Shengnan Zhang

Study of a fully vacuum-sealed calcium atomic beam tube for optical frequency standard

Optical frequency standard is regarded as the future quantum frequency standard. With no laser cooling system, the calcium atomic beam optical frequency standard has the potential to be miniaturized and engineered, while providing a better performance than cesium clock and hydrogen clock. This paper presents the design and fabrication of a fully vacuum-sealed calcium beam tube with a smaller size and lighter weight. So far one testing has operated continuously for more than 15 months. We believe this study has laid a foundation for practicability and engineering of the calcium atomic beam optical frequency standard.

Hanle Detection for Optical Clocks

Considering the strong inhomogeneous spatial polarization and intensity distribution of spontaneous decay fluorescence due to the Hanle effect, we propose and demonstrate a universe Hanle detection configuration of electron-shelving method for optical clocks. Experimental results from Ca atomic beam optical frequency standard with electron-shelving method show that a designed Hanle detection geometry with optimized magnetic field direction, detection laser beam propagation and polarization direction, and detector position can improve the fluorescence collection rate by more than one order of magnitude comparing with that of inefficient geometry. With the fixed 423 nm fluorescence, the improved 657 nm optical frequency standard signal intensity is presented. The potential application of the Hanle detection geometry designed for facilitating the fluorescence collection for optical lattice clock with a limited solid angle of the fluorescence collection has been discussed. The Hanle detection geometry is also effective for ion detection in ion optical clock and quantum information experiments. Besides, a cylinder fluorescence collection structure is designed to increase the solid angle of the fluorescence collection in Ca atomic beam optical frequency standard.

A transportable calcium atomic beam optical clock

Achievement on atomic optical clock, which has reached to 10-18 level uncertainty, accelerates the research of the fundamental physics. But enormous volume size limits the application of optical clock outside the lab. Here, we report a transportable Calcium atomic beam optical clock with higher stability than general microwave clocks. In this system, we use the electron-shelving method to greatly improve the signal-to-noise of 657 nm signal. After locking, the stability of transportable Calcium atomic beam optical clock is 3.0 × 10-14 at 1 s and decreases to 2.9 × 10-15 at 200 s with continuous working time 11000 s by self-evaluation. Then we have used a 750 MHz repetition frequency optical frequency comb to transfer Calcium atomic beam optical clock frequency to microwave and compare with Hydrogen maser. The frequency stability of Calcium atomic beam optical clock versus hydrogen maser reaches to 1.62 × 10-12 at 1 s, which is close to the frequency stability of Hydrogen maser. The whole system is specially designed for robustness. Besides, a new full-sealed Calcium atomic beam vacuum system without flanges is constructed.

Distribution of populations in excited states of electrodeless discharge lamp of Rb atoms

The intensity of fluorescence spectral lines of Rb atoms in the region of 350–1110 nm is measured in eletrodeless discharge lamp. The population ratio between the excited states is calculated according to the spontaneous transition probabilities with rate equations. At the same time, the population density of energy level is also obtained. The results provide the potential applications of electrodeless discharge lamp in atomic filter and optical frequency reference at higher excited states without a pumping laser.

Experimental study of a miniaturized calcium atomic beam tube for small optical frequency standard

With the advantage of relatively simple structure, the optical frequency standard based on calcium thermal atomic beam has the potential to be miniaturized and engineered. We develop a new fully vacuum-sealed calcium atomic beam tube, which divergence angle of calcium atomic beam is narrower and background noise of 423 nm laser-induced fluorescence is much lower than its predecessor. This paper describes experimental study of this new tube using the 657 nm clock transition laser and 423 nm detection laser.

Dispersion detection of optical clock transition in thermal atomic beam

In order to develop small optical clock for various transportable applications with better stability and accuracy than that of commercial small Cs clock and H-maser clock, our group has proposed a small Ca atomic beam clock scheme with electron-shelving detection. Its potential stability was experimentally demonstrated recently. Here, we first demonstrated the dispersive detection of 657 nm transition via the Faraday rotation effect of 423 nm transition based on a thermal Ca atomic beam, this alternative detection method can effectively detect the clock signal by the ground state atoms after the clock laser interrogation. The Faraday anomalous dispersion optical filter of 423 nm transition can reach a maximum transmission of 10%. With a magnetic field of 84 G, the observed 3.3% of the ground state atoms has been excited to the σ± level of metastable 3P1(m=±1) state, which are separated 176.6 MHz to m=0 level.

Frequency stabilization of 423 nm laser for calcium-beam optical frequency standard

The Ca beam optical frequency standard has the potential to be small optical clock like small Cs beam clock in microwave regime. For improving the signal-to-noise ratio of the clock transition spectroscopy, the electron shelving detection, which uses the fast ¹S<inf>0</inf>-¹P<inf>1</inf> 423nm transition to detect the narrow ¹S<inf>0</inf>-³P<inf>1</inf> 657nm clock transition, was applied for Ca beam optical frequency standard. Therefore the frequency stability of 423 nm laser to some extent determines the performance of the optical frequency standard. In this paper, we stabilize the 423 nm laser frequency to Ca beam signal with side-band locking technique and estimate the preliminary stability of 423 nm laser which is 1.4×10⁻¹¹ at 1 s and 2×10⁻¹² at 100 s.

Precision measurement of the hyperfine structure of the 85Rb 6P 3/2 state

Partly subjected to the stability of optical frequency source, the magnetic dipole and electric quadrupole hyperfine constants of the ⁸⁵Rb 6P<inf>3/2</inf> State have been reported to be 8.163(4) MHz and 8.191(27) MHz, respectively. To pursue the higher precision, here we have constructed two more robust ⁸⁵Rb spectrometers with a preliminary stability of 2.3×10⁻¹³/√τ based on the modulation transfer spectroscopy technique of ⁸⁵Rb 5S<inf>1/2</inf>–6P<inf>3/2</inf> transition. By the heterodyne experiment between the two ⁸⁵Rb spectrometers, we have measured the hyperfine separations of ⁸⁵Rb 6P<inf>3/2</inf>, and obtained the magnetic dipole constant and the electric quadrupole constant which are 8.220 MHz and 5.148 MHz respectively, not considering the systematic errors yet.

A small calcium beam optical frequency standard with fully-sealed vacuum tube

In this paper a small Calcium beam optical frequency standard utilizing fully-sealed Calcium beam vacuum tube owning smaller size compared with vacuum system with flanges is demonstrated. Adopting the electron shelving detection method, we detect the clock transition 657 nm saturation absorbing spectroscopy whose linewidth is 500 kHz. The instability of this small optical frequency standard is 6.8×10−14 at 1 second and 1.8×10−15 after 1000 seconds of averaging by comparing with another small Calcium beam optical frequency standard whose reported instability is 3.0×10−14 at 1 second. The optical part of the whole system is specially designed for robustness and miniaturization.

Miniaturized calcium beam optical frequency standard using fully-sealed vacuum tube with 10 −15 instability

We implement a miniaturized calcium beam optical frequency standard using specially-designed fully-sealed vacuum tube, and realize the comparison with another calcium beam optical clock whose vacuum tube is sealed by flanges. The electron shelving detection method is adopted to improve the signal-to-noise ratio of the clock transition spectroscopy, and the readout laser is locked by modulation-free frequency locking technology based on Doppler effect. Injection locking is carried out to boost the power of the 657 nm master clock transition laser, thus ensuring the comparison. The fractional instability of the miniaturized calcium beam optical frequency standard using fully-sealed vacuum tube is 1.8×10-15 after 1600 s of averaging. Total volume of the system except for electronics is about 0.3 m3. To our knowledge, its the first time to realize the optical frequency standard using fully-sealed vacuum tube. This work will promote the miniaturization and transportability of the optical clock based on atomic beam.