可搬运钙离子光钟的集成化激光冷却系统

Integrated Laser Cooling System for Transportable ^(40)Ca^(+) Ion Optical Clock

时间单位“秒”是人类计量最为精准的国际单位,目前国际单位制(SI)“秒”的定义是基于微波跃迁的铯原子(^(133)Cs)喷泉钟。光钟的参考跃迁频率为光学频率,理论上精度会比微波钟高4个数量级,因此可能成为未来的国际“秒”定义。可搬运光钟的研制使光钟迈进实际应用,除了在时间基准以及卫星导航方面的应用,也在验证洛伦兹不变性、基本物理常数是否随时间变化、引力波和暗物质探测等方面具有广泛的应用。针对可搬运钙离子光钟,设计了一套集成化激光冷却系统,利用Pound-Drever-Hall方法将钙离子光钟397nm和866nm冷却激光的频率锁定在一个超低热膨胀系数的Fabry-Perot振腔上,以获得窄线宽和低频率漂移的稳频激光。同时,实验中采用了基于FPGA的高分辨率时序控制系统,并采用“预冷和冷却”两步Doppler冷却方式更好地冷却离子。通过以上方法,将可搬运钙离子光钟的宏运动引起的二阶Doppler频移的不确定度抑制到小系数10^(-18)水平。

The“second”is the most accurate unit in the international system of units(SI)and is defined on the microwave transition of cesium atom(^(133)Cs)fountain clock.Optical clocks with quantum transition frequencies at higher optical frequencies are theoretically 4 orders of magnitude more accurate than microwave clocks,so it may become the future definition of the international“second”.The transportable optical clocks have led to more and more practical applications.In addition to applications in time benchmarking and satellite navigation,it is also a new means to verify Lorentz invariance,fundamental physical constants change with time,gravitational waves and dark matter detection.In this paper,an integrated laser cooling system for transportable ^(40)Ca^(+)ion optical clock is designed.The frequency of 397nm and 866nm cooling laser is locked in a Fabrie-Perot resonator with ultra-low thermal expansion coefficient(ULE)by Pound-Drever-Hall method to obtain a frequency stabilized laser with narrow linewidth and low frequency shift.At the same time,a high time resolution timing control system based on FPGA is used in the experiment,and the two steps of“pre-cooling and cooling”methods are used to better cool the ion.The uncertainty of the second order Doppler shift caused by the transportable ^(40)Ca^(+)ion optical clock secular motion is reduced to the fractional 10-18 level.