10 Essen L, Parry J. Atomic standard of frequency and time interval. Nature, 1955, 176: 280-284？？
[4]
11 Ramsey N. A molecular beam resonance method with separated oscillating fields. Phys Rev, 1950, 78: 695-699？？
[5]
12 Chu S, Hollberg L, Bjorkholm J, et al. Three-dimensional viscous confinement and cooling of atoms by resonance radiation pressure. PhysRev Lett, 1985, 55: 48-51？？
[6]
13 Zacharias J, Yates G, Haun R. An atomic frequency standard. Proc IRE, 1955, 43: 364-368
[7]
15 Parker T. Long-term comparison of caesium fountain primary frequency standards. Metrologia, 2010, 47: 1-10？？
8 13th CGPM. Resolution 1 (CR.103): SI unit of time (second). Metrologia, 1968, 4: 41-45
[18]
9 Rabi I, Millman S, Zacharias J, et al. The molecular beam resonance method for measuring nuclear magnetic moments: The magneticmoments of 3Li6, 3Li7 and 9F19. Phys Rev, 1939, 55: 526-535
[19]
14 Clairon A, Laurent P, Santarelli G, et al. A cesium fountain frequency standard: Preliminary results. IEEE Inst Meas, 1995, 44: 128-131？？
[20]
17 Li T, Li M, Lin P, et al. Improvements and new evaluation of NIM4 cesium fountain clock in 2005 and construction of NIM5 clock. ChinPhys Lett, 2007, 24: 1177-1179
[21]
18 Li T, Li M, Lin P, et al. The transportable cesium fountain clock NIM5: Its construction and performance. In: Proceedings of the 7th FrequencyStandards and Metrology, 2008. 314-320
[22]
20 Li T, Li M, Lin P, et al. NIM4 cesium atomic fountain primary frequency standard: Performance and evaluation. IEEE UFFC-FCS2004,2004. 431-432
[23]
21 Gao Y, Gao X, Li T, et al. The generation of new TA(NIM), which is steered by NIM5 cesium fountain clock. Metrologia, 2008, 45:34-37
[24]
22 Udem T, Reichert J, Hansch T, et al. Accurate measurement of large frequency differences with a mode-locked laser. Opt Lett, 1999, 24:881-883？？
24 Holzwart R, Udem T, Hansch T, et al. Optical frequency synthesize for precision spectroscopy. Phys Rev Lett, 2000, 85: 2264-2267？？
[27]
25 Ma L, Bi Z, Diddams S, et al. Optical frequency synthesis and comparison with uncertainty at the 10-19 level. Science, 2004, 303:1843-1845？？
[28]
26 Ma L, Robertsson L, Zucco M, et al. First international comparison of femtosecond laser combs at the international bureau of weights andmeasures. Opt Lett, 2004, 29: 641-643？？
[29]
28 Chou C, Hume D, Rosenband T, et al. Frequency comparison of two high-accuracy Al+ clocks. Phys Rev Lett, 2010, 104: 070802？？
[30]
29 Campbell G K, Ludlow A D, Ye J, et al. The absolute frequency of the 87Sr optical clock transition. Metrologia, 2008, 45: 539-548？？
[31]
30 Katori H, Ido T, Isoya Y, et al. Magneto-optical trapping and cooling of strontium atoms down to the photon recoil temperature. Phys RevLett, 1999, 82: 1116-1120
[32]
31 Tokamoto M, Katori H. Spectroscopy of the 1S0-3P0 clock transition of 87Sr in an optical lattice. Phys Rev Lett, 2003, 91: 223001？？
[33]
33 Wang S K, Wang Q, Lin Y G, et al. Cooling and trapping 88Sr atom with 461 nm laser. Chin Phys Lett, 2009, 26: 093202？？
