Welcome To IQE！
Tel：010-6275-3208

**1. Research Background of Compact Calcium Beam Optical Clock**

Atomic clock is so far the most accurate time and frequency equipment, it is widely used in global satellite navigation, timekeeping etc. Theaccuracy and stabilityof the best atomic clocks (optical atomic clocks) has entered 10^{-19}level currently [1], thus these excellent performance atomic clocks are gradually being used in precision physical measurements and astrophysical observations [2, 3]. But the further applications of optical lattice atomic clocks and ion optical clocks are limited by their large sizes and complicated systems, our group is devoted to the research of the compact calcium beam optical clock atoms, thermal atomic beam is taken as the the quantum reference, this system does not need complicated laser cooling and trapping construction, the volume of physical part can be largely reduceed.

The research of the compact calcium beam optical clock is to make the excellent performance optical clock miniaturizized, integrated and transportable, expanding the application range of the optical clock andensuring the excellent performance (stability and accuracy) at the same time, to ensure that the optical clock can be applied into global satellite navigation system, timekeeping, space optical clock, optical clocks networks, geophysical astrophysical observations, precise physical measurements and other fields, this research is promising and significant.

Compared to the traditional method of direct detection of clock transition [4], our group put forward electronic shelving detection scheme applied into the atomic beam in 2006 [5], this method greatly improves the signal-to-noise ratio of the clock transition spectroscopy, and it has been widely used in atomic beam optical clocks [6-9]. In 2016, our group reported the compact calcium beam optical clock by self-estimation, the instability of which is 3 × 10

Figure 1. Relevant energy level of

Figure 1 is the energy level of

The clock transition spectroscopy is as shown in figure 2, the Q value is about 10

Figure 2. Clock transition spectroscopy of 657 nm.

Figure 3 is the photograph of our compact calcium beam optical clock, the entire optical parts (excluding the Pound - Drever - Hall system) is built on an 90 cm × 60 cm optical board, and enclosed in a black box, reducing the airflow and scattering light. The whole system uses only two external cavity diode lasers, a set of atomic beam vacuum tube, and a set of super-high finesse optical resonators.

Figure 3. Photograph of the compact calcium beam optical clock.

In order to further promote miniaturization and transportability, we adopt a smaller, better robust fully-sealed vacuum tube, which is more conducive to the transportability. At present, we have realized the compact calcium beam optical frequency standard based on fully-sealed vacuum tube, and carried out the comparsion of two optical frequency standards. As shown in Fig. 4, the instability is 1.8 × 10

Figure 4. Instability of the compact calcium beam optical

frequency standard based on fully sealed vacuum tube.

Our further work will focus on the stability and miniaturization of the system, the instability at 1 s is expected to enter 10

**3. Compact Rb Optical Clock**

We proposed a compact 420nm Rb optical clock based on the transition line 5S_{1/2}--6P_{3/2 }of ^{85}Rb with a stability of 1.2*10^{-14}/t1/2, which is better than all the Rb optical clock.

Here, we choose the 420nm transition for three reasons. First, compared with other transitions, for example, D1 and D2 lines of the first excited state of Rb, the natural linewidth of 6P_{3/2 }is narrower, and the absolute frequency of the 420nm transition is higher, which are beneficial to the final fractional frequency stability. Second, due to the high oscillator strength and high atomic density, the signal-to-noise ratio of 420 nm spectrum is higher than that of the 532 nm I2optical frequency transition. Besides, the shorter effective interaction length and diode laser lead to a more compact system. Third, the 420 nm Rb optical frequency standard can be further employed as the pumping laser for the 1529 nm Rb active optical clock.

Fig. 185Rb energy level Fig. 2 experimental steup Fig. 3 frequency stability curve

**Reference:**

[1] S. L. Campbell，R. B. Hutson, G. E. Marti, A. Goban, N. Darkwah Oppong, R. L. McNally, L. Sonderhouse, J. M. Robinson, W. Zhang, B. J. Bloom, J. Ye, "A Fermi-degenerate three-dimensional optical lattice clock", Science **358**, 90-94 (2017)

[2] T. Takano, M. Takamoto, I. Ushijima, N. Ohmae, T. Akatsuka, A. Yamaguchi, Y. Kuroishi, H. Munekane, B. Miyahara, and H. Katori, "Geopotential measurements with synchronously linked optical lattice clocks," Nat. Photonics **10**(10), 662-666 (2016).

[3] S. Kolkowitz, I. Pikovski, N. Langellier, M. D. Lukin, R. L. Walsworth, and J. Ye, "Gravitational wave detection with optical lattice clocks," Physics Review D**94**(12), 124043 (2016).

[4] P. Kersten, F. Mensing, U. Sterr, and F. Riehle, "A transportable optical calcium frequency standard," Applied Physics B**68**(1), 27-38 (1999).

[5] K. Huang, J. Zhang, D. Yu, Z. Chen, W. Zhuang, and J. Chen, "Application of electron-shelving detection via 423 nm transition in calcium-beam optical frequency standard," Chinese Phys. Lett.**23**(12), 3198-3201 (2006).

[6] J. J. Mcferran and A. N. Luiten, "Fractional frequency instability in the 10^{-14} range with a thermal beam optical frequency reference," Journal of Optical Society of American B **27**(2), 277-285 (2010).

[7] X. Zhang, S. Zhang, Z. Jiang, M. Li, H. Shang, F. Meng, W. Zhuang, A. Wang, and J. Chen, "A transportable calcium atomic beam optical clock," in*Proceedings of IEEE International Frequency Control Symposium *(IEEE, 2016), pp. 1-4.

[8] J. Olson, R. Fox, R. Brown, T. Fortier, T. Sheerin, R. Stoner, C. W. Oates, and A. D. Ludlow, "High-stability laser using Ramsey-Borde interferometry," in*Proceedings of IEEE International Frequency Control Symposium *(IEEE, 2017), pp. 32-33.

[9] H. Shang, X. Zhang, S. Zhang, D. Pan, H. Chen, J. Chen, "Miniaturized calcium beam optical frequency standard using fully sealed vacuum tube with 10-15 instability," Opt. Express**25**(24), 30459-30467 (2017).

[3] S. Kolkowitz, I. Pikovski, N. Langellier, M. D. Lukin, R. L. Walsworth, and J. Ye, "Gravitational wave detection with optical lattice clocks," Physics Review D

[4] P. Kersten, F. Mensing, U. Sterr, and F. Riehle, "A transportable optical calcium frequency standard," Applied Physics B

[5] K. Huang, J. Zhang, D. Yu, Z. Chen, W. Zhuang, and J. Chen, "Application of electron-shelving detection via 423 nm transition in calcium-beam optical frequency standard," Chinese Phys. Lett.

[6] J. J. Mcferran and A. N. Luiten, "Fractional frequency instability in the 10

[7] X. Zhang, S. Zhang, Z. Jiang, M. Li, H. Shang, F. Meng, W. Zhuang, A. Wang, and J. Chen, "A transportable calcium atomic beam optical clock," in

[8] J. Olson, R. Fox, R. Brown, T. Fortier, T. Sheerin, R. Stoner, C. W. Oates, and A. D. Ludlow, "High-stability laser using Ramsey-Borde interferometry," in

[9] H. Shang, X. Zhang, S. Zhang, D. Pan, H. Chen, J. Chen, "Miniaturized calcium beam optical frequency standard using fully sealed vacuum tube with 10-15 instability," Opt. Express