainty and instability than the Cesium frequency standard in operation. Precision time and frequency standard can be the reliable reference for timekeeping, fundamental physics and even further scientific research.There are two main kinds of optical clocks. One is the ionic optical clock, which confined single-ion in a quadruple trap and it can reach the uncertainty of 10-18 magnitude; confined by the single ion can be interrogated in each experimental cycle, the instability is relatively higher for short averaging time. Another kind of optical clocks is the neutral atomic optical clock, which trapped thousands of atoms in the optical lattice. In the benefit of the proposal of optical lattice, large number of atoms are trapped in Lamb-Dicke region and separated in different lattice sites, which can not only suppress the Doppler shift and recoil shift, but also reduce the collisional frequency shift. The interrogation of thousands of atoms at the same time ensure that the instability to be several magnitude lower than that of ionic clocks. Nowadays, both the uncertainty and instability of strontium (Sr) optical lattice clocks reach at the level of 10-18 magnitude. While the instability of ytterbium (Yb) optical lattice clocks have already reach to 10-18, but the uncertainty is still need to be reduced.The State Key Laboratory of Precision Spectroscopy and Department of Physics in East China Normal University has built two ytterbium optical lattice clocks Yb-1 and Yb-2. And on the basis of achieving the two stages laser cooling and trapping, we loaded the cold atoms into optical lattice, and also interrogated the narrow clock-transition spectrum. Recently, we realize the frequency locking of the ultranarrow linewidth clock laser to the cold atoms transition of both Yb-1 and Yb-2.In this work, we first present the improvements of the Yb-1 optical lattice clock. We improved the vacuum system by adding a titanium sublimation pump to sustain the vacuum environment of the MOT (Magneto-Optical-Trap) combined with two ionic pumps. And thus the lifetime of the MOT is increased, so does the transfer efficiency from the 1st stage MOT to the 2nd stage MOT. We also carefully optimize the 2nd stage MOT, and measured the dependence of the cold atom temperature, atom number and density on the 2nd stage cooling laser detuning, intensity and MOT magnetic field gradient. According to the experimental results, we can find the optimal experimental conditions and achieved ultracold atoms of 6μK, which is approach to the Doppler limit temperature of this transition, and loaded the cold atoms into optical lattice for spectrum interrogation. The linewidth of the Yb-1 and Yb-2 clock transition spectrum are 56 Hz and 6 Hz, respectively.Collisional frequency shift is one of the main aspects that restrict the reducing of uncertainty to a lower magnitude in the estimation of the systematic uncertainty. So how to minimize and eliminate the collisional frequency shift is a crucial topic to be considered. In this work, we theoretical analyzed the collisional frequency shift in 17IYb optical lattice clock. Restrained by the Pauli Exclusion Principle, the identical fermion cannot occupy the same state, thus there exist no collisional frequency shift in fermion optical lattice clocks. On the benefit of this, fermion optical lattice clocks are popular in many laboratories. But recent research manifest that there exist collisional frequency shift in fermion optical lattice clocks, which is caused by the inhomogeneous excitation. This inhomogeneous-excitation collisional frequency shift mainly depends on the cold atom temperature and the misalignment angle between the lattice laser and the clock laser. According to the theoretical analysis of the inhomogeneous excitation in Sr optical lattice clock, and suppressing the collisional shift in a strong interacting optical lattice by tightly confining atoms in an array of quasi-one dimensional (1D) potentials formed by a 2D optical lattice, and the collisional frequency shift is restricted in 10-17 magnitude.However, the ytterbium atoms are usually hotter than the strontium atoms after second-stage laser cooling. For instance, the typical temperature of cold fermionic Sr atoms is about several μK with its Doppler limit temperature of 0.4 μK, and the temperature of the fermionic Yb atoms is a few tens of μK with its Doppler limit temperature of 4.5 μK. It means that the analysis of inhomogeneous-excitation collisional frequency shift in Yb optical clocks will be more important than that in Sr optical clocks. Therefore we theoretically analysis the dependences of the inhomogeneous-excitation shift on the ground state fraction under different cold atom temperatures, atom numbers, lattice trap depths and unequal transverse and longitudinal temperatures in 171Yb optical lattice clocks. We also estimate the collisional frequency shift under certain conditions and suggest that the most efficient method to minimize the collisional frequency is further cooling. The results show that the uncertainty of the ytterbium clocks, contributed by the inhomogeneous excitation, can be reduced to 10-19 or even lower under certain conditions.We also realized the frequency locking of both the Yb-1 and Yb-2 to the clock transition spectrum. The short term stability of the local clock laser is superior to that of cold atoms systems, but the long term stability is on the contrary. Therefore locking the local clock laser frequency to the atomic transition spectrum can realize the whole clock system long term stable operation. The instability is 1.47×10-13/(?)τ and 7.35×10-15@ 400s for Yb-1;4.54x 10-15/(?)τ and 1×10-16@ 1000s for Yb-2, here τ is the averaging time.The DS 345 signal generator is used to synchronize the sample clock of Yb-1 and Yb-2. Therefore the two clocks can be controlled to start and stop exactly. Synchronization interrogating the clock-transition spectrum of the two clocks can eliminate the common-mode noise, and achieve high stability for long term operation. Finally, we estimate the frequency shift and systematic uncertainty of both Yb-1 and Yb-2, including the collisional frequency shift, black body radiation shift, Zeeman shift and Stark shift. These are important for further improvement of the performance of the clocks. |
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