小型化锶光钟物理系统的研制

光钟物理系统的小型化是制约可搬运光钟及空间冷原子光钟发展的重要因素.主要介绍了小型化锶原子光钟物理系统的研制实验.采用真空腔内置反亥姆霍兹线圈,构建一个小电流、低功耗及小体积的磁光阱.实验中测得真空线圈通电电流仅为2 A时,磁光阱中心区域轴向磁场梯度可达到43 Gs/cm,完全满足锶原子多普勒冷却与俘获对磁场梯度的要求.目前已经成功将锶原子光钟物理系统体积缩小至60 cm×20 cm×15 cm,约为实验室原锶光钟物理系统体积的1/10,并且实现了锶原子的一级冷却,测得俘获区冷原子团的直径为1.5 mm,温度约为10.6 mK.锶同位素~(88)Sr和~(87)Sr的冷原子数目分别为1.6×10~6和1.5×10~5.重抽运激光707和679 nm的加入,消除了冷原子在~3P_2和~3P_0两能态上的堆积,最终可将冷原子数目提高5倍以上.

Miniaturization of physics system in Sr optical clock

The compactness and robustness of the vacuum setup are the major limitations to develop transportable and space-borne optical clocks. The first step in the engineering challenge is to reduce volume and weight with respect to a stationary system. In this paper, we present the realization of a miniaturized vacuum system by building two anti-Helmholtz coils inside the vacuum magneto-optical-trap (MOT) chamber. The built-in coils are specially designed to minimize the distance between the coils, and in this way it is possible to reduce the current needed to realize a typical magnetic gradient of 40 Gs/cm required for blue MOT. When the MOT coil current is 2 A, an axial magnetic field gradient of 43 Gs/cm is obtained in the center of the MOT, which is enough for the first stage Doppler cooling. This novel design allows us to reduce size, weight and power consumption with respect to a traditional laser cooling apparatus, and simultaneously avoid complicating the water cooling equipment. Our vacuum system has a size of 60 cm×20 cm×15 cm, about 1/10 of the original system in the laboratory. In addition, the circularly polarized Zeeman slowing laser is sent to counter propagating atomic beam, and atoms at a few hundred meters per second are now routinely slowed down to velocities of tens of meters per second. As a result, about 16.4% of the atoms are actually trapped into the blue MOT. The final temperature of the blue MOT is approximately 10.6 mK, and the internal diameter is 1.5 mm by observing the expansion of the atoms from the MOT. The populations of cold atoms finally trapped in the MOT are 1.6×10⁶ of ⁸⁸Sr and 1.5×10⁵ of ⁸⁷Sr. The ¹S₀ → ¹P₁ transition used for the blue MOT is not perfectly closed due to the decay channel of the 5p¹P₁ → 4d¹D₂, and a part of atoms are stored in the ³P₂ and ³P₀ states. To prevent the atoms from losing, 707 and 679 nm repumping lasers are employed to recycle these atoms in the ³P₁ state, and then the atoms decay to the ground state ¹S₀. Now the typical number of loaded atoms dramatically increases by 5 times compared with before. The slowing efficiency of Zeeman slower is also optimized for the operation with deceleration related to the parameter of magnet length, which uses more effectively available magnetic field distribution, in contrast to the usual constant deceleration mode. Our future work will focus on constructing a Zeeman slower combined with permanent magnets or an electric magnet for better tuning of the magnetic field.